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VOSI 1.1 REC

 1 \documentclass[11pt,letter]{ivoa} 2 \input tthdefs 3 4 \usepackage{listings} 5 \lstloadlanguages{XML} 6 \lstset{flexiblecolumns=true,basicstyle=\footnotesize,tagstyle=\ttfamily,showspaces=false} 7 8 \title{IVOA Support Interfaces} 9 10 \ivoagroup{Grid and Web Services Working Group} 11 12 \author{Grid and Web Services Working Group} 13 14 \editor{Matthew Graham} 15 \editor{Guy Rixon} 16 \editor{Patrick Dowler} 17 \editor{Brian Major} 18 19 \previousversion{VOSI-1.0} 20 21 \begin{document} 22 \begin{abstract} 23 This document describes the minimum interface that a web service requires to participate in the IVOA. Note that this is not required of standard VO services developed prior to this specification, although uptake is strongly encouraged on any subsequent revision. All new standard VO services, however, must feature a VOSI-compliant interface. 24 \end{abstract} 25 26 27 \section*{Acknowledgments} 28 29 This document has been developed with support from the National Science Foundation's Information Technology Research Program under Cooperative Agreement AST0122449 with The Johns Hopkins University, from the UK Particle Physics and Astronomy Research Council (PPARC), from the European Commission's (EC) Sixth Framework Programme via the Optical Infrared Coordination Network (OPTICON), and from EC's Seventh Framework Programme via its eInfrastructure Science Repositories initiative. 30 31 This work is based on discussions and actions from the 2003 IVOA meeting in Strasbourg and further discussions on registry functionality at JHU late in 2003. Later inputs came from a local meeting at JHU in Sept. 2004. William O'Mullane and Ani Thakar were the editors and primary authors for these early versions. 32 33 The decision to split the interfaces into a mandatory set and optional logging interfaces was taken by GWS-WG at the IVOA meeting of May 2006. 34 35 \section*{Conformance-related definitions} 36 37 The words MUST'', SHALL'', SHOULD'', MAY'', RECOMMENDED'', and 38 OPTIONAL'' (in upper or lower case) used in this document are to be 39 interpreted as described in IETF standard RFC2119 \citep{std:RFC2119}. 40 41 The \emph{Virtual Observatory (VO)} is a 42 general term for a collection of federated resources that can be used 43 to conduct astronomical research, education, and outreach. 44 The \href{http://www.ivoa.net}{International 45 Virtual Observatory Alliance (IVOA)} is a global 46 collaboration of separately funded projects to develop standards and 47 infrastructure that enable VO applications. 48 49 50 \section{Introduction} 51 \label{sec:introduction} 52 53 This document describes a set of common basic functions that VO web services provide in the form of a standard support interface in order to allow for the effective management of the VO. There are three basic support functions that this document describes: The reporting of capability metadata, the reporting of service availability metadata, and the reporting of table metadata (if applicable). 54 55 VO service standards previous to VOSI may not be forced to retrospectively implement VOSI (although that should be encouraged). Nonetheless, all new VO service standards (or updated existing ones) must enforce the VOSI implementation. 56 57 58 \subsection{Role within the VO Architecture} 59 60 The IVOA Architecture \citep{note:VOARCH} provides a high-level view of how IVOA standards work together to connect users and applications with providers of data and services, as depicted in the diagram in Figure ~\ref{fig:archdiag}. 61 62 \begin{figure} 63 \centering 64 65 66 \includegraphics[width=0.9\textwidth]{archdiag.png} 67 \caption{VOSI in the IVOA Architecture. VOSI is the standard that defines the basic functions that all VO services should provide in order to support management of the VO.} 68 \label{fig:archdiag} 69 \end{figure} 70 71 In this architecture, users employ a variety of tools (from the User 72 Layer) to discover and access archives and services -- that is, 73 resources -- of interest (represented in the Resource Layer). A registry 74 plays a role in discovery by harvesting metadata that describe archives 75 and services and making them searchable from a central service. The VOSI 76 interface provides a means for a service to provide some of this 77 metadata itself; this allows a registry to pull the metadata from the 78 service rather than relying on a human to provide it (e.g. by typing the 79 data into a registration form manually). This mechanism can make it 80 easier to collect highly detailed metadata (e.g. descriptions of columns 81 in a catalog) that might not be practically provided otherwise. As some 82 of this metadata describes the service interface and how it behaves, 83 other applications can use this information for controlling how they use 84 the service. Even when the service is discovered'' through some means other than a registry, an application can still understand how to use the service by querying for this information directly. (See Appendix \ref{appendix:harvesting} for a more detailed description of this use case.) 85 86 Once a user discovers data and services of interest, she will want to engage them in an analysis process. Success requires that the selected services are actually up and running properly as a down service can cause automated processing to fail completely. Registry and workflow services can assist with this by tracking the availability of services and alerting users about downtime. We envision that VOSI will allow VO projects to better track the overall health of the VO ecosystem. 87 88 \section{Interface bindings} 89 \label{sec:bindings} 90 91 This section explains how services are to report VO support metadata through a standard interface. 92 93 The standard interface returns metadata without changing the state of the service with which it is associated. This standard requires a REST binding of VOSI even when applied to services that do not have a RESTful interface. 94 95 In the REST binding, the support interfaces shall have distinct URLs in the HTTP scheme and shall be accessible by the GET operation in the HTTP protocol. The response to an HTTP POST, PUT or DELETE to these resources is not defined by this specification. However, if an implementation has no special action to perform for these requests, the normal response would be a HTTP 405 "Method not allowed" status code. Both the capabilities and availability bindings must be available to anonymous requests. 96 97 The endpoints and interface types for the support interface shall be defined in the service's registration using one \xmlel{capability} element for each interface. The values of the \xmlel{standardID} attribute for these Capabilities are given in section~\ref{sec:endpoints}. 98 99 When using the REST binding, any HTTP URLs may be used. The client must find the appropriate URLs from the service's entry in the VO registry and, in general, should not try and infer the URLs from any other URLs for that service. However, standards for specific services may put extra constraints on the form of the URLs. 100 101 \section{Metadata specification} 102 \label{sec:metadata} 103 104 There are various classes of metadata that might be returned by a service through its standard interface: 105 106 \begin{itemize} 107 \item those describing its functional capabilities 108 \item those describing its operational behaviour - availability, reliability, etc. 109 \item those describing tabular data handled by the service 110 \item those describing other aspects of the service 111 \end{itemize} 112 113 This section defines how each of these classes is represented. The following typographic convention is used to represent a XML element defined within a particular namespace: 114 115 $$\hbox{\xmlel{http://some.name.space\#elementName}}$$ 116 117 For example, \xmlel{http://www.ivoa.net/xml/VOResource/v1.0\#resource} indicates a XML element named \xmlel{resource} that is described in the XML schema associated with the 'http://www.ivoa.net/xml/VOResource/v1.0' namespace -- in this case, this would be VOResource.xsd \citep{std:VOR}. 118 119 \subsection{Capability metadata} 120 121 Note: 122 'Capability' is unfortunately an overloaded term in the VO referring to both a functional aspect of a service and also particular pieces of metadata defined by various XML schema. When referring to an XML element called \xmlel{capability}, it shall be typeset in italic typewriter. Its parent namespace may also be included (using the syntax described above) if it is ambiguous which XML schema is being referred to. 123 124 This interface provides the service metadata in the form of a list of Capability descriptions. Each of these descriptions is an XML element that: 125 126 \begin{itemize} 127 \item states that the service provides a particular, IVOA-standard function; 128 \item lists the interfaces for invoking that function; 129 \item records any details of the implementation of the function that are not defined as default or constant in the standard for that function. 130 \end{itemize} 131 132 For example, one Capability might describe a cone search function and another the TAP implementation but these two might well apply to the same service. 133 134 An entry for a service in the resource registry - i.e., its VOResource - contains the Dublin Core resource metadata (identifier, curation information, content description, etc.) followed by the service's capability descriptions (expressed as a series of \xmlel{http://www.ivoa.net/xml/VOResource/v1.0\#Ca\-pa\-bi\-lity} elements). Effectively, the resource metadata describes the service to human users and the capability list describes it to software. Therefore, the latter list has two uses: 135 136 \begin{itemize} 137 \item it may be read by a client application to find out how to invoke the service. This presumes that the service has been already been selected and the VOSI endpoint located. 138 \item it may be read by the registry itself to compile the registry entry for the service. In this case, the resource metadata are entered into the registry directly and the service metadata are then read from the service. Since the service implementation usually knows its capabilities, this removes the need for a human to type them into the registry. 139 \end{itemize} 140 141 The service metadata shall be represented as an XML document with the root element:\\ 142 $$\hbox{\xmlel{http://www.ivoa.net/xml/VOSICapabilities/v1.0\#capabilities}}$$\\ 143 (See Appendix \ref{appendix:capabilities} for the definition of the VOSICapabilities XML schema.) This element must contain one or more child capability elements that describe the capabilities of the service. Given that the capability element is defined to be of type\\ 144 $$\hbox{\xmlel{http://www.ivoa.net/xml/VOResource/v1.0\#Capability}}$$\\ 145 a capability element may be represented by a legal sub-type of\\ 146 $$\hbox{\xmlel{http://www.ivoa.net/xml/VOResource/v1.0\#Capability}}$$\\ 147 in which case, the capability element must use an \xmlel{xsi:type} attribute to identify the sub-type (see section 2.2.1 of VOResource \citep{std:VOR}). 148 149 \begin{admonition}{Note} 150 The value of the capability element's \xmlel{standardID} attribute is used to indicate the service's support for particular standard protocols (such as Simple Image Access, Simple Cone Search, etc.). In the case of some protocols, the support for the standard is further characterized by additional metadata provided by a standard XML schema extension of \xmlel{Capability} for that protocol. The extension metadata is enabled by adding a \xmlel{xsi:type} attribute to the capability element set to the Capability sub-type value defined in the extension schema for that protocol (see example below). 151 152 The VOResource list of capabilities should include capabilities describing VOSI endpoints as specified in section \ref{sec:endpoints}. 153 \end{admonition} 154 155 In the REST binding, the service metadata shall be a single web resource with a registered URL. The date and time at which the metadata last changed shall be obtained from the Last-Modified HTTP Header keyword sent in the response to a GET or HEAD request to the registered URI. 156 157 All VO services should provide this interface. 158 159 \subsection{Availability metadata} 160 161 This interface indicates whether the service is operable and the reliability of the service for extended and scheduled requests. The availability shall be represented as an XML document in which the root element is \xmlel{http://www.ivoa.net/xml/Availability/v1.0\#availability}. This element shall contain child elements providing the following information: 162 163 \begin{itemize} 164 \item \xmlel{available} -- whether the service is currently accepting requests 165 \item \xmlel{upSince} -- duration for which the service has been continuously available 166 \item \xmlel{downAt} -- the instant at which the service is next scheduled to be unavailable 167 \item \xmlel{backAt} -- the instant at which the service is scheduled to become available again after down time; 168 \item \xmlel{note} -- textual note, e.g. explaining the reason for unavailability. 169 \end{itemize} 170 171 The elements \xmlel{upSince}, \xmlel{downAt}, \xmlel{backAt} and \xmlel{note} are optional. The \xmlel{available} element is mandatory. There may be more than one \xmlel{note} element. 172 173 The XML document shall conform to the schema given in appendix \ref{appendix:availability} of this specification. 174 175 When reporting availability, the service should do a good check on its underlying parts to see if it is still operational and not just make a simple return from a web server, e.g., if it relies on a database it should check that the database is still up. If any of these checks fail, the service should set available to false in the availability output. 176 177 If a service is to be online but unavailable for work (e.g., when a service with a work queue intends to shut down after draining the queue) then the service should set available to false. 178 179 There are no special elements in the availability document for the contact details of the service operator. These details may be given as a note element if they are known to the service. 180 181 In the REST binding, the availability shall be a single web resource with a registered URL 182 183 All VO services shall provide this interface. 184 185 \subsection{Table metadata} 186 187 Some services deal with tabular data. These data may be the target of ADQL queries, as in TAP \citep{std:TAP}, or they may be the output of other operations, as in SIAP queries. In each case, it is useful if the service describes the details of the tables concerned. It is more useful if this description can be captured in the resource registry. 188 189 The VODataService standard \citep{std:VODS11} defines XML elements for describing a set of tables. These elements can be included in a resource document for a service. 190 191 A service which uses tables in its interface should define a VOSI endpoint from which table metadata can be read. The table metadata for all content of a service shall be represented as an XML document of which the root element is of type\\ 192 $$\hbox{\xmlel{http://www.ivoa.net/xml/VODataService/v1.1\#TableSet}}$$\\ 193 The table metadata for a single table (see below) shall be represented as an 194 XML document of which the root element is of type\\ 195 $$\hbox{\xmlel{http://www.ivoa.net/xml/VODataService/v1.1\#Table}}$$\\ 196 This element may contain any mix of elements allowed by the VODataService XML schema. 197 198 In the REST binding, the TableSet metadata shall be a hierarchical web resource with a registered URL. There are two levels of TableSet metadata detail that may be returned from the base \textit{tables} endpoint. Maximum detail (max) is the complete metadata including all details of the Table elements. Minimum detail (min) includes the full set of Table elements with names and optional descriptions, but does not include Table elements of type Column or ForeignKey. 199 200 If the parameter \textit{detail} accompanies the request with values of either \textit{max} or \textit{min}, the service should take this as a suggestion as to what level of detail to return, but may return either level. If the parameter \textit{detail} is not present, the service may choose the level of metadata detail to return. 201 202 For example, the request: 203 204 \begin{verbatim} 205 GET http://example.net/srv/tables?detail=min 206 \end{verbatim} 207 208 should be taken as a suggestion to return the minimum level of TableSet metadata detail. 209 210 Services with a large number of tables and/or columns cannot normally respond with a usable TableSet document that has the maximum level of detail so may wish to always respond with the minimum level of detail. 211 212 The REST endpoint must also support a child resource for each table described in the TableSet document. The child resource must be named as it appears in the name of the corresponding child Table element. For example: 213 214 \begin{verbatim} 215 GET http://example.net/srv/tables/ivoa.ObsCore 216 \end{verbatim} 217 218 would return a Table document describing the ivoa.ObsCore table in full detail, starting: 219 220 \begin{verbatim} 221 222 ivoa.ObsCore 223 224 ... 225 \end{verbatim} 226 227 \subsection{Non-service metadata (non-normative)} 228 229 There may be other metadata associated with a service than the capability metadata described above. 230 231 \begin{itemize} 232 \item Every service has the Dublin Core resource metadata \citep{std:DUBLINCORE}. 233 \item Some services are associated with registered applications. 234 \item Some services are associated with registered data collections. 235 \end{itemize} 236 237 None of these are explicitly provided for in this version of VOSI. Some might be covered in later versions of VOSI. 238 239 \section{Registration of VOSI endpoints} 240 \label{sec:endpoints} 241 242 The endpoints for the service and availability metadata shall be included in the registration of each service that provides them. 243 244 \begin{tabular}{l l l l l} 245 \label{tab:registration} 246 Endpoint type & standardID value \\ 247 availability & \nolinkurl{ivo://ivoa.net/std/VOSI#availability} \\ 248 capabilities & \nolinkurl{ivo://ivoa.net/std/VOSI#capabilities} \\ 249 tables (1.0) & \nolinkurl{ivo://ivoa.net/std/VOSI#tables} \\ 250 tables (1.1) & \nolinkurl{ivo://ivoa.net/std/VOSI#tables-1.1} \\ 251 \end{tabular} 252 253 An availability endpoint shall be represented by an element named \xmlel{capability}, of type \xmlel{http://www.ivoa.net/xml/VOResource/v1.0\#Capability} (defined by the standard VOResource XML schema \citet{std:VOR}). The value of the \xmlel{standardID} attribute of the capability shall be \nolinkurl{ivo://ivoa.net/std/VOSI#availability}. 254 255 A capabilities endpoint should be represented by an element named \xmlel{capability}, of type \xmlel{http://www.ivoa.net/xml/VOResource/v1.0\#Capability}. If such a capability is provided then the value of the \xmlel{standardID} attribute must be \nolinkurl{ivo://ivoa.net/std/VOSI#capabilities}. 256 257 A tables endpoint should be represented by an element named \xmlel{capability}, of type \xmlel{http://www.ivoa.net/xml/VOResource/v1.0\#Capability}. If such a capability is provided then the value of the \xmlel{standardID} attribute must be \nolinkurl{ivo://ivoa.net/std/VOSI\#tables}, or, for version 1.1, \nolinkurl{ivo://ivoa.net/std/VOSI\#tables-1.1}. 258 259 With all three VOSI functions, the \xmlel{capability} element that describes the function must contain an \xmlel{interface} element of a type semantically appropriate for the binding of the function to the service; the \xmlel{accessURL} element within the \xmlel{interface} element indicates the endpoint for the VOSI function. For the REST binding, this \xmlel{accessURL} element must set the \xmlel{use} attribute to \texttt{"full"}. Furthermore, for the REST binding, this document recommends using the \xmlel{http://www.ivoa.net/xml/VODataService/v1.1\#ParamHTTP} interface type to encode VOSI endpoints (see the examples given in section~\ref{sec:examples}). 260 261 The capabilities and availability endpoints must not require any credentials to view. Thus, the \xmlel{interface} registry entries for capabilities and availability must not contain a securityMethod element. 262 263 \section{Example VOSI responses} 264 \label{sec:examples} 265 266 \subsection{Example 1: SIA 1.0 capabilities} 267 268 A sample response from a capabilities resource describing an SIA service. 269 270 \begin{lstlisting}[language=XML,basicstyle=\footnotesize] 271 272 277 278 279 280 281 http://adil.ncsa.uiuc.edu/siaform.html 282 283 284 285 286 287 288 290 291 http://adil.ncsa.uiuc.edu/cgi-bin/voimquery?survey=f& 292 293 294 295 Pointed 296 297 298 299 360.0 300 301 180.0 302 303 304 305 306 307 360.0 308 309 180.0 310 311 312 313 314 315 5000 316 317 5000 318 319 320 321 100000000 322 323 5000 324 325 326 327 328 329 330 331 http://adil.ncsa.uiuc.edu/cgi-bin/voimquery/capabilities 332 333 334 335 336 337 338 339 340 http://adil.ncsa.uiuc.edu/cgi-bin/voimquery/availability 341 342 343 344 345 346 \end{lstlisting} 347 348 349 \subsection{Example 2: TAP tables, maximum detail} 350 351 A sample response from a tables resource describing a TAP service in full (maximum) detail. 352 353 \begin{lstlisting}[language=XML] 354 355 360 361 362 cfht 363 364 cfht.deepU 365 366 cfhtlsID 367 adql:VARCHAR 368 369 370 survey 371 adql:VARCHAR 372 373 374 field 375 adql:VARCHAR 376 377 378 pointing 379 adql:VARCHAR 380 381 382 selectionFilter 383 adql:VARCHAR 384 385 386 387 388 TAP_SCHEMA.keys 389 390 key_id 391 unique key to join to TAP_SCHEMA.key_columns 392 393 adql:VARCHAR 394 395 396 from_table 397 the table with the foreign key 398 adql:VARCHAR 399 400 401 target_table 402 the table with the primary key 403 adql:VARCHAR 404 405 406 407 408 409 410 \end{lstlisting} 411 412 \subsection{Example 3: Tables, minimum detail} 413 414 A sample response from a tables resource with no Column or ForeignKey elements. 415 416 \begin{lstlisting}[language=XML] 417 418 423 424 425 cfht 426 427 cfht.deepU 428 429 430 431 432 TAP_SCHEMA 433 434 TAP_SCHEMA.tables 435 436 437 TAP_SCHEMA.columns 438 439 440 TAP_SCHEMA.keys 441 442 443 TAP_SCHEMA.key_columns 444 445 446 447 448 \end{lstlisting} 449 450 \subsection{Example 4: Child table resource} 451 452 A sample response from a child table resource (e.g. http://example.net/srv/tables/TAP\_SCHEMA.columns). 453 454 \begin{lstlisting}[language=XML] 455 456 460 TAP_SCHEMA.columns 461 462 table_name 463 the table this column belongs to 464 VARCHAR 465 466 467 column_name 468 the column name 469 VARCHAR 470 471 472 utype 473 lists the utypes of columns in the tableset 474 VARCHAR 475 476 477 ucd 478 lists the UCDs of columns in the tableset 479 VARCHAR 480 481 482 unit 483 lists the unit used for column values in the tableset 484 VARCHAR 485 486 487 description 488 describes the columns in the tableset 489 VARCHAR 490 491 492 datatype 493 lists the ADQL datatype of columns in the tableset 494 VARCHAR 495 496 497 size 498 lists the size of variable-length columns in the tableset 499 INTEGER 500 501 502 principal 503 a principal column; 1 means 1, 0 means 0 504 INTEGER 505 506 507 indexed 508 an indexed column; 1 means 1, 0 means 0 509 INTEGER 510 511 512 std 513 a standard column; 1 means 1, 0 means 0 514 INTEGER 515 516 517 TAP_SCHEMA.tables 518 519 table_name 520 table_name 521 522 523 524 \end{lstlisting} 525 526 \subsection{Example 5: Availability} 527 528 A sample response from an availability resource. 529 530 \begin{lstlisting}[language=XML] 531 532 true 533 service is accepting queries 534 535 \end{lstlisting} 536 537 \appendix 538 539 \section{The Complete VOSICapabilities Schema} 540 \label{appendix:capabilities} 541 \begin{lstlisting}[language=XML] 542 550 551 552 553 A schema for formatting service capabilities as returned by a 554 capabilities resource, defined by the IVOA Support Interfaces 555 specification (VOSI). 556 557 See http://www.ivoa.net/Documents/latest/VOSI.html. 558 559 560 561 563 564 567 568 569 570 571 A listing of capabilities supported by a service 572 573 574 575 576 577 579 580 581 582 A capability supported by the service. 583 584 585 586 A protocol-specific capability is included by specifying a 587 vr:Capability sub-type via an xsi:type attribute on this 588 element. 589 590 591 592 593 594 595 596 597 598 599 600 601 \end{lstlisting} 602 603 \section{The Complete VOSIAvailability Schema} 604 \label{appendix:availability} 605 \begin{lstlisting}[language=XML] 606 614 615 616 617 A schema for formatting availability metadata as returned by an 618 availability resource defined in the IVOA Support Interfaces 619 specification (VOSI). 620 621 See http://www.ivoa.net/Documents/latest/VOSI.html. 622 623 624 625 628 629 630 631 632 633 634 635 636 Indicates whether the service is currently available. 637 638 639 640 641 642 643 644 The instant at which the service last became available. 645 646 647 648 649 650 651 652 The instant at which the service is next scheduled to become 653 unavailable. 654 655 656 657 658 659 660 661 The instant at which the service is scheduled to become available 662 again after a period of unavailability. 663 664 665 666 667 669 670 671 A textual note concerning availability. 672 673 674 675 676 677 678 679 680 681 \end{lstlisting} 682 683 \section{The Complete VOSITables Schema} 684 \label{appendix:tables} 685 \begin{lstlisting}[language=XML] 686 695 696 697 < xsd:documentation> 698 A schema for formatting table metadata as returned by a 699 tables resource, defined by the IVOA Support Interfaces 700 specification (VOSI). 701 702 See http://www.ivoa.net/Documents/latest/VOSI.html. 703 704 705 706 708 709 712 713 714 715 A description of the table metadata supported by the 716 service associated with a VOSI-enabled resource. 717 718 719 720 721 724 725 726 727 A description of a single table supported by the 728 service associated with a VOSI-enabled resource. 729 730 731 732 733 734 \end{lstlisting} 735 736 \section{Use Case for Capability Harvesting (non-normative)} 737 \label{appendix:harvesting} 738 739 In the section \ref{sec:introduction}, we summarized the role that the metadata retrieval functions play in the discovery of services. In particular, it mentions that a registry can harvest this information from a service's VOSI interface to save the provider from entering the information explicitly into a web form. In this appendix, we describe this use case in more detail, including both the publishing of the metadata and its typical use by service clients. 740 741 Some publishing registries \citep{std:RI1} provide a publicly accessible publishing tool: it allows any data service provider to register'' his service by providing the necessary metadata adequate to describe it. Such a tool typically provides a form that the provider fills out to enter all the metadata; the form processor uses those inputs to format a VOResource record that describes the service based on that metadata. Prior to the registry's support for the VOSI metadata functions, entering all of the metadata (particularly, fully describing all of the table columns) would often be laborious; consequently, providers are effectively discouraged from providing the mostly optional information. 742 743 With support for the VOSI metadata functions, the registry's publishing tool can now offer an alternative mechanism for providing much of the information. After generally describing the service via core metadata (e.g., its title, identifier, general description, contact information, etc.), the registry can offer the option of entering the VOSI URLs that provide the capability and table metadata. In the case of TAP, where these VOSI functions are mandated as part of the TAP interface, it would only be necessary for the user to enter the TAP service's base URL (in VOResource parlance, the access URL''). In either case, the tool would access the VOSI URLs, pull over the metadata, integrate it into the core metadata, and show the provider the combined results before publishing it to the registry. The tool might also ask the provider if this data is expected to change over time and thus whether the VOSI URLs should be polled regularly to update the service description held by the registry. Alternatively, the tool may may allow the provider to quickly update the service description via a single button click that causes the tool to re-access the VOSI endpoints and refresh service description held by the registry. 744 745 We note that pulling certain metadata from the service itself is expected to save the provider time and effort because the information is in large part inherently available to the service implementation. This most obviously applies to the table metadata: the provider's underlying database will normally have access to table schemas which can be used to provide, for instance, detailed descriptions of all the tables and their columns. It is less natural, perhaps, for the service to have access to the capability information; however, with the growing use of service toolkits that allow a provider to quickly deploy compliant IVOA services, it is possible that the capability information could naturally be assembled from the configuration information that was used to set up the service. Of course, if the provider is forced to create static XML documents manually to implement the VOSI functions, it's unlikely that this has saved her any time over entering the metadata into a publishing tool. 746 747 A key goal of the VOSI metadata functions is to encourage the capture of metadata that is useful for discovering and selecting services that a user will want to work with. For example, a user may wish to find services that access a table containing redshift values. Or, a user may wish to find Simple Spectral Services (a particular capability) that are fully compliant. A second goal is to make that metadata available so that the user can plan its use of the service: for example, the user, through some tool, might browse through the table column descriptions to figure out how to form her query. In this latter use, the client tool can either use the registry as the source of this information or the service itself via its VOSI functions. The question that arises for the client tool developer then is, which source -- the registry or the service -- should be preferred? 748 749 This VOSI specification does not recommend the use of one source over the other. The choice, in general, will depend on the context of a particular client tool and what it is trying to do, and the preferences of developers may indeed evolve over time as, say, VOSI support becomes more ubiquitous. At least initially, client tools -- particularly general ones that can engage different kinds of service protocols -- will likely prefer to use the registry for the source of capability and table metadata. The main reason would be that not all services will be implemented to support VOSI. By going to the registry in this case, the client gets this metadata for both services where it was retrieved via VOSI and where it was entered explicitly into a publishing tool by the provider. Under certain circumstances however, say, where the client works with just one kind of service protocol like TAP in which VOSI support is mandated for compliance, the VOSI interface might be the preferred source. In particular, if the service URL was obtained by the client through some means other than the discovery in a registry, then it would not be necessary for the client to go to the registry to understand what can be done with the service; the tool can get this information from the service itself. 750 751 We have implied in the above discussion that the capability and table metadata are the same whether they are retrieved from the registry or from the service. It is possible, however, that the service could change -- new capabilities or table columns could be added -- and the registry could (at least temporarily, depending on the registry) get out of sync with the service. This circumstance may occur rarely; nevertheless, if being up-to-date is important, then the client may need to be more sophisticated in its retrieval. That is, it could retrieve the resource description from the registry; then if the description indicates support for VOSI, the VOSI URLs would be accessed to get the latest, up-to-date information. 752 753 \section{Changes from Previous Versions} 754 \label{appendix:changes} 755 756 \subsection{Changes since REC-VOSI-20110531} 757 758 Added alternate root element (table) to VOSITables schema. 759 760 Extended VOSITables REST binding to include detail level and parameter and child table resource for scalability. 761 762 Defined \#tables-1.1 standardID. 763 764 Removed references to SOAP bindings. 765 766 \subsection{Changes since PR-20101206} 767 768 Added Appendix B, use case discussion 769 770 Formatting comments from RFC addressed: typos, etc. 771 772 \subsection{Changes since PR-20100311} 773 774 Inclusion of IVOA Architecture text 775 776 Restructuring and clarification in response to RFC comments 777 778 Inclusion of VOSITables schema in appendix 779 780 Second example added for a TAP service response 781 782 \subsection{Changes since WD-20090825} 783 784 Mandate the use of VOSICapabilities to return capabilities 785 786 S2.1: added non-normative note about capability sub-types; added example capabilities metadata 787 788 Recommend the inclusion of VOSI interfaces in capability metadata 789 790 S2.5: When returning capabilities metadata, require VOSI (REST) accessURLs to have use="full"; recommend this use of ParamHTTP. 791 792 Rename Availability schema to VOSIAvailability; added VOSICapabilities schema. 793 794 \subsection{Changes since WD-20081030} 795 796 The REST binding is made mandatory for all kinds of service. Details of the SOAP binding, including its WSDL contract, are removed. 797 798 The definition of the root element for the table-metadata document is corrected. Instead of requiring the tableset element from VODataService 1.1 (which element does not exist in that schema), the text now requires an element of type TableSet. 799 800 801 \bibliography{ivoatex/ivoabib} 802 803 804 \end{document}