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% Protocol version used in document version and as version number label
% prescribed for software.
\newcommand{\sampversion}{1.2}

%%
%%  Header of the document...
%%

% Provide a title for your document
\title{SAMP --- Simple Application Messaging Protocol}

% Give date and version number
\date{2010-07-19}
\version{\sampversion}

% Choose one document type from below
%\ivoatype{IVOA Note}
\ivoatype{IVOA Working Draft}
%\ivoatype{IVOA Proposed Recommendation}
%\ivoatype{IVOA Recommendation}

% Name Working group
\ivoagroup{Applications}

% Give author list: separate different authors with \\
% You can add email addresses with links \url{mailto:yourname@ivoa.net}
\author{M.~Taylor (m.b.taylor@bristol.ac.uk)\\
        T.~Boch (boch@astro.u-strasbg.fr)\\
        M.~Fitzpatrick (fitz@noao.edu)\\
        A.~Allan (aa@astro.ex.ac.uk)\\
        L.~Paioro (luigi@lambrate.inaf.it)\\
        J.~Taylor (jontayler@gmail.com)\\
        D.~Tody (dtody@nrao.edu)}
\editor{T.~Boch, M.~Fitzpatrick, M.~Taylor}

\urlthisversion{\url{http://www.ivoa.net/Documents/WD/App/WD-SAMP-1.2-20100719.html}}
\urllastversion{\url{http://www.ivoa.net/Documents/latest/SAMP.html}}
\previousversion{
   1.0: \url{http://www.ivoa.net/Documents/WD/App/SAMP-20080625.html}\\
   1.1: \url{http://www.ivoa.net/Documents/PR/App/SAMP-20081121.html}\\
   1.11: \url{http://www.ivoa.net/Documents/REC/App/SAMP-20090421.html}}

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\begin{document}

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\begin{abstract}
SAMP is a messaging protocol that enables astronomy software tools to 
interoperate and communicate.

IVOA members have recognised that building a monolithic tool that attempts to fulfil all 
the requirements of all users is impractical, and it is a better use of our 
limited resources to enable individual tools to work together better. 
One element of this is defining common file formats for the exchange of data 
between different applications. Another important component is a messaging 
system that enables the applications to share data and take advantage of each 
other's functionality. SAMP builds on the success of a prior 
messaging protocol, PLASTIC, which has been in use since 2006 in 
over a dozen astronomy applications and has proven popular with 
users and developers.
It is also intended to form a framework for more general messaging 
requirements.
\end{abstract}

\section*{Status of this Document}

This is a Working Draft.
It is not appropriate to refer to it except as a work in progress.

Comments, questions and discussions relating to this document
may be posted to the SAMP subgroup of the Applications Working Group
at \url{mailto:apps-samp@ivoa.net}.

Changes since earlier versions may be found in Appendix \ref{sect:changes}.

This document has been produced by the IVOA Applications Working Group.


\tableofcontents

\section{Introduction}

\subsection{Non-Technical Preamble} \label{sect:nonTechPreamble}
SAMP, the Simple Application Messaging Protocol, is a standard
for allowing software tools to exchange control and data information,
thus facilitating tool interoperability, and so allowing users
to treat separately developed applications as an integrated suite.
An example of an operation that SAMP might facilitate is passing a
source catalogue from one GUI application to another, and 
subsequently allowing sources marked by the user in one of those 
applications to be visible as such in the other.

The protocol has been designed, and implementations developed, within
the context of the IVOA, but the design is not specific either to 
the Virtual Observatory (VO) or to Astronomy.  It is used in practice 
for both VO and non-VO work with astronomical tools, and is in  
principle suitable for non-astronomical purposes as well.

The SAMP standard itself is neither a dependent, nor a dependency,
of other VO standards, but it provides valuable glue between user-level
applications which perform different VO-related tasks, and hence
contributes to the integration of Virtual Observatory functionality
from a user's point of view.  SAMP interoperability is a feature of
most existing tools which operate within the User Layer of the IVOA
Architecture.

The semantics of messages that can be exchanged using SAMP are defined
by contracts known as MTypes (message-types), which are defined
by developer agreement outside of this standard.  The list of MTypes
used for common astronomical and VO purposes can be found near 
\url{http://www.ivoa.net/samp/}; many of these make use of standards from 
elsewhere in the IVOA Architecture, including VOTable, VOResource, 
Simple Spectral Access, UCD and Utype.


\subsection{History}
SAMP, the Simple Application Messaging Protocol,
 is a direct descendent of the PLASTIC protocol, which in turn grew 
--- in the European VOTech \cite{votech} framework --- 
from the interoperability work of the Aladin \cite{2000A+AS..143...33B} and 
VisIVO \cite{2007HiA....14..622B} teams. We also note the 
contribution of the team behind the earlier XPA protocol \cite{xpa}. For more information 
on PLASTIC's history and purpose see the IVOA Note {\em PLASTIC --- a protocol for 
desktop application interoperability} \cite{plastic}
and the PLASTIC SourceForge site \cite{plastic-sf}.

SAMP has similar aims to PLASTIC, but 
incorporates lessons learnt from two years of practical experience and ideas 
from partners who were not involved in PLASTIC's initial design. 

Broadly speaking, SAMP is an abstract framework for
loosely-coupled, asynchronous, RPC-like and/or event-based communication,
based on a central service providing multi-directional publish/subscribe
message brokering.
The message semantics are extensible and use structured but weakly-typed data.
These concepts are expanded on below.
It attempts to make as few assumptions as possible about the transport
layer or programming language with which it is used.
It also defines a ``Standard Profile'' which specifies how to 
implement this framework using XML-RPC \cite{xmlrpc} as the transport layer.
The result of combining this Standard Profile with the rest of the SAMP
standard is deliberately similar in design to PLASTIC, and the intention is
that existing PLASTIC applications can be modified to use SAMP 
instead without great effort.

\subsection{Requirements and Scope}
SAMP aims to be a simple and extensible protocol that is platform- and 
language-neutral. 
The emphasis is on a 
simple protocol with a very shallow learning curve in order to encourage
as many application authors as possible to adopt it.
SAMP is intended to do what you need most of the time. The SAMP authors 
believe that this is the best way to foster innovation and collaboration in
astronomy applications.

It is important to note therefore that SAMP's scope is reasonably modest; it is 
not intended to be the perfect messaging solution for all situations.
In particular SAMP itself has 
no support for transactions, guaranteed message delivery, message integrity or
messaging beyond a single machine.
However, by layering the SAMP architecture on top of suitable
messaging infrastructures such capabilities could be provided.
These possibilities are not discussed further in this document,
but the intention is to provide an architecture which is sufficiently
open to allow for such things in the future with little change to the
basics.

\subsection{Types of Messaging}
\label{sect:typeOfMsging}

        SAMP is currently limited to inter-application desktop messaging
with the idea that the basic framework presented here is extensible to meet
future needs, and so it is beyond the scope of this document to outline the
many types of messaging systems in use today (these are covered in detail
in many other documents).  While based on established messaging models,
SAMP is in many ways a hybrid of several basic messaging concepts; the
protocol is however flexible enough that later versions should be able to
interact fairly easily with other messaging systems because of the shared
messaging models.

The messaging concepts used within SAMP include:

\begin{description}
\item[Publish/Subscribe Messaging:]
    A publish/subscribe (pub/sub) messaging system supports an event driven
    model where information producers and consumers participate in message
    passing.  SAMP applications ``publish'' a message, while consumer
    applications ``subscribe'' to messages of interest and consume events.
    The underlying messaging system routes messages from producers 
    to consumers based on
    the message types in which an application has registered an interest.
\item[Point-to-Point Messaging:]
    In point to point messaging systems, messages are routed to an
    individual consumer which maintains a queue of ``incoming'' messages.  In
    a traditional message queue, applications send messages to a specified
    queue and clients retrieve them.  In SAMP, the message system manages
    the delivery and routing of messages, but also permits the concept of a
    directed message meant for delivery to a specific application.  SAMP
    does not, however, guarantee the order of message delivery as with a
    traditional message queue.
\item[Event-based Messaging:]
    Event-based systems are systems in which producers generate events, and in
    which messaging middleware delivers events to consumers based upon a
    previously specified interest.  One typical usage pattern of these systems
    is the publish/subscribe paradigm, however these systems are also widely
    used for integrating loosely coupled application components.  SAMP allows
    for the concept that an ``event'' occurred in the system and that these
    message types may have requirements different from messages where the
    sender is trying to invoke some action in the network of applications.
\item[Synchronous vs. Asynchronous Messaging:]
    As the term is used in this document, a ``synchronous'' message is one
    which blocks the sending application from further processing until a
    reply is received.  However, SAMP messaging is based on ``asynchronous''
    message and response in that the delivery of a message and its
    subsequent response are handled as separate activities by the
    underlying system.  With the exception of the synchronous message
    pattern supported by the system, sending or replying to a message using
    SAMP allows an application to return to other processing while the
    details of the delivery are handled separately.
\end{description}


\subsection{About this Document}
This document contains the following main sections describing the SAMP protocol
and how to use it.
Section \ref{sect:architecture}
covers the requirements, basic concepts and overall architecture of SAMP.
Section \ref{sect:apis} defines abstract (i.e.\ independent of language,
platform and transport protocol) interfaces which clients and hubs 
must offer to participate in SAMP messaging, along with data types
and encoding rules required to use them.
Section \ref{sect:profile}
explains how the abstract API can be mapped to specific network operations
to form an interoperable messaging system, and defines the ``Standard Profile'',
based on XML-RPC, which gives a particular set of such mappings.
Section \ref{sect:mtypes}
describes the use of the MType keys used to denote message semantics,
and outlines an MType vocabulary.

The key words ``MUST'', ``MUST NOT'', ``REQUIRED'', ``SHALL'', ``SHALL NOT'', 
``SHOULD'', ``SHOULD NOT'', ``RECOMMENDED'',  ``MAY'', and ``OPTIONAL'' 
in this document are to be interpreted as described in RFC 2119 \cite{rfc2119}.

\section{Architectural Overview}
\label{sect:architecture}
This section provides a high level view of the SAMP protocol.

\subsection{Nomenclature}
In the text that follows these terms are used: 

\begin{description}
\item[Hub:]
    A broker service for routing SAMP Messages.
\item[Client:]
    An application that talks to a Hub using SAMP. May be a
    Sender, Recipient, or both.
\item[Sender:]
    A Client that sends a SAMP Message to one or more
    Recipients via the Hub.
\item[Recipient:]
    A Client that receives a SAMP Message from
    the Hub. This may have originated from another Client or be from the
    Hub itself.
\item[Message:]
    A communication sent from a Sender to a Recipient
    via a SAMP Hub.  Contains an MType and zero or more named parameters.
    May or may not provoke a Response.
\item[Response:]
    A communication which may be returned from a Recipient to a Sender
    in reply to a previous Message.  A Response may contain returned values
    and/or error information.  In the terminology of this document,
    a Response is not itself a Message.  A Response is also known as a
    Reply in this document.
\item[MType:]
    A string defining the semantics of a Message and of its arguments and
    return values (if any).  Every Message contains exactly one MType,
    and a Message is only delivered to Clients subscribed to that MType.
\item[Subscription:]
    A Client is said to be Subscribed to a given MType if it has
    declared to the Hub that it is prepared to receive Messages
    with that MType.
\item[Callable Client:]
    A Client to which the Hub is capable of performing callbacks.
    Clients are not obliged to be Callable, but only Callable Clients
    are able to receive Messages or asynchronous Responses.
\item[Broadcast:]
    To send a SAMP Message to all Subscribed Clients 
    excluding the Sender.
\item[Profile:]
    A set of rules which map the abstract API defined by SAMP to a set of
    I/O operations which may be used by Clients to send and receive
    actual Messages.
\end{description}


\subsection{Messaging Topology}
SAMP has a hub-based architecture (see Figure~\ref{fig:samp-archi}). The hub is a single service used to route all 
messages between clients. This makes application discovery more 
straightforward in that each client only needs to locate the hub, and the services
provided by the hub are intended to simplify the actions of the client. A disadvantage of this 
architecture is that the hub may be a message bottleneck and potential single point of failure. 
The former means that SAMP may not be suitable for extremely high
throughput requirements;
the latter may be mitigated by an appropriate strategy for hub restart if failure is likely.
\begin{figure}
\begin{center}
\includegraphics[scale=0.5]{samp-archi}
\caption{The SAMP hub architecture}
\label{fig:samp-archi}
\end{center}
\end{figure}

Note that the hub is defined as a service interface which may have any of
a number of implementations.  It may be an independent application running as
a daemon, an adapter interface layered on top of an existing messaging
infrastructure, or a service provided by an application which is itself
one of the hub's clients.


\subsection{The Lifecycle of a Client}

A SAMP client goes through the following phases: 
\begin{enumerate}
	\item Determine whether a hub is running by using the appropriate hub discovery 
	mechanism.
	\item If so, use the hub discovery mechanism to work out how to communicate
        with the hub.
	\item Register with the hub.
	\item Store metadata such as client name, description and icon in the 
	hub.
	\item Subscribe to a list of MTypes to define messages which may be 
	received.
	\item Interrogate the hub for metadata of other clients.
	\item Send and/or receive messages to/from other clients via the hub.
	\item Unregister with the hub.
\end{enumerate}
Phases 4--7 are all optional and may be repeated in any order. 

By subscribing to the MTypes described in Section \ref{sect:hub-mtypes}
a client may, if it wishes, keep track of the details of other clients' 
registrations, metadata and subscriptions.

\subsection{The Lifecycle of a Hub}
A SAMP hub goes through the following phases: 

\begin{enumerate}
	\item Locate any existing hub by using the appropriate hub discovery mechanism.
	\begin{enumerate}
		\item Check whether the existing hub is alive.
		\item If so, exit.
	\end{enumerate}
	\item If no hub is running, or a hub is found but is not 
	functioning, write/overwrite the hub discovery record and start up.
	\item Await client registrations. When a client makes a legal 
	registration, assign it a public ID,
    and add the application to the table of registered 
	clients under the public ID. Broadcast a message
	announcing the registration of a 
	new client.
	\item When a client stores metadata in the hub, broadcast a message 
        announcing the change
	and make the metadata available.
	\item When a client updates its list of subscribed MTypes, broadcast a 
	message announcing the change and make the subscription information
        available
	\item When the hub receives a message for relaying, pass it on to 
	appropriate recipients which are subscribed to the message's MType.
	Broadcast messages are sent to all 
	subscribed clients except the sender, messages with a specified recipient
    are sent to that recipient if it is subscribed.
	\item Await client unregistrations. When a client unregisters, broadcast 
	a message announcing the unregistration 
	and remove the client from the table of registered clients. 
	\item If the hub is unable to communicate with a client, it may 
	unregister it as described in phase 7.
	\item When the hub is about to shutdown, broadcast a message to 
	all subscribed clients.
	\item Delete the hub discovery record.
\end{enumerate}
Phases 3--8 are responses to events which may occur 
multiple times and in any order.

The MTypes broadcast by the hub to inform clients of changes in its state
are given in Section \ref{sect:hub-mtypes}.

Readers should note that, given this scheme, race conditions may occur.
A client might for instance try to register with a hub which has just shut down,
or attempt to send to a recipient which has already unregistered.
Specific profiles MAY define best-practice rules in order to best manage these
conditions,
but in general clients should be aware that SAMP's lack of guaranteed
message delivery and timing means that unexpected conditions are possible.

\subsection{Message Delivery Patterns}
\label{sect:delivery-outline}

Messages can be sent according to three patterns, differing in 
whether and how a response is returned to the sender:
\begin{enumerate}
	\item Notification
	\item Asynchronous Call/Response
	\item Synchronous Call/Response
\end{enumerate}
The Notification pattern is strictly one-way while in the Call/Response
patterns the recipient returns a response to the sender.

If the sender expects to receive some useful data as a result of the
receiver's processing, or if it wishes to find out whether and
when the processing is completed, it should use one of the Call/Response
variants.  If on the other hand the sender has no interest in what
the recipient does with the message once it has been sent, it
may use the Notification pattern.  Notification, since it involves
no communication back from the recipient to the sender, uses
fewer resources.
Although typically ``event''-type messages will be sent using Notify
and ``request-for-information''-type messages will be sent using
Call/Response, the choice of which delivery pattern to use is
entirely distinct from the content of the message, and is up to
the sender; any message (MType) may be sent using any of the above
patterns.
Apart from the fact of returning or not returning a response,
the recipient SHOULD process messages in exactly the same way 
regardless of which pattern is used.

From the receiver's point of view there are only two cases:
Notification and Asynchronous Call/Response.
However, the hub provides a convenience
method which simulates a synchronous call from the sender's point of view.
The purpose of this is to simplify the use of the protocol
in situations such as scripting environments which cannot easily handle
callbacks.  However, it is RECOMMENDED to use the asynchronous pattern
where possible due to its greater robustness.

\subsection{Extensible Vocabularies}
\label{sect:vocab}

At several places in this document structured information is 
conveyed by use of a controlled but extensible vocabulary.
Some examples are the application metadata keys (Section \ref{sect:app-metadata}),
message encoding keys (Section \ref{sect:msg-encode}) and 
Standard Profile lockfile tokens (Section \ref{sect:lockfile}).

Wherever this pattern is used, the following rules apply.
This document defines certain well-known keys with defined meanings.
These may be OPTIONAL or REQUIRED as documented, but if present
MUST be used by clients and hubs in the way defined here.  
All such well-known keys start with the string ``{\tt samp.}''.

Clients and hubs are however free to introduce and use non-well-known
keys as they see fit.  Any string may be used for such a non-standard key, 
with the restriction that it MUST NOT start with the string ``{\tt samp.}''.

The general rule is that hubs and clients which encounter keys which
they do not understand SHOULD ignore them, propagating them to 
downstream consumers if appropriate.
As far as possible, where new keys are introduced they SHOULD be
such that applications which ignore them will continue to behave
in a sensible way.

Hubs and clients are therefore able to communicate information 
additional to that defined in the current version of this document
without disruption to those which do not understand it.
This extensibility may be of use to applications which have mutual 
private requirements outside the scope of this specification,
or to enable experimentation with new features.
If the SAMP community finds such experiments useful,
future versions of this document may bring such functionality within
the SAMP specification itself by defining new keys in the 
``{\tt samp.}'' namespace.  The ways in which these vocabularies 
are used means that such extensions should be possible with 
minimal upheaval to the existing specification and implementations.


\subsection{Use of Profiles}
\label{sect:profiles}

The design of SAMP is based on the abstract interfaces
defined in Section~\ref{sect:apis}.  On its own however, this does
not include the detailed instructions required by application developers
to achieve interoperability.  To achieve that, application developers
must know how to map the operations in the abstract SAMP interfaces 
to specific I/O
(in most cases, network) operations.  It is these I/O operations
which actually form the communication between applications.
The rules defining this mapping from interface to I/O operations
are what constitute a SAMP ``Profile''
(the term ``Implementation'' was considered for this purpose, but rejected
because it has too many overlapping meanings in this context).

There are two ways in which such a Profile can be specified as far as
client application developers are concerned:
\begin{enumerate}
\item By describing exactly what bytes are to be sent using what wire
      protocols for each SAMP interface operation
\item By providing one or more language-specific libraries with calls 
      which correspond to those of the SAMP interface
\end{enumerate}
Although either is possible, SAMP is well-suited for approach (1) above
given a suitable low-level transport library.
This is the case since the operations are quite low-level, 
so client applications can easily
perform them without requiring an independently developed SAMP library.
This has the additional advantages that central effort does not have to
be expended in producing language-specific libraries, and that
the question of ``unsupported'' languages does not arise.

Section~\ref{sect:profile} describes a Profile along the lines of (1) above,
based on XML-RPC. 
This can be used directly by client and hub developers, 
in conjunction with the abstract interface description
in Section~\ref{sect:apis}, to write interoperable applications.
This is at present the only SAMP Profile which has been defined.

Splitting the abstract interface and Profile descriptions 
in this way
separates the basic design principles from the details
of how to apply them, and it opens the door for other Profiles
serving other use cases in the future. 


\section{Abstract APIs and Data Types}
\label{sect:apis}

\subsection{Hub Discovery Mechanism}
\label{sect:hub-discovery}

In order to keep track of which hub is running, a hub discovery mechanism, capable 
of storing information about how to determine the existence of and communicate with 
a running hub, is needed. This is a Profile-specific matter and
a specific prescription is described in Section \ref{sect:lockfile}.

\subsection{Communicating with the Hub}
The details of how a client communicates with the hub are Profile-specific.
A specific prescription is described in Section \ref{sect:profile}.


\subsection{SAMP Data Types}
\label{sect:samp-data-types}

For all hub/client communication, including the actual content of messages,
SAMP uses three conceptual data types:
\begin{enumerate}
\item {\tt string} --- a scalar value consisting of a sequence of characters;
       each character is an ASCII character with hex code
       09, 0a, 0d or 20--7f
\item {\tt list} --- an ordered array of data items
\item {\tt map} --- an unordered associative array of key-value pairs,
      in which each key is a {\tt string} and each value is a data item
\end{enumerate}
These types can in principle be nested to any level, so that the elements
of a list or the values of a map may themselves be strings, lists or maps.

There is no reserved representation for a null value, and it is 
illegal to send a null value in a SAMP context even if the underlying
transport protocol permits this.  However a zero-length
string or an empty list or map may, where appropriate, be used to 
indicate an empty value.

Although SAMP imposes no maximum on the length of a string,
particular transport protocols or implementation considerations 
may effectively do so; in general, hub and client implementations 
are not expected to deal with data items of unlimited size.
General purpose MTypes SHOULD therefore be specified so that
bulk data is not sent within the message.  In general it is
preferred to define a message parameter as the URL or filename
of a potentially
large file rather than as the inline text of the file itself.

At the protocol level there is no provision for typing of scalars.
Unlike many Remote Procedure Call (RPC) protocols SAMP does not distinguish syntactically
between strings, integers, floating point values, booleans etc.
This minimizes the restrictions on what underlying transport
protocols may be used, and avoids a number of problems associated with
using typed values from untyped languages such as 
Python and Perl.
The practical requirement to transmit these types is addressed however
by the next section.

\subsection{Scalar Type Encoding Conventions}
\label{sect:scalar-types}

Although the protocol itself defines {\tt string} as the only scalar type,
some MTypes will wish to define parameters or return
values which have non-string semantics,
so conventions for encoding these as {\tt string}s
are in practice required. 
Such conventions only need to be understood by the sender and 
recipient of a given message and so can be established on a per-MType basis, 
but to avoid unnecessary duplication of effort
this section defines some commonly-used
type encoding conventions.

We define the following BNF productions:
\begin{verbatim}
  <digit>        ::= "0" | "1" | "2" | "3" | "4" | "5" | "6"
                   | "7" | "8" | "9"
  <digits>       ::= <digit> | <digits> <digit>
  <float-digits> ::= <digits> | <digits> "." | "." <digits>
                   | <digits> "." <digits>
  <sign>         ::= "+" | "-"
\end{verbatim}
With reference to the above we define the following type encoding conventions:
\begin{itemize}
\item \verb,<SAMP int> ::= [ <sign> ] <digits>,\\
   An integer value is encoded using its decimal representation with
   an OPTIONAL preceding sign and with no
   leading, trailing or embedded whitespace.
   There is no guarantee about the largest or smallest values which can
   be represented, since this will depend on the processing environment
   at decode time. \\
%
\item \verb,<SAMP float> ::= [ <sign> ] <float-digits>,\\
      \verb,                 [ "e" | "E" [ <sign> ] <digits> ],\\
   A floating point value is encoded as a mantissa with an OPTIONAL
   preceding sign followed by an OPTIONAL exponent part
   introduced with the character ``{\tt e}'' or ``{\tt E}''.
   There is no guarantee about the largest or smallest values which can
   be represented or about the number of digits of precision which are
   significant, since these will depend on the processing environment
   at decode time. \\
%
\item \verb,<SAMP boolean> ::= "0" | "1",\\
   A boolean value is represented as an integer: zero represents false,
   and any other value represents true.
   1 is the RECOMMENDED value to represent true.
\end{itemize}

The numeric types are based on the syntax of the C programming language,
since this syntax forms the basis for typed data syntax in many other
languages.
There may be extensions to this list in future versions of this standard.

Particular MType definitions may use these conventions or devise
their own as required.  Where the conventions in this list are used, 
message documentation SHOULD make it clear using a form of
words along the lines ``this parameter contains a {\em SAMP int\/}''.


\subsection{Registering with the Hub}
\label{sect:registration}

A client registers with the hub to:
\begin{enumerate} 
    \item establish communication with the hub
    \item advertise its presence to the hub and to other clients
    \item obtain registration information
\end{enumerate}
The registration information is in the form of a {\tt map} containing
data items which the client may wish to use during the SAMP session.
The hub MUST fill in values for the following keys in the returned {\tt map}:
\begin{description}
\item[{\tt samp.hub-id}] ---
   The client ID which is used by the hub when it sends messages itself 
   (rather than forwarding them from other senders).
   For instance, this ID will be used when the hub sends the 
   {\tt samp.hub.event.shutdown} message.
\item[{\tt samp.self-id}] ---
   The client ID which identifies the registering client.
\end{description}
These keys form part of an extensible vocabulary as explained in
Section \ref{sect:vocab}.
In most cases a client will not require either of the above IDs for
normal SAMP operation, but they are there for clients which do wish
to know them.
Particular Profiles may require additional entries in this map.

Immediately following registration, the client will typically
perform some or all of the following OPTIONAL operations:
\begin{itemize}
    \item supply the hub with metadata about itself, using the
    {\tt declareMetadata()} call
    \item tell the hub how it wishes the hub to communicate with it,
    if at all (the mechanism for this is profile-dependent, and it may
    be implicit in registration)
    \item inform the hub which MTypes it wishes to subscribe to, using the
    {\tt declareSubscriptions()} call
\end{itemize}


\subsection{Application Metadata}
\label{sect:app-metadata}

A client may store metadata in the form of a {\tt map} of key-value pairs in the hub 
for retrieval by other clients. Typical metadata might be the human-readable 
name of the application, a description and a URL for its icon, but other values 
are permitted. The following keys are defined for well-known metadata items:
\begin{description}
\item[{\tt samp.name}] --- A one word title for the application.
\item[{\tt samp.description.text}] --- A short description of the 
application, in plain text.
\item[{\tt samp.description.html}] --- A description of the application, 
in HTML.
\item[{\tt samp.icon.url}] --- The URL of an icon in png, gif or jpeg format. 
\item[{\tt samp.documentation.url}] --- The URL of a documentation web page.
\end{description}
All of the above are OPTIONAL, but {\tt samp.name} is strongly RECOMMENDED.
These keys form the basis of an extensible vocabulary as explained in
Section \ref{sect:vocab}.


\subsection{MType Subscriptions}
\label{sect:subscription}

As outlined above, an MType is a string which defines the semantics of 
a message.  MTypes have a hierarchical form.  Their syntax is given
by the following BNF:
\begin{verbatim}
  <mchar>  ::= [0-9a-z] | "-" | "_"
  <atom>   ::= <mchar> | <atom> <mchar>
  <mtype>  ::= <atom> | <mtype> "." <atom>
\end{verbatim}
Examples might be ``{\tt samp.hub.event.shutdown}'' or ``{\tt file.load}''.

A client may {\em subscribe\/} to one or more MTypes to indicate 
which messages it is willing to receive.  A client will only ever
receive messages with MTypes to which it has subscribed.
In order to do this it passes a subscriptions {\tt map} to the hub.
Each key of this map is an MType string to which the client wishes
to subscribe, and the corresponding value is a map which may contain
additional information about that subscription.  Currently, no keys
are defined for these per-MType maps, so typically they will be empty
(have no entries).  The use of a map here is to permit experimentation
and perhaps future extension of the SAMP standard.

As a special case, simple wildcarding is permitted in subscriptions.
The keys of the subscription map may actually be of the form
\verb|<msub>|, where
\begin{verbatim}
  <msub>   ::= "*" | <mtype> "." "*"
\end{verbatim}
Thus a subscription key ``{\tt file.event.*}'' means that a client wishes
to receive any messages with MType which begin ``{\tt file.event.}''.
This does not include ``{\tt file.event}''.
A subscription key ``{\tt *}'' subscribes to all MTypes.
Note that the wildcard ``{\tt *}'' character may only appear
at the end of a subscription key, and that this indicates 
subscription to the entire subtree.

More discussion of MTypes, including their semantics, is given in
Section \ref{sect:mtypes}.

\subsection{Message Encoding}
\label{sect:msg-encode}

A message is an abstract container for the information we wish to send
to another application.  The message itself is that data which should
arrive at the receiving application.  It may be transmitted along
with some external items (e.g.\ sender, recipient and message identifiers)
required to ensure proper delivery or handling.

A message is encoded for SAMP transmission as a {\tt map} with the following REQUIRED keys:
\begin{description}
\item[{\tt samp.mtype}] ---
  A {\tt string} giving the MType which defines the meaning of the message.
  The MType also, via external documentation, defines the names, types and 
  meanings of any parameters and return values.
  MTypes are discussed in more detail in 
  Section \ref{sect:mtypes}.
\item[{\tt samp.params}] ---
  A {\tt map} containing the values for the message's named parameters.
  These give the data required for the receiver to act on the message,
  for instance the URL of a given file.  The names, types and semantics 
  of these parameters are determined by the MType.
  Each key in this map is the name of a parameter, and the corresponding
  value is that parameter's value.
\end{description}
These keys form the basis of an extensible vocabulary as explained in
Section \ref{sect:vocab}.


\subsection{Response Encoding}
\label{sect:response-encode}

A response is what may be returned from a recipient to a sender giving
the result of processing a message (though in the case of the Notification
delivery pattern, no such response is generated or returned).
It may contain MType-specific return values, or error information, or both.

A response is encoded for SAMP transmission as a {\tt map} with the following keys:
\begin{description}
\item[{\tt samp.status}] (REQUIRED) ---
   A {\tt string} summarising the result of the processing.
   It may take one of the following defined values:
   \begin{description}
   \item[{\tt samp.ok}:]
      Processing successful.
      The {\tt samp.result}, but not the {\tt samp.error} entry
      SHOULD be present.
   \item[{\tt samp.warning}:]
      Processing partially successful.
      Both {\tt samp.result} and {\tt samp.error} entries SHOULD be present.
   \item[{\tt samp.error}:]
      Processing failed.
      The {\tt samp.error}, but not the {\tt samp.result} entry
      SHOULD be present.
   \end{description}
   These values form the basis of an extensible vocabulary as explained in
   Section \ref{sect:vocab}.
\item[{\tt samp.result}] (REQUIRED in case of full or partial success) ---
   A {\tt map} containing the values for the message's named return values.
   The names, types and semantics of these returns are determined by
   the MType.
   Each key in this map is the name of a return value, and the corresponding
   value is the actual value.
   Note that even for MTypes which define no return values, the value of this
   entry MUST still be a {\tt map} (typically an empty one).
\item[{\tt samp.error}] (REQUIRED in case of full or partial error) ---
   A {\tt map} containing error information.
   The following keys are defined for this map:
   \begin{description}
      \item[{\tt samp.errortxt}] (REQUIRED) ---
         A short string describing what went wrong.
         This will typically be delivered to the user of the sender application.
      \item[{\tt samp.usertxt}] (OPTIONAL) ---
         A free-form string containing any additional text an application wishes
         to return.  This may be a more verbose error description meant to be  
         appended to the {\tt samp.errortxt} string,
         however it is undefined how this 
         string should be handled when received.
      \item[{\tt samp.debugtxt}] (OPTIONAL) ---
         A longer string which may contain more detail on what went wrong.
         This is typically intended for debugging purposes, and may for instance
         be a stack trace.
      \item[{\tt samp.code}] (OPTIONAL) ---
         A string containing a numeric or textual code identifying the error,
         perhaps intended to be parsable by software.
   \end{description}
   These keys form the basis of an extensible vocabulary as explained in
   Section \ref{sect:vocab}.
\end{description}
These keys form the basis of an extensible vocabulary as explained in
Section \ref{sect:vocab}.


\subsection{Sending and Receiving Messages}
\label{sect:delivery}

As outlined in Section~\ref{sect:delivery-outline}, 
three messaging patterns are supported, differing
according to whether and how the response is returned to the sender. 
For a given MType 
there may be a messaging pattern that is most typically used, but there is 
nothing in the protocol that ties a particular MType to a particular messaging 
pattern; any MType may legally be sent using any delivery pattern.

From the point of view of the sender, there are three ways in which a message
may be sent,
and from the point of view of the recipient there are two ways in which
one may be received.  These are described as follows.
\begin{description}
\item[Notification:]  In the notification pattern, communication is only
in one direction:
\begin{enumerate}
\item The sender sends a message to the hub for delivery to one or more
   recipients.
\item The hub forwards the message to those requested recipients which are subscribed.
\item No reply from the recipients is expected or possible.
\end{enumerate}
Notifications can be sent to a given recipient or broadcast to all 
recipients.  The notification pattern for a single recipient is illustrated in 
Figure~\ref{fig:notification}.
\begin{figure}[!h]
\begin{center}
\includegraphics[scale=0.5]{samp-notification}
\caption{Notification pattern}
\label{fig:notification}
\end{center}
\end{figure}
%
\item[Asynchronous Call/Response:]
In the asynchronous call pattern, {\em message tags\/} and
{\em message identifiers\/} are used 
to tie together messages and their replies:
\begin{enumerate}
\item The sender sends a message to the hub for delivery to one or more
    recipients, supplying along with the message a tag string of its
    own choice, {\em msg-tag\/}.  In return it receives a unique 
    identifier string, {\em msg-id\/}.
\item The hub forwards the message to the appropriate recipients,
    supplying along with the message an identifier string, {\em msg-id\/}.
\item Each recipient processes the message, and sends its response
    back to the hub along with the ID string {\em msg-id\/}.
\item Using a callback, the hub passes the response back to the 
    original sender along with the ID string {\em msg-tag\/}.
\end{enumerate}
The sender is free to use any value for the {\em msg-tag\/}.
There is no requirement on the form of the hub-generated {\em msg-id\/}
(it is not intended to be parsed by the recipient), but it MUST be
sufficient for the hub to pair messages with their responses reliably,
and to pass the correct {\em msg-tag\/} back with the response 
to the sender\footnote{
   One way a hub might implement this is to generate {\em msg-id\/}
   by concatenating the sender's client ID and the {\em msg-tag\/}.
   When any response is received the hub can then unpack the accompanying
   {\em msg-id\/} to find out who the original sender was and what
   {\em msg-tag\/} it used.  In this way the hub can determine
   how to pass each response back to its correct sender without needing
   to maintain internal state concerning messages in progress.
   Hub and client implementations may wish to exploit this freedom 
   in assigning message IDs for other purposes as well, 
   for instance to incorporate timestamps or checksums.
}.
In most cases the sender will not require the {\em msg-id\/}, since
the {\em msg-tag\/} is sufficient to match calls with responses.
For this reason, the sender need not retain the {\em msg-id\/} and
indeed need not wait for it, avoiding a hub round trip at send time.
The only case in which the sender may require the {\em msg-id\/} is
if it needs to communicate later with the recipient about the message
that was sent, for instance as part of a progress report.
Asynchronous calls may be sent to a given recipient or broadcast to all
recipients.  In the latter case, the sender SHOULD be prepared to deal
with multiple responses to the same call.
The asynchronous pattern is illustrated in Figure~\ref{fig:async}.
\begin{figure}[!h]
\begin{center}
\includegraphics[scale=0.45]{samp-asynchronous}
\caption{Asynchronous Call/Response pattern}
\label{fig:async}
\end{center}
\end{figure}
%
\item[Synchronous Call/Response]
A synchronous utility method is provided by the hub, mainly for
the convenience of
environments where dealing with asynchronicity might be a problem.
The hub will provide synchronous behaviour to the sender,
interacting with the receiver in exactly the same way as for the
asynchronous case above.
\begin{enumerate}
\item The sender sends a message to the hub for delivery to a given recipient,
      optionally specifying as well a maximum time it is prepared to wait.
      The sender's call blocks until a response is available.
\item The hub forwards the message to the recipient,
    supplying along with the message an ID string, {\em msg-id\/}.
\item The recipient processes the message, and sends its response
    back to the hub along with the ID string {\em msg-id\/}.
\item The hub passes back the response as the return value from the original
    blocking call made by the sender.  If no response is received within
    the sender's specified timeout the blocking call will terminate with 
    an error.  The hub is not guaranteed to wait indefinitely; 
    it MAY in effect impose its own timeout.
\end{enumerate}
There is no broadcast counterpart for the synchronous call.
This pattern is illustrated in Figure~\ref{fig:sync}.
\begin{figure}[!h]
\begin{center}
\includegraphics[scale=0.45]{samp-synchronous}
\caption{Synchronous Call/Response pattern}
\label{fig:sync}
\end{center}
\end{figure}
%
\end{description}

Note that the two different cases from the receiver's point of view,
{\em Notification\/} and {\em Call/Response\/}, 
differ only in whether a response is returned to the hub.
In other respects the receiver SHOULD process the message in 
exactly the same way for both patterns.

Although it is REQUIRED by this standard that client applications
provide a Response for every Call that they receive, there is no
way that the hub can enforce this.  Senders using the Synchronous
or Asynchronous Call/Response patterns therefore should be aware 
that badly-behaved recipients might fail to respond, leading to calls
going unanswered indefinitely.  The timeout parameter in the
Synchronous Call/Response pattern provides some protection from
this eventuality; users of the Asynchronous Call/Response pattern
may or may not wish to take their own steps.


\subsection{Operations a Hub Must Support}
\label{sect:hubOps}
This section describes the operations that a hub MUST support and the associated
data that MUST be sent and received.
The precise details of how these operations 
map onto method names and signatures is Profile-dependent.
The mapping for the Standard Profile is given in Section \ref{sect:mappingXMLRPC}.
\begin{itemize}
	\item \verb|map reg-info = register()|\\
    Method called by a client wishing to register 
	with the hub.
        The form of {\tt reg-info} is given in Section \ref{sect:registration}.
	Note that the form of this call may vary according to the requirements 
	of the particular Profile in use. For instance authentication tokens
    may be passed in one or both directions to complete registration.
    \\
	\item \verb|unregister()|\\
    Method called by a client wishing to unregister from the hub
    \\
	\item \verb|declareMetadata(map metadata)|\\
    Method called 
	by a client to declare its metadata.
	May be called zero or more times to update hub state; the most recent 
	call is the one which defines the client's currently declared metadata.
        The form of the {\tt metadata} map is given in
        Section \ref{sect:app-metadata}.
    \\
	\item \verb|map metadata = getMetadata(string client-id)|\\
    Returns the metadata
	information for the client whose public ID is {\tt client-id}.
        The form of the {\tt metadata} map is given in 
        Section \ref{sect:app-metadata}.
    \\
	\item \verb|declareSubscriptions(map subscriptions)|\\ 
    Method called by
	a callable client to declare the MTypes it wishes to subscribe to.
	May be called zero or more times to update hub state; the most recent 
	call is the one which defines the client's currently subscribed MTypes.
        The form of the {\tt subscriptions} map is given in
        Section \ref{sect:subscription}.
    \\
	\item \verb|map subscriptions = getSubscriptions(string client-id)|\\
    Returns the subscribed MTypes
        for the client whose public ID is {\tt client-id}.
        The form of the {\tt subscriptions} map is given in
        Section \ref{sect:subscription}.
    \\
	\item \verb|list client-ids = getRegisteredClients()|\\
    Returns the list of public ids of all other registered clients.
    The caller's ID ({\tt samp.self-id} from Section \ref{sect:registration}) 
    is not included,
    but the hub's ID ({\tt samp.hub-id} from Section \ref{sect:registration}) is.
    \\
    \item \verb|map client-subs = getSubscribedClients(string mtype)|\\
    Returns a map with an entry for all other registered clients which are
    subscribed to the MType {\tt mtype}.
    The key for each entry is a subscribed client ID, 
    and the value is a (possibly empty) {\tt map} 
    providing further information on its subscription to {\tt mtype} 
    as described in Section \ref{sect:subscription}.
    An entry for the caller is not included, even if it is subscribed.
    {\tt mtype} MUST NOT include wildcards.
    \\
	\item \verb|notify(string recipient-id, map message)|\\
        Sends a message using the Notification pattern
        to a given recipient.
        The form of the {\tt message} map is given 
        in Section \ref{sect:msg-encode}.
        An error results if the recipient is not subscribed to the message's
        MType.
    \\
	\item \verb|list recipient-ids = notifyAll(map message)|\\ 
        Sends a message using the Notification pattern
        to all other clients which are subscribed to the message's MType.
        The form of the {\tt message} map is given 
        in Section \ref{sect:msg-encode}.
        The return value is a {\tt list} of the client IDs of the clients
        to which an attempt to send the message is made.
    \\
	\item \verb|string msg-id = call(string recipient-id, string msg-tag,|\\
              \verb|                     map message)|\\
        Sends a message using the Asynchronous Call/Response pattern
        to a given recipient.
        The form of the {\tt message} map is given 
        in Section \ref{sect:msg-encode}.
        An error results if the recipient is not subscribed to the message's
        MType, or if the invoking client is not Callable.
    \\
	\item \verb|map calls = callAll(string msg-tag, map message)|\\
        Sends a message using the Asynchronous Call/Response pattern
        to all other clients which are subscribed to the message's MType.
        The form of the {\tt message} map is given 
        in Section \ref{sect:msg-encode}.
        The returned value is a {\tt map} in which the keys are the client IDs
        of clients to which an attempt to send the message is made,
        and the values are the associated {\tt msg-id} strings.
        An error results if the invoking client is not Callable.
    \\
	\item \verb|map response = callAndWait(string recipient-id,|\\
              \verb|                           map message, string timeout)|\\
        Sends a message using the Synchronous Call/Response pattern
        to a given recipient.
        The forms of the {\tt message} and {\tt response} maps are given 
        in Sections \ref{sect:msg-encode} and \ref{sect:response-encode}.
        The {\tt timeout} parameter is interpreted as a {\em SAMP int\/}
        (Section \ref{sect:scalar-types}) giving
        the maximum number of seconds the client wishes to wait.
        If the response takes longer than that to arrive this method SHOULD
        terminate anyway with an error (it MUST not return a {\tt response} indicating
        error).
        Any response arriving from the recipient after that will be discarded.
        If {\tt timeout}$<=0$ then no artificial timeout is imposed.
        An error results if the recipient is not subscribed to the message's
        MType.
    \\
	\item \verb|reply(string msg-id, map response)|\\
    Method
	called by a client to send its response to a given message.
        The form of the {\tt response} map is given 
        in Section \ref{sect:response-encode}.
\end{itemize}

Of these operations, only {\tt callAndWait()} involves blocking communication
with another client.
The others SHOULD be implemented in such a way that clients can expect
them to complete, and where appropriate return a value, on a timescale
short compared to user response time.

\subsection{Operations a Callable Client Must Support}
\label{sect:clientOps}

This section lists the operations which a client MUST support in order
to be classified as callable.
The hub uses these operations when it wishes to 
pass information to a callable client.
Note that callability is OPTIONAL for clients; special (Profile-dependent) steps
may be required for a client to inform the hub how it can be contacted,
and thus become callable.  Clients which are not callable can send
messages using the Notify or Synchronous Call/Response patterns,
but are unable to receive messages or to use Asynchronous Call/Response,
since these operations rely on client callbacks from the hub.

The precise details of how these operations 
map onto method names and signatures is Profile-dependent.
The mapping for the Standard Profile is given in Section \ref{sect:mappingXMLRPC}.
\begin{itemize}
	\item \verb|receiveNotification(string sender-id, map message)|\\
	Method called by the hub when dispatching a notification to its
	recipient.
        The form of the {\tt message} map is given 
        in Section \ref{sect:msg-encode}.
    \\
	\item \verb|receiveCall(string sender-id, string msg-id, map message)|\\
	Method called by the hub when dispatching a call to its recipient. The client
	MUST at some later time make a matching call to {\tt reply()} on the hub.
        The form of the {\tt message} map is given 
        in Section \ref{sect:msg-encode}.
    \\
	\item \verb|receiveResponse(string responder-id, string msg-tag,|\\
	      \verb|                map response)|\\
	Method used by the hub to dispatch to the sender the response of an
	earlier asynchronous call.
        The form of the {\tt response} map is given 
        in Section \ref{sect:response-encode}.
\end{itemize}

\subsection{Error Processing}
\label{sect:faults}

Errors encountered by clients when processing Call/Response-pattern messages 
themselves (in response to a syntactically legal {\tt receiveCall()}
operation) SHOULD be signalled by returning appropriate content in
the response map sent back in the matching {\tt reply()} call,
as described in Section \ref{sect:response-encode}.

In the case of failed calls of
the operations defined in Sections \ref{sect:hubOps} and \ref{sect:clientOps},
for instance syntactically invalid parameters or communications failures,
hubs and clients SHOULD use the usual error reporting mechanisms 
of the transport protocol in use.
Note in particular that a {\tt samp.status=samp.error}-type Response
(Section \ref{sect:response-encode})
is only generated by a client processing a received Message, 
it MUST NOT be used by the hub to signal a failed call.



%% Mark Taylor's part %%
\section{Standard Profile}
\label{sect:profile}


Section \ref{sect:apis} provides an abstract definition of 
the operations and data structures used for SAMP messaging.
As explained in Section~\ref{sect:profiles}, 
in order to implement this architecture some concrete choices about 
how to instantiate these concepts are required.

This section gives the details of a SAMP Profile based on the 
XML-RPC specification \cite{xmlrpc}.  
Hub discovery is via a lockfile in the user's home directory.

XML-RPC is a simple general purpose Remote Procedure Call
protocol based on sending XML documents using HTTP POST
(it resembles a very lightweight version of SOAP).
Since the mappings from SAMP concepts
such as API calls and data types to their XML-RPC equivalents is very 
straightforward, it is easy for application authors to write 
compliant code without use of any SAMP-specific library code.
An XML-RPC library, while not essential, will make coding much easier;
such libraries are available for many languages.


\subsection{Data Type Mappings}

The SAMP argument and return value data types described in
Section \ref{sect:samp-data-types} map straightforwardly onto XML-RPC
data types as follows:
\begin{center}
\begin{tabular}{lcl}
SAMP type    &                       & XML-RPC element \\
\hline
{\tt string} & --- & {\tt <string>} \\
{\tt list}   & --- & {\tt <array>} \\
{\tt map}    & --- & {\tt <struct>} \\
\end{tabular}
\end{center}
The {\tt <value>} children of {\tt <array>} and {\tt <struct>} elements
themselves contain children of type {\tt <string>}, {\tt <array>} or
{\tt <struct>}.

Note that other XML-RPC scalar types ({\tt <i4>}, {\tt <double>} etc)
are not used; even where the semantic sense of a value matches
one of those types it MUST be encoded as an XML-RPC {\tt <string>}.


\subsection{API Mappings}
\label{sect:mappingXMLRPC}

The operation names in the SAMP hub and client abstract APIs
(Sections \ref{sect:hubOps} and \ref{sect:clientOps}) very nearly 
have a one to one mapping with those in the Standard Profile XML-RPC APIs.
The Standard Profile API MUST be implemented as described in Sections
\ref{sect:hubOps} and \ref{sect:clientOps} with the following REQUIRED adjustments:
\begin{enumerate}
\item The XML-RPC method names (i.e.\ the contents of the XML-RPC
      {\tt <methodName>} elements) are formed by 
      prefixing the hub and client abstract API operation names with
      ``{\tt samp.hub.}'' or ``{\tt samp.client.}'' respectively.
\item The {\tt register()} operation takes the following form:
      \begin{itemize}
      \item \verb|map reg-info = register(string samp-secret)|
      \end{itemize}
      The argument is the {\tt samp-secret} value read from the lockfile
      (see Section \ref{sect:lockfile}).
      The returned {\tt reg-info} map contains an additional entry 
      with key {\tt samp.private-key} whose value is a string
      generated by the hub.
\item {\em All\/} other hub and client methods take the 
      {\tt private-key} as their first argument.
\item A new method, {\tt setXmlrpcCallback()} is added to the hub API.
      \begin{itemize}
      \item \verb|setXmlrpcCallback(string private-key, string url)|
      \end{itemize}
      This informs the hub of the XML-RPC endpoint on which the client 
      is listening for calls from the hub.
      The client is not considered Callable unless and until 
      it has invoked this method.
\item Another new method, {\tt ping()} is added to the hub API.
      This may be called by registered or unregistered applications
      (as a special case the {\tt private-key} argument may be omitted),
      and can be used to determine whether the hub is responding to requests.
      Any non-error return indicates that the hub is running.
\end{enumerate}

The {\tt private-key} string referred to above serves two purposes.
First it identifies the client in hub/client communications.
Some such identifier is required, since XML-RPC calls have no other way of
determining the sender's identity.
Second, it prevents application spoofing, since the private key is
never revealed to other applications, so that one application cannot
pose as another in making calls to the hub.

The usual XML-RPC fault mechanism is used to respond to invalid
calls as described in Section \ref{sect:faults}.
The XML-RPC {\tt <fault>}'s {\tt <faultString>} element SHOULD contain a
user-directed message as appropriate and the {\tt <faultCode>} value
has no particular significance.


\subsection{Lockfile and Hub Discovery}
\label{sect:lockfile}

Hub discovery is performed by examining a lockfile to determine
hub connection parameters, specifically the XML-RPC endpoint at
which the hub can be found, and a ``secret'' token which affords
some measure of security.  To discover the hub, a client must
therefore:
\begin{enumerate}
  \item Determine where to find the lockfile
        (\ref{sect:lockfileLoc})
  \item Read the lockfile to obtain the hub connection parameters
        (\ref{sect:lockfileText})
\end{enumerate}

\subsubsection{Lockfile Location}
\label{sect:lockfileLoc}

The default location of the lockfile is the file named ``{\tt .samp}''
in the user's home directory.  However the content of the environment
variable named SAMP\_HUB can be used to override this default.

The value of the SAMP\_HUB environment variable is of the form
\verb|<samphub-value>|, as defined by the following BNF production:
\begin{verbatim}
   <samphub-value>  ::=  <hub-location>
   <hub-location>   ::=  <stdlock-prefix> <stdlock-url>
   <lockurl-prefix> ::=  "std-lockurl:"
   <stdlock-url>    ::=  (any URL)
\end{verbatim}
The \verb|<stdlock-url>| will typically, but not necessarily, be a
file-type URL (as described in RFC 1738, section 3.10 \cite{rfc1738}).
So for instance to indicate that the
lockfile to be used will be the file ``{\tt /tmp/samp1}'', you would set
\begin{verbatim}
   SAMP_HUB=std-lockurl:file:///tmp/samp1
\end{verbatim}

Although no other form of the \verb|<hub-location>| value is defined
here, the intention is that the SAMP\_HUB environment variable MAY
be used with prefixes other than ``{\tt std-lockurl:}'' to indicate
use of other, non-Standard, profiles.

Therefore to locate the lockfile, a Standard Profile-compliant client MUST
determine whether an environment variable named SAMP\_HUB exists;
if so, the client MUST examine the variable's value;
if the value begins with the prefix ``{\tt std-lockurl:}''
the client MUST interpret the remainder of the value as a URL
whose content is the text of the lockfile to be used for hub discovery.
If no SAMP\_HUB environment variable exists, the client MUST
use the file ``.samp'' in the user's home directory as the
lockfile to be used for hub discovery.
If the variable exists, but its value begins with a different prefix,
the client MAY interpret that in some non-Standard way for hub discovery.

Rules for a Standard Profile-compliant hub to use when writing lockfiles 
are similar, but if a hub is unable or unwilling to write a lockfile 
such that it can be read using the above procedure, it MUST signal
an error at the startup and then abort.  For practical reasons, a hub
will probably only be able to write a lockfile indicated by a {\tt file}-type
URL, not for instance an arbitrary {\tt http}-type one.

The lockfile SHOULD normally be created with permissions which allow
only its owner to read it.  This provides a measure of security in
that only processes with the same permissions as the hub process
(hence presumably running under the same user ID) will be able to
register with the hub, since only they will be able to provide the
secret token, obtained from the lockfile, which is required for registration.
Thus under normal circumstances all participants in a SAMP conversation can be
presumed to be owned by the same user, and therefore not malicious.\footnote{
   Of course they may be owned by the same user and still be malicious,
   but in this case SAMP represents no additional security risk.
}
If a lockfile is made available in some other way, for instance via
an unprotected {\tt http}-type URL in order to facilitate use of
the same hub by multiple users on different hosts, a security risk
will arise. In that case, a protection through an authentication and/or
authorization mechanism might be adopted by the hub implementations, for
instance exploiting TLS cryptographic protocol.

The existence or readability of the lockfile MAY be taken 
(e.g.\ by a hub deciding whether to start or not) 
to indicate that a hub is running.
However it is RECOMMENDED to attempt to contact the hub
at the given XML-RPC URL (e.g.\ by calling {\tt ping()})
to determine whether it is actually alive.

The ``home directory'' referred to above is a somewhat system-dependent 
concept: we define it as the value of the {\tt HOME} environment variable on
Unix-like systems and as the value of the {\tt USERPROFILE} environment
variable on Microsoft Windows\footnote{
   Note to Java developers: contrary to what you might expect,
   the {\tt user.home} system property on Windows does {\em not} give you the
   value of {\tt USERPROFILE}.
   See \url{http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4787931}.
}.  
``Environment variable'' is itself potentially a system-dependent concept,
but it is clear how to interpret it for all platforms on which we 
currently expect SAMP to be used, so no further explanation is provided
here.

In version 1.11 of the standard, the lockfile was always in the
``{\tt .samp}'' file in the user's home directory.
The option of setting the SAMP\_HUB environment variable to override
this has been introduced to allow more flexibility;
for instance one user can run multiple unconnected hubs,
or multiple users can share the same hub.
If no SAMP\_HUB environment variable is defined, client and hub 
behaviour is exactly as in version 1.11.


\subsubsection{Lockfile Content}
\label{sect:lockfileText}

The format of the lockfile is given by the following BNF productions:
\begin{verbatim}
   <file>          ::=   <lines>
   <lines>         ::=   <line> | <lines> <line>
   <line>          ::=   <line-content> <EOL> | <EOL>
   <line-content>  ::=   <comment> | <assignment>
   <comment>       ::=   "#" <any-string>
   <assignment>    ::=   <name> "=" <any-string>
   <name>          ::=   <token-string>
   <token-string>  ::=   <token-char> | <token-string> <token-char>
   <any-string>    ::=   <any-char> | <any-string> <any-char>
   <EOL>           ::=   "\r" | "\n" | "\r" "\n"
   <token-char>    ::=   [a-zA-Z0-9] | "-" | "_" | "."
   <any-char>      ::=   [\x20-\x7f]
\end{verbatim}
The only parts which are significant to SAMP clients/hubs are
(a) existence of the file and (b) {\tt <assignment>} lines.

A legal lockfile MUST provide (in any order) unique assignments for the
following tokens:
\begin{description}
\item[{\tt samp.secret}] ---
      An opaque text string which must be passed to the hub to permit
      registration.
\item[{\tt samp.hub.xmlrpc.url}] ---
      The XML-RPC endpoint for communication with the hub.
\item[{\tt samp.profile.version}] ---
      The version of the SAMP Standard Profile implemented by the hub
      (``\sampversion'' for the version described by this document).
\end{description}
These keys form the basis of an extensible vocabulary as explained in
Section \ref{sect:vocab}.
Other blank, comment or assignment lines may be included as desired.

An example lockfile might therefore look like this:
\begin{quote}\tt
   \# SAMP lockfile written 2008-29-02T17:45:01 \\
   \# Required keys: \\
   samp.secret=734144fdaab8400a1ec2 \\
   samp.hub.xmlrpc.url=http://andromeda.star.bris.ac.uk:8001/xmlrpc \\
   samp.profile.version=\sampversion \\
   \# Info stored by hub for some private reason: \\
   com.yoyodyne.hubid=c80995f1
\end{quote}

\subsubsection{Hub Discovery Sequences}

The hub discovery sequences are therefore as follows:

\begin{itemize}
\item Client startup:
   \begin{itemize}
   \item Determine hub existence as above
   \item If no hub, client MAY start its own hub
   \item Acquire {\tt samp.secret} value from lockfile
   \item If pre-existing or own hub is running, call {\tt register()} and
         zero or more of {\tt setXmlrpcCallback()}, {\tt declareMetadata()},
         {\tt declareSubscriptions()}
   \end{itemize}
\item Hub startup:
   \begin{itemize}
   \item Determine hub existence as above
   \item If hub is running, exit
   \item Otherwise, start up XML-RPC server
   \item Write lockfile with mandatory assignments including XML-RPC endpoint
   \end{itemize}
\item Hub shutdown:
   \begin{itemize}
   \item Remove lockfile (it is RECOMMENDED to first check that this
         is the lockfile written by self)
   \item Notify candidate clients that shutdown will occur
   \item Shut down services
   \end{itemize}
\end{itemize}

A hub implementation SHOULD make its best effort to perform the
shutdown sequence above even if it terminates as a result of some
error condition.

Note that manipulation of a file is not atomic, so that race conditions
are possible.  For instance a client or hub examining the lockfile
may read it after it has been created but before it has been populated
with the mandatory assignments, or two hubs may look for a lockfile
simultaneously, not find one, and both decide that they should
therefore start up, one presumably overwriting the other's lockfile.
Hub and client implementations should be aware of such possibilities,
but may not be able to guarantee to avoid them or their consequences.
In general this is the sort of risk that SAMP and its
Standard Profile are prepared to take --- an eventuality which will occur
sufficiently infrequently that it is not worth significant
additional complexity to avoid.
In the worst case a SAMP session may fail in some way, and will have
to be restarted.


\subsection{Examples}

Here is an example in pseudo-code of how an application might locate and
register with a hub, and send a message requiring no response to other
registered clients.
{\examplesize
\begin{verbatim}
   # Locate and read the lockfile.
   string hubvar-value = readEnvironmentVariable("SAMP_HUB");
   string lock-location = getLockfileLocation(hubvar-value);
   map lock-info = readLockfile(lock-location);

   # Extract information from lockfile to locate and register with hub.
   string hub-url = lock-info.getValue("samp.hub.xmlprc.url");
   string samp-secret = lock-info.getValue("samp.secret");

   # Establish XML-RPC connection with hub
   # (uses some generic XML-RPC library)
   xmlrpcServer hub = xmlrpcConnect(hub-url);

   # Register with hub.
   map reg-info = hub.xmlrpcCall("samp.hub.register", samp-secret);
   string private-key = reg-info.getValue("samp.private-key");

   # Store metadata in hub for use by other applications.
   map metadata = ("samp.name" -> "dummy",
                   "samp.description.text" -> "Test Application",
                   "dummy.version" -> "0.1-3");
   hub.xmlrpcCall("samp.hub.declareMetadata", private-key, metadata);

   # Send a message requesting file load to all other 
   # registered clients, not wanting any response.
   map loadParams = ("filename" -> "/tmp/foo.bar");
   map loadMsg = ("samp.mtype" -> "file.load",
                  "samp.params" -> loadParams);
   hub.xmlrpcCall("samp.hub.notifyAll", private-key, loadMsg);

   # Unregister
   hub.xmlrpcCall("samp.hub.unregister", private-key);
\end{verbatim}
}

The first few XML-RPC documents sent over the wire for this exchange 
would look something like the following.  
The registration call from the client to the hub:
{\examplesize
\begin{verbatim}
   POST /xmlrpc HTTP/1.0
   User-Agent: Java/1.5.0_10
   Content-Type: text/xml
   Content-Length: 189

   <?xml version="1.0"?>
   <methodCall>
     <methodName>samp.hub.register</methodName>
     <params>
       <param><value><string>734144fdaab8400a1ec2</string></value></param>
     </params>
   </methodCall>
\end{verbatim}
}
which leads to the response:
{\examplesize
\begin{verbatim}
   HTTP/1.1 200 OK
   Connection: close
   Content-Type: text/xml
   Content-Length: 464

   <?xml version="1.0"?>
   <methodResponse>
     <params><param><value><struct>
       <member>
         <name>samp.private-key</name>
         <value><string>client-key:1a52fdf</string></value>
       </member>
       <member>
         <name>samp.hub-id</name>
         <value><string>client-id:0</string></value>
       </member>
       <member>
         <name>samp.self-id</name>
         <value><string>client-id:4</string></value>
       </member>
     </struct></value></param></params>
   </methodResponse>
\end{verbatim}
}
The client can then declare its metadata:
the response to this call has no useful content so can be ignored or discarded.
{\examplesize
\begin{verbatim}
   POST /xmlrpc HTTP/1.0
   User-Agent: Java/1.5.0_10
   Content-Type: text/xml
   Content-Length: 600

   <?xml version="1.0"?>
   <methodCall>
     <methodName>samp.hub.declareMetadata</methodName>
     <params>
       <param><value><string>app-id:1a52fdf-2</string></value></param>
       <param><value><struct>
         <member>
           <name>samp.name</name>
           <value><string>dummy</string></value>
         </member>
         <member>
           <name>samp.description.text</name>
           <value><string>Test application</string></value>
         </member>
         <member>
           <name>dummy.version</name>
           <value><string>0.1-3</string></value>
         </member>
       </struct></value></param>
     </params>
   </methodCall>
\end{verbatim}
}
The message itself is sent from the client to the hub as follows:
{\examplesize
\begin{verbatim}
   POST /xmlrpc HTTP/1.0
   User-Agent: Java/1.5.0_10
   Content-Type: text/xml
   Content-Length: 523

   <?xml version="1.0"?>
   <methodCall>
     <methodName>samp.hub.notifyAll</methodName>
     <params>
       <param><value><string>app-id:1a52fdf-2</string></value></param>
       <param><value><struct>
         <member>
           <name>samp.mtype</name>
           <value>file.load</value>
         </member>
         <member>
           <name>samp.params</name>
           <value><struct>
             <name>filename</name>
             <value>/tmp/foo.bar</value>
           </struct></value>
         </member>
       </struct></value></param>
     </params>
   </methodCall>
\end{verbatim}
}
Again, there is no interesting response.



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\section{MTypes: Message Semantics and Vocabulary}
\label{sect:mtypes}

        A message contains an MType string that
defines the semantic meaning of the message, for example a request for
another application to load a table.
The concept behind the MType
is similar to that of a UCD \cite{ucd} 
in that a small vocabulary is sufficient to
describe the expected range of concepts required by a messaging system
within the current scope of the SAMP protocol.
Developers are free to introduce new MTypes for use within applications
without restriction; new MTypes intended to be used for Hub messaging or
other administrative purposes within the messaging system should be discussed
within the IVOA for approval as part of the SAMP standard.

\subsection{The Form of an MType}

        MType syntax is formally defined in Section \ref{sect:subscription}.
Like a UCD, an MType is made up of {\em atoms}.
These are not only meaningful to the developer, but form the central
concept of the message.  
Because the capabilities one application is searching for
are loosely coupled with the details of what another may provide,
there is not a rigorous definition of the {\em behavior} that
an MType must provoke in a receiver.  Instead, the MType defines a specific
semantic message such as ``display an image'', and it is up to the receiving
application to determine how it chooses to do the display (e.g.\ a rendered
greyscale image within an application or displaying the image in a web
browser might both be valid for the recipient and faithful to the meaning
of the message).

        The ordering of the words in an MType SHOULD normally use the
object of the message followed by the action to be performed (or the
information about that object).  For example, the use of ``{\tt image.display}''
is preferred to ``{\tt display.image}'' in order to keep the number of top-level
words (and thus message classes) like `image' small, but still allow for a
wide variety of messages to be created that can perform many useful actions
on an image.  If no existing MType exists for the required purpose,
developers can agree to the use of a new MType such as
``{\tt image.display.extnum}'' if, e.g., the ability to display a specific image
extension number warrants a new MType.


\subsection{The Description of an MType}
\label{sect:mtype-doc}

In order that senders and recipients can agree on what is meant by 
a given message, the meaning of an MType must be clearly documented.
This means that for a given MType the following information must be
available:
\begin{enumerate}
\item The MType string itself
\item A list of zero or more named parameters
\item A list of zero or more named returned values
\item A description of the meaning of the message
\end{enumerate}
For each of the named parameters, and each of the returned values,
the following information must be provided:
\begin{itemize}
\item name
\item data type ({\tt map}, {\tt list} or {\tt string}
    as described in Section \ref{sect:samp-data-types}) and if appropriate
    scalar sub-type (see Section \ref{sect:scalar-types}) 
\item meaning
\item whether it is OPTIONAL (considered REQUIRED unless stated otherwise)
\item OPTIONAL parameters MAY specify what default will be used
         if the value is not supplied
\end{itemize}
Together, this is much the same information as should be given for
documentation of a public interface method in a weakly-typed programming
language.

The parameters and return values associated with each MType 
form extensible vocabularies as explained in Section \ref{sect:vocab},
except that there is no reserved ``{\tt samp.}'' namespace.

Note that it is possible for the MType to have no returned values.
This is actually quite common if the MType does not represent a
request for data.  It is not usually necessary to define a status-like
return value (success or failure),
since this information can be conveyed as the value of the {\tt samp.status}
entry in the call response as described in Section \ref{sect:response-encode}.

\subsection{MType Vocabulary: Extensibility and Process}

The set of MTypes forms an extensible vocabulary along the lines of
Section \ref{sect:vocab}.
The relatively small set of MTypes in the ``{\tt samp.}'' namespace is
defined in Section \ref{sect:admin-mtypes} of this document,
but applications will need to use a wider range of MTypes to exchange
useful information.
Although clients are formally permitted to
define and use any MTypes outside of the reserved ``{\tt samp.}'' namespace,
for effective interoperability there must be
public agreement between application authors on this unreserved vocabulary
and its semantics.

Since addition of new MTypes is expected to be ongoing, 
MTypes from this broader vocabulary will
be documented
outside of this document to avoid the administrative
overhead and delay associated with the IVOA Recommendation Track \cite{docstd}.
At time of writing, the procedures for maintaining the list of
publicly-agreed MTypes are quite informal.
These procedures remain under review,
however the current list and details of best practice for adding to it 
are, and will remain, available in some form from the URL
\url{http://www.ivoa.net/samp/}.


\subsection{Core MTypes}
\label{sect:admin-mtypes}

This section defines those MTypes currently in the ``{\tt samp.}'' hierarchy.
These are the ``administrative''-type MTypes which are core to the
SAMP architecture or widely applicable to SAMP applications.

% define command for describing an MType.
% arguments are:
%    qualified name,
%    parameter list,
%    return values,
%    description, 
\newcommand{\mtypedef}[4]{
   \begin{description}
     \item[{\tt #1}:]\dbreakd
     \begin{description}
       \item[Arguments:]\dbreakd\begin{description}#2\end{description}
       \item[Return Values:]\dbreakd\begin{description}#3\end{description}
       \item[Description:]\dbreak #4
     \end{description}
   \end{description}
}

% define command for describing an MType argument.
% arguments are:
%    name,
%    type,
%    short description
\newcommand{\mtypearg}[3]{\item[{\tt #1} ({\tt #2}) ---] #3}

\newcommand{\mtypenoargs}{\item[{\em none}]}

\subsubsection{Hub Administrative Messages}
\label{sect:hub-mtypes}

The following MTypes are for messages which SHOULD be broadcast by the hub
in response to changes in hub state.  By subscribing to these messages,
clients are able to keep track of the current set of registered applications
and of their metadata and subscriptions.
In general, non-hub clients SHOULD NOT send these messages.
%
\mtypedef{samp.hub.event.shutdown}
         {\mtypenoargs}
         {\mtypenoargs}
         {The hub SHOULD broadcast this message just before it exits.
          The hub SHOULD make every effort to broadcast this message even in
          case of an exit due to an error condition.}
%
\mtypedef{samp.hub.event.register}
         {\mtypearg{id}{string}{Public ID of newly registered client}}
         {\mtypenoargs}
         {The hub SHOULD broadcast this message every time a client 
          successfully registers.}
%
\mtypedef{samp.hub.event.unregister}
         {\mtypearg{id}{string}{public ID of unregistered client}}
         {\mtypenoargs}
         {The hub SHOULD broadcast this message every time a client
          unregisters.}
%
\mtypedef{samp.hub.event.metadata}
         {\mtypearg{id}{string}{public ID of client declaring metadata}
          \mtypearg{metadata}{map}{new metadata declared by client}}
         {\mtypenoargs}
         {The hub SHOULD broadcast this message every time a client
          declares its metadata.
          The {\tt metadata} argument is exactly as passed using the
          {\tt declareMetadata()} method.}
%
\mtypedef{samp.hub.event.subscriptions}
         {\mtypearg{id}{string}{public ID of subscribing client}
          \mtypearg{subscriptions}{map}{new subscriptions declared by client}}
         {\mtypenoargs}
         {The hub SHOULD broadcast this message every time a client 
          declares its subscriptions.
          The {\tt subscriptions} argument is exactly as passed using the
          {\tt declareSubscriptions()} method, and hence may contain wildcarded
          MType strings.}
%
\mtypedef{samp.hub.disconnect}
         {\mtypearg{reason}{string}{(OPTIONAL)
                                    Short text message indicating the reason
                                    that the disconnection is being forced}}
         {\mtypenoargs}
         {The hub SHOULD send this message to a client if the hub intends to disconnect that
          client forcibly.  This indicates that no further communication
          from that client is welcome, and any such attempts may be
          expected to fail.
          The hub may wish to disconnect clients forcibly as a result of
          some hub timeout policy or for other reasons.}

\subsubsection{Client Administrative Messages}

The following messages are generic messages defined for client use.
%
\mtypedef{samp.app.ping}
         {\mtypenoargs}
         {\mtypenoargs}
         {Diagnostic used to indicate whether an application is 
          currently responding.
          No ``status''-like return value is defined,
          since in general any response will indicate aliveness,
          and the normal {\tt samp.status} key in the response may be
          used to indicate any abnormal state.}
%
\mtypedef{samp.app.status}
         {\mtypearg{txt}{string}{Textual indication of status}}
         {\mtypenoargs}
         {General purpose message to indicate application status.}
%
\mtypedef{samp.app.event.shutdown}
         {\mtypenoargs}
         {\mtypenoargs}
         {Indicates that the sending application is going to shut down.
          Note that sending this message is not a substitute for unregistering
          with the hub --- registered clients about to shut down 
          SHOULD always explicitly unregister.}
%
\mtypedef{samp.msg.progress}
         {\mtypearg{msgid}{string}{Message ID of a previously received message}
          \mtypearg{txt}{string}{Textual indication of progress}
          \mtypearg{percent}{string}{(OPTIONAL)
                                     SAMP float value giving the approximate 
                                     percentage progress}
          \mtypearg{timeLeft}{string}{(OPTIONAL)
                                      SAMP float value giving the estimated
                                      time to completion in seconds}}
         {\mtypenoargs}
         {Reports on progress of a message previously received by the sender
          of this message.  Such progress reports MAY be sent at intervals
          between the receipt of the message and sending a reply.
          Note that the {\tt msg-id} of the earlier message must be passed to
          identify it --- the sender of the earlier message (the recipient of
          this one) will have to have retained it from the return value of the
          relevant {\tt call*()} method to match progress reports with
          requests.}

\newpage
\appendix

\section{Changes between PLASTIC and SAMP}

In order to facilitate the transition from PLASTIC to SAMP from an
application developer's point of view, we summarize in this Appendix 
the main changes. 
In some cases the reasons for these are summarized as well.

\begin{description}
\item[Language Neutrality:]
   PLASTIC contained some Java-specific ideas and details, in particular
   an API defined by a Java interface, use of Java RMI-Lite as a 
   transport protocol option, and a lockfile format
   based on java Property serialization.
   No features of SAMP are specific to, or defined with reference to, Java
   (or to any other programming language).
\item[Profiles:]
   The formal notion of a SAMP Profile replaces the choices of transport
   protocol in PLASTIC.  In practice since the Standard Profile is the only
   one currently defined, this means that XML-RPC is currently 
   the only transport protocol which can be used to communicate in SAMP.
\item[Nomenclature:]
   Much of the terminology has changed between PLASTIC and SAMP,
   in some cases to provide better consistency with common usage in
   messaging systems.
   There is not in all cases a one-to-one correspondence betweeen PLASTIC and
   SAMP concepts, but a partial translation table is as follows:
   \begin{center}
   \begin{tabular}{ll}
   PLASTIC                &  SAMP  \\
   \hline
   message                &  MType         \\
   support a message      &  subscribe to an MType  \\
   registered application &  client \\
   synchronous request    &  synchronous call/response  \\
   asynchronous request   &  notification 
   \end{tabular}
   \end{center}
\item[MTypes:]
   In PLASTIC message semantics were defined using opaque URIs such as
   {\tt ivo://votech.org/hub/event/HubStopping}.  
   These have now been replaced by
   a vocabulary of structured MTypes such as {\tt samp.hub.event.shutdown}.
\item[Asynchrony:]
   Responses from messages in PLASTIC were returned synchronously, using
   blocking methods at both sender and recipient ends.  As well as 
   inhibiting flexibility, this risked timeouts for long processing times
   at the discretion of the underlying transport.
   The basic model in SAMP relies on asynchronous responses, though 
   a synchronous fa\c{c}ade hub method is also provided for 
   convenience of the sender.
   Client toolkits may also wish to provide client-side synchronous 
   fa\c{c}ades based on fully asynchronous messaging.
\item[Registration:]
   In PLASTIC clients registered with a single call which acquired a
   hub connection and declared callback information, message subscriptions, 
   and some metadata.
   In SAMP, these four operations have been decomposed into separate calls.
   As well as being tidier, this offers benefits such as meaning that
   the subscriptions and metadata can be updated during the lifetime of
   the connection.
\item[Client Metadata:]
   PLASTIC stored some application metadata (Name) in the hub and provided
   acess to others (Description, Icon URL, \ldots) using custom messages.
   SAMP stores it all in the hub providing better extensibility and consistency
   as well as improving metadata provision for non-callable applications
   and somewhat reducing traffic and burden on applications.
\item[Named Parameters:]
   The parameters for PLASTIC messages were identified by sequence
   (forming a list), while
   the parameters for SAMP MTypes are identified by name (forming a map).
   As well as improving documentability, this makes it much more convenient
   to allow for optional parameters or to introduce new ones.
   The same arrangement applies to return values.
\item[Recipient Targetting:]
   PLASTIC featured methods for sending messages to all or to an explicit list 
   of recipients.  In practice the list variants were rarely used except to
   send to a single recipient.
   SAMP has methods for sending to all or to a single recipient.
\item[Typing:]
   Data types in PLASTIC were based partly on Java and partly on XML-RPC types.
   There was not a one-to-one correspondence between types in the Java-RMI
   transport and the XML-RPC one, which encouraged confusion.
   Parameter types included integer, floating point and boolean as well as
   string, which proved problematic to use correctly from some 
   weakly-typed languages.
   SAMP uses a more restricted set of types 
   (namely string, list and map) at the protocol level,
   along with some auxiliary rules for encoding numbers and booleans as strings.
\item[Lockfile:]
   The lockfile in SAMP's standard profile is named {\tt .samp},
   its format is defined explicitly rather than with reference
   to Java documentation, and there is better provision for its 
   location in a language-independent way on MS Windows systems.
   In many cases however, the same lockfile location/parsing code 
   will work for both SAMP and PLASTIC except for the different filenames
   (``.samp'' vs.\ ``.plastic'').
\item[Public/Private ID:]
   In PLASTIC a single, public ID was used to label and identify 
   applications during communications directed to the hub
   or to other applications.
   This meant that applications could easily, if they wished, 
   impersonate other applications.  SAMP's standard profile
   uses different IDs for public labelling and private identification,
   which means that such ``spoofing'' is no longer a danger.
\item[Errors:]
   SAMP has provision to return more structured error information 
   than PLASTIC did.
\item[Extensibility:]
   Although PLASTIC was in some ways extensible, SAMP provides more hooks
   for future extension, in particular by pervasive use of the 
   {\em extensible vocabulary\/} pattern.
\end{description}

% Annexe 2 : discuss how we can make SAMP-compatible and PLASTIC-compatible
% applications talk together 
% \section{SAMP/PLASTIC interoperability}
% This section will detail how interoperability between SAMP-compatible and
% PLASTIC-compatible clients can be achieved, using hubs able to translate PLASTIC 
% messages into SAMP MTypes.

\section{Change History}
\label{sect:changes}

Changes to SAMP between Working Draft version 1.0 (2008-06-25) and
Recommendation version 1.11 (2009-04-21):
\begin{itemize}
\item Return values of {\tt callAll} and {\tt notifyAll} operations changed;
      they now return information about clients receiving the messages
      (Section \ref{sect:hubOps}).
\item Characters allowed in {\tt string} type restricted to avoid
      problems transmitting over XML;
      was 0x01--0x7f, now 0x09, 0x0a, 0x0d, 0x20--0x7f
      (Section \ref{sect:samp-data-types}).
\item New hub administrative message {\tt samp.hub.disconnect}
      (Section \ref{sect:hub-mtypes}).
\item Empty placeholder appendix on SAMP/PLASTIC interoperability removed.
\item Wording clarified and made more explicit in a few places.
\item Typos fixed, including incorrect BNF in Section \ref{sect:subscription}.
\item Author list re-ordered.
\item Editorial changes and clarifications following RFC period.
\item MType Vocabulary section now directs readers to
      {\tt http://www.ivoa.net/samp/} to find current MType list and process.
\end{itemize}

Changes to SAMP between Recommendation version 1.11 (2009-04-21) and
version 1.2:
\begin{itemize}
\item Use of new SAMP\_HUB environment variable lockfile location option
      documented in section \ref{sect:lockfile}.
\item Added Non-Technical Preamble section \ref{sect:nonTechPreamble}
      as per agreement for all new/revised IVOA documents.
\end{itemize}

%%%%%%%% Bibliography %%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% include all references
\nocite{*}
% unsorted style for bibliography (?)
\bibliographystyle{unsrt}
% bibliography file is called samp-bibliography[.bib]
\bibliography{samp-bibliography}

\end{document}

% $Id$
