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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-26) exists of draft-ietf-httpbis-p6-cache-22 ** Obsolete normative reference: RFC 4627 (Obsoleted by RFC 7158, RFC 7159) ** Obsolete normative reference: RFC 5226 (Obsoleted by RFC 8126) ** Obsolete normative reference: RFC 5988 (Obsoleted by RFC 8288) == Outdated reference: A later version (-26) exists of draft-ietf-httpbis-p4-conditional-22 == Outdated reference: A later version (-26) exists of draft-ietf-httpbis-p5-range-22 == Outdated reference: A later version (-26) exists of draft-ietf-httpbis-p7-auth-22 Summary: 3 errors (**), 0 flaws (~~), 5 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group M. Nottingham 3 Internet-Draft May 8, 2013 4 Intended status: Informational 5 Expires: November 9, 2013 7 Home Documents for HTTP APIs 8 draft-nottingham-json-home-03 10 Abstract 12 This document proposes a "home document" format for non-browser HTTP 13 clients. 15 Note to Readers 17 This draft should be discussed on the apps-discuss mailing list; see 18 [apps-discuss]. 20 Status of this Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at http://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on November 9, 2013. 37 Copyright Notice 39 Copyright (c) 2013 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 55 1.1. Notational Conventions . . . . . . . . . . . . . . . . . . 3 56 2. JSON Home Documents . . . . . . . . . . . . . . . . . . . . . 3 57 3. Resource Objects . . . . . . . . . . . . . . . . . . . . . . . 5 58 3.1. Resolving Templated Links . . . . . . . . . . . . . . . . 6 59 4. Resource Hints . . . . . . . . . . . . . . . . . . . . . . . . 6 60 4.1. allow . . . . . . . . . . . . . . . . . . . . . . . . . . 7 61 4.2. formats . . . . . . . . . . . . . . . . . . . . . . . . . 7 62 4.3. accept-patch . . . . . . . . . . . . . . . . . . . . . . . 7 63 4.4. accept-post . . . . . . . . . . . . . . . . . . . . . . . 8 64 4.5. accept-ranges . . . . . . . . . . . . . . . . . . . . . . 8 65 4.6. accept-prefer . . . . . . . . . . . . . . . . . . . . . . 8 66 4.7. docs . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 67 4.8. precondition-req . . . . . . . . . . . . . . . . . . . . . 9 68 4.9. auth-req . . . . . . . . . . . . . . . . . . . . . . . . . 9 69 4.10. status . . . . . . . . . . . . . . . . . . . . . . . . . . 9 70 5. Representation Hints . . . . . . . . . . . . . . . . . . . . . 10 71 6. Creating and Serving Home Documents . . . . . . . . . . . . . 10 72 6.1. Managing Change in Home Documents . . . . . . . . . . . . 10 73 6.2. Evolving and Mixing APIs with Home Documents . . . . . . . 11 74 6.3. Documenting APIs that use Home Documents . . . . . . . . . 11 75 7. Consuming Home Documents . . . . . . . . . . . . . . . . . . . 11 76 8. Security Considerations . . . . . . . . . . . . . . . . . . . 12 77 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 78 9.1. HTTP Resource Hint Registry . . . . . . . . . . . . . . . 12 79 9.2. HTTP Representation Hint Registry . . . . . . . . . . . . 12 80 9.3. Media Type Registration . . . . . . . . . . . . . . . . . 12 81 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 82 10.1. Normative References . . . . . . . . . . . . . . . . . . . 13 83 10.2. Informative References . . . . . . . . . . . . . . . . . . 13 84 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 14 85 Appendix B. Frequently Asked Questions . . . . . . . . . . . . . 14 86 B.1. Why not Microformats? . . . . . . . . . . . . . . . . . . 14 87 B.2. Why doesn't the format allow references or inheritance? . 15 88 B.3. What about authentication? . . . . . . . . . . . . . . . . 15 89 B.4. What about "Faults" (i.e., errors)? . . . . . . . . . . . 15 90 B.5. How Do I find the schema for a format? . . . . . . . . . . 15 91 B.6. How do I express complex query arguments? . . . . . . . . 15 92 Appendix C. Open Issues . . . . . . . . . . . . . . . . . . . . . 16 93 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16 95 1. Introduction 97 There is an emerging preference for non-browser Web applications 98 (colloquially, "HTTP APIs") to use a link-driven approach to their 99 interactions to assure loose coupling, thereby enabling extensibility 100 and API evolution. 102 This is based upon experience with previous APIs that specified 103 static URI paths (such as 104 "http://api.example.com/v1.0/widgets/abc123/properties"), which have 105 resulted in brittle, tight coupling between clients and servers. 107 Sometimes, these APIs are documented by a document format like [WADL] 108 that is used as a design-time description; i.e., the URIs and other 109 information they describe are "baked into" client implementations. 111 In contrast, a "follow your nose" API advertises the resources 112 available to clients using link relations [RFC5988] and the formats 113 they support using internet media types [RFC6838]. A client can then 114 decide - at run time - which resources to interact with based upon 115 its capabilities (as described by link relations), and the server can 116 safely add new resources and formats without disturbing clients that 117 are not yet aware of them. 119 As such, clients need to be able to discover this information quickly 120 and efficiently use it to interact with the server. Just as with a 121 human-targeted "home page" for a site, we can create a "home 122 document" for a HTTP API that describes it to non-browser clients. 124 Of course, an HTTP API might use any format to do so; however, there 125 are advantages to having a standard home document format. This 126 specification suggests one for consideration, using the JSON format 127 [RFC4627]. 129 1.1. Notational Conventions 131 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 132 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 133 document are to be interpreted as described in [RFC2119]. 135 2. JSON Home Documents 137 A JSON Home Document uses the format described in [RFC4627] and has 138 the media type "application/json-home". 140 Its content consists of a root object with a "resources" property, 141 whose member names are link relation types (as defined by [RFC5988]), 142 and values are Resource Objects, defined below. 144 For example: 146 GET / HTTP/1.1 147 Host: example.org 148 Accept: application/json-home 150 HTTP/1.1 200 OK 151 Content-Type: application/json-home 152 Cache-Control: max-age=3600 153 Connection: close 155 { 156 "resources": { 157 "http://example.org/rel/widgets": { 158 "href": "/widgets/" 159 }, 160 "http://example.org/rel/widget": { 161 "href-template": "/widgets/{widget_id}", 162 "href-vars": { 163 "widget_id": "http://example.org/param/widget" 164 }, 165 "hints": { 166 "allow": ["GET", "PUT", "DELETE", "PATCH"], 167 "formats": { 168 "application/json": {} 169 }, 170 "accept-patch": ["application/json-patch"], 171 "accept-post": ["application/xml"], 172 "accept-ranges": ["bytes"] 173 } 174 } 175 } 176 } 178 Here, we have a home document that links to a resource, "/widgets/" 179 with the relation "http://example.org/rel/widgets". It also links to 180 an unknown number of resources with the relation type 181 "http://example.org/rel/widget" using a URI Template [RFC6570], along 182 with a mapping of identifiers to a variable for use in that template. 184 It also gives several hints about interacting with the latter 185 "widget" resources, including the HTTP methods usable with them, the 186 patch formats they accept, and the fact that they support partial 187 requests [I-D.ietf-httpbis-p5-range] using the "bytes" range- 188 specifier. 190 It gives no such hints about the "widgets" resource. This does not 191 mean that it (for example) doesn't support any HTTP methods; it means 192 that the client will need to discover this by interacting with the 193 resource, and/or examining the documentation for its link relation 194 type. 196 Effectively, this names a set of behaviors, as described by a 197 resource object, with a link relation type. This means that several 198 link relations might apply to a common base URL; e.g.: 200 { 201 "resources": { 202 "http://example.org/rel/search-by-id": { 203 "href-template": "/search?id={widget}", 204 "href-vars": { 205 "widget_name": "http://example.org/param/widget" 206 } 207 }, 208 "http://example.org/rel/search-by-name": { 209 "href-template": "/search?name={widget_name}", 210 "href-vars": { 211 "widget_name": "http://example.org/param/widget_name" 212 } 213 } 214 } 215 } 217 3. Resource Objects 219 A Resource Object links to resources of the defined type using one of 220 two mechanisms; either a direct link (in which case there is exactly 221 one resource of that relation type associated with the API), or a 222 templated link, in which case there are zero to many such resources. 224 Direct links are indicated with an "href" property, whose value is a 225 URI [RFC3986]. 227 Templated links are indicated with an "href-template" property, whose 228 value is a URI Template [RFC6570]. When "href-template" is present, 229 the Resource Object MUST have a "href-vars" property; see "Resolving 230 Templated Links". 232 Resource Objects MUST have exactly one of the "href" and "href-vars" 233 properties. 235 In both forms, the links that "href" and "href-template" refer to are 236 URI-references [RFC3986] whose base URI is that of the JSON Home 237 Document itself. 239 Resource Objects MAY also have a "hints" property, whose value is an 240 object that uses named Resource Hints (see Section 4) as its 241 properties. 243 3.1. Resolving Templated Links 245 A URI can be derived from a Templated Link by treating the "href- 246 template" value as a Level 3 URI Template [RFC6570], using the "href- 247 vars" property to fill the template. 249 The "href-vars" property, in turn, is an object that acts as a 250 mapping between variable names available to the template and absolute 251 URIs that are used as global identifiers for the semantics and syntax 252 of those variables. 254 For example, given the following Resource Object: 256 "http://example.org/rel/widget": { 257 "href-template": "/widgets/{widget_id}", 258 "href-vars": { 259 "widget_id": "http://example.org/param/widget" 260 }, 261 "hints": { 262 "allow": ["GET", "PUT", "DELETE", "PATCH"], 263 "formats": { 264 "application/json": {} 265 }, 266 "accept-patch": ["application/json-patch"], 267 "accept-post": ["application/xml"], 268 "accept-ranges": ["bytes"] 269 } 270 } 272 If you understand that "http://example.org/param/widget" is an 273 numeric identifier for a widget (perhaps by dereferencing that URL 274 and reading the documentation), you can then find the resource 275 corresponding to widget number 12345 at 276 "http://example.org/widgets/12345" (assuming that the Home Document 277 is located at "http://example.org/"). 279 4. Resource Hints 281 Resource hints allow clients to find relevant information about 282 interacting with a resource beforehand, as a means of optimising 283 communications, as well as advertising available behaviours (e.g., to 284 aid in laying out a user interface for consuming the API). 286 Hints are just that - they are not a "contract", and are to only be 287 taken as advisory. The runtime behaviour of the resource always 288 overrides hinted information. 290 For example, a resource might hint that the PUT method is allowed on 291 all "widget" resources. This means that generally, the user has the 292 ability to PUT to a particular resource, but a specific resource 293 might reject a PUT based upon access control or other considerations. 294 More fine-grained information might be gathered by interacting with 295 the resource (e.g., via a GET), or by another resource "containing" 296 it (such as a "widgets" collection) or describing it (e.g., one 297 linked to it with a "describedby" link relation). 299 This specification defines a set of common hints, based upon 300 information that's discoverable by directly interacting with 301 resources. See Section 9.1 for information on defining new hints. 303 4.1. allow 305 o Resource Hint Name: allow 306 o Description: Hints the HTTP methods that the current client will 307 be able to use to interact with the resource; equivalent to the 308 Allow HTTP response header. 309 o Specification: [this document] 311 Content MUST be an array of strings, containing HTTP methods. 313 4.2. formats 315 o Resource Hint Name: formats 316 o Description: Hints the representation types that the resource 317 produces and consumes, using the GET and PUT methods respectively, 318 subject to the 'allow' hint. 319 o Specification: [this document] 321 Content MUST be an object, whose keys are media types, and values are 322 objects containing Representation Hints (see Section 5). 324 4.3. accept-patch 326 o Resource Hint Name: accept-patch 327 o Description: Hints the PATCH [RFC5789] request formats accepted by 328 the resource for this client; equivalent to the Accept-Patch HTTP 329 response header. 331 o Specification: [this document] 333 Content MUST be an array of strings, containing media types. 335 When this hint is present, "PATCH" SHOULD be listed in the "allow" 336 hint. 338 4.4. accept-post 340 o Resource Hint Name: accept-post 341 o Description: Hints the POST request formats accepted by the 342 resource for this client. 343 o Specification: [this document] 345 Content MUST be an array of strings, containing media types. 347 When this hint is present, "POST" SHOULD be listed in the "allow" 348 hint. 350 4.5. accept-ranges 352 o Resource Hint Name: accept-ranges 353 o Description: Hints the range-specifiers available to the client 354 for this resource; equivalent to the Accept-Ranges HTTP response 355 header [I-D.ietf-httpbis-p5-range]. 356 o Specification: [this document] 358 Content MUST be an array of strings, containing HTTP range- 359 specifiers. 361 4.6. accept-prefer 363 o Resource Hint Name: accept-prefer 364 o Description: Hints the preferences [I-D.snell-http-prefer] 365 supported by the resource. Note that, as per that specifications, 366 a preference can be ignored by the server. 367 o Specification: [this document] 369 Content MUST be an array of strings, contain preferences. 371 4.7. docs 373 o Resource Hint Name: docs 374 o Description: Hints the location for human-readable documentation 375 for the relation type of the resource. 376 o Specification: [this document] 378 Content MUST be a string containing an absolute-URI [RFC3986] 379 referring to documentation that SHOULD be in HTML format. 381 4.8. precondition-req 383 o Resource Hint Name: precondition-req 384 o Description: Hints that the resource requires state-changing 385 requests (e.g., PUT, PATCH) to include a precondition, as per 386 [I-D.ietf-httpbis-p4-conditional], to avoid conflicts due to 387 concurrent updates. 388 o Specification: [this document] 390 Content MUST be an array of strings, with possible values "etag" and 391 "last-modified" indicating type of precondition expected. 393 4.9. auth-req 395 o Resource Hint Name: auth-req 396 o Description: Hints that the resource requires authentication using 397 the HTTP Authentication Framework [I-D.ietf-httpbis-p7-auth]. 398 o Specification: [this document] 400 Content MUST be an array of objects, each with a "scheme" property 401 containing a string that corresponds to a HTTP authentication scheme, 402 and optionally a "realms" property containing an array of zero to 403 many strings that identify protection spaces that the resource is a 404 member of. 406 For example, a Resource Object might contain the following hint: 408 { 409 "auth-req": [ 410 { 411 "scheme": "Basic", 412 "realms": ["private"] 413 } 414 ] 415 } 417 4.10. status 419 o Resource Hint Name: status 420 o Description: Hints the status of the resource. 421 o Specification: [this document] 423 Content MUST be a string; possible values are: 425 o "deprecated" - indicates that use of the resource is not 426 recommended, but it is still available. 427 o "gone" - indicates that the resource is no longer available; i.e., 428 it will return a 410 Gone HTTP status code if accessed. 430 5. Representation Hints 432 TBD 434 6. Creating and Serving Home Documents 436 When making a home document available, there are a few things to keep 437 in mind: 439 o A home document is best located at a memorable URI, because its 440 URI will effectively become the URI for the API itself to clients. 441 o Home documents can be personalised, just as "normal" home pages 442 can. For example, you might advertise different URIs, and/or 443 different kinds of link relations, depending on the client's 444 identity. 445 o Home documents SHOULD be assigned a freshness lifetime (e.g., 446 "Cache-Control: max-age=3600") so that clients can cache them, to 447 avoid having to fetch it every time the client interacts with the 448 service. 449 o Custom link relation types, as well as the URIs for variables, 450 should lead to documentation for those constructs. 452 6.1. Managing Change in Home Documents 454 The URIs used in home documents MAY change over time. However, 455 changing them can cause issues for clients that are relying on cached 456 home documents containing old links. 458 To mitigate the impact of such changes, servers SHOULD consider: 460 o Reducing the freshness lifetime of home documents before a link 461 change, so that clients are less likely to refer to an "old" 462 document. 463 o Regarding the "old" and "new" URIs as equally valid references for 464 an "overlap" period. 465 o After that period, handling requests for the "old" URIs 466 appropriately; e.g., with a 404 Not Found, or by redirecting the 467 client to the new URI. 469 6.2. Evolving and Mixing APIs with Home Documents 471 Using home documents affords the opportunity to change the "shape" of 472 the API over time, without breaking old clients. 474 This includes introducing new functions alongside the old ones - by 475 adding new link relation types with corresponding resource objects - 476 as well as adding new template variables, media types, and so on. 478 It's important to realise that a home document can serve more than 479 one "API" at a time; by listing all relevant relation types, it can 480 effectively "mix" different APIs, allowing clients to work with 481 different resources as they see fit. 483 6.3. Documenting APIs that use Home Documents 485 Another use case for "static" API description formats like WSDL and 486 WADL is to generate documentation for the API from them. 488 An API that uses the home document format correctly won't have a need 489 to do so, provided that the link relation types and media types it 490 uses are well-documented already. 492 7. Consuming Home Documents 494 Clients might use home documents in a variety of ways. 496 In the most common case - actually consuming the API - the client 497 will scan the Resources Object for the link relation(s) that it is 498 interested in, and then to interact with the resource(s) referred to. 499 Resource Hints can be used to optimise communication with the client, 500 as well as to inform as to the permissible actions (e.g., whether PUT 501 is likely to be supported). 503 Note that the home document is a "living" document; it does not 504 represent a "contract", but rather is expected to be inspected before 505 each interaction. In particular, links from the home document MUST 506 NOT be assumed to be valid beyond the freshness lifetime of the home 507 document, as per HTTP's caching model [I-D.ietf-httpbis-p6-cache]. 509 As a result, clients SHOULD cache the home document (as per 510 [I-D.ietf-httpbis-p6-cache]), to avoid fetching it before every 511 interaction (which would otherwise be required). 513 Likewise, a client encountering a 404 Not Found on a link SHOULD 514 obtain a fresh copy of the home document, to assure that it is up-to- 515 date. 517 8. Security Considerations 519 Clients need to exercise care when using hints. For example, a naive 520 client might send credentials to a server that uses the auth-req 521 hint, without checking to see if those credentials are appropriate 522 for that server. 524 9. IANA Considerations 526 9.1. HTTP Resource Hint Registry 528 This specification defines the HTTP Resource Hint Registry. See 529 Section 4 for a general description of the function of resource 530 hints. 532 In particular, resource hints are generic; that is, they are 533 potentially applicable to any resource, not specific to one 534 application of HTTP, nor to one particular format. Generally, they 535 ought to be information that would otherwise be discoverable by 536 interacting with the resource. 538 Hint names MUST be composed of the lowercase letters (a-z), digits 539 (0-9), underscores ("_") and hyphens ("-"), and MUST begin with a 540 lowercase letter. 542 Hint content SHOULD be described in terms of JSON [RFC4627] 543 constructs. 545 New hints are registered using the Expert Review process described in 546 [RFC5226] to enforce the criteria above. Requests for registration 547 of new resource hints are to use the following template: 549 o Resource Hint Name: [hint name] 550 o Description: [a short description of the hint's semantics] 551 o Specification: [reference to specification document] 553 Initial registrations are enumerated in Section 4. 555 9.2. HTTP Representation Hint Registry 557 TBD 559 9.3. Media Type Registration 561 TBD 563 10. References 565 10.1. Normative References 567 [I-D.ietf-httpbis-p6-cache] 568 Fielding, R., Nottingham, M., and J. Reschke, "Hypertext 569 Transfer Protocol (HTTP/1.1): Caching", 570 draft-ietf-httpbis-p6-cache-22 (work in progress), 571 February 2013. 573 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 574 Requirement Levels", BCP 14, RFC 2119, March 1997. 576 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 577 Resource Identifier (URI): Generic Syntax", STD 66, 578 RFC 3986, January 2005. 580 [RFC4627] Crockford, D., "The application/json Media Type for 581 JavaScript Object Notation (JSON)", RFC 4627, July 2006. 583 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 584 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 585 May 2008. 587 [RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010. 589 [RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., 590 and D. Orchard, "URI Template", RFC 6570, March 2012. 592 10.2. Informative References 594 [I-D.ietf-httpbis-p4-conditional] 595 Fielding, R. and J. Reschke, "Hypertext Transfer Protocol 596 (HTTP/1.1): Conditional Requests", 597 draft-ietf-httpbis-p4-conditional-22 (work in progress), 598 February 2013. 600 [I-D.ietf-httpbis-p5-range] 601 Fielding, R., Lafon, Y., and J. Reschke, "Hypertext 602 Transfer Protocol (HTTP/1.1): Range Requests", 603 draft-ietf-httpbis-p5-range-22 (work in progress), 604 February 2013. 606 [I-D.ietf-httpbis-p7-auth] 607 Fielding, R. and J. Reschke, "Hypertext Transfer Protocol 608 (HTTP/1.1): Authentication", draft-ietf-httpbis-p7-auth-22 609 (work in progress), February 2013. 611 [I-D.snell-http-prefer] 612 Snell, J., "Prefer Header for HTTP", 613 draft-snell-http-prefer-18 (work in progress), 614 January 2013. 616 [MICROFORMATS] 617 microformats.org, "Microformats", 618 . 620 [RFC5789] Dusseault, L. and J. Snell, "PATCH Method for HTTP", 621 RFC 5789, March 2010. 623 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type 624 Specifications and Registration Procedures", BCP 13, 625 RFC 6838, January 2013. 627 [WADL] Hadley, M. and Sun Microsystems, "Web Application 628 Description Language", 629 . 631 [apps-discuss] 632 IETF, "IETF Apps-Discuss Mailing List", 633 . 635 Appendix A. Acknowledgements 637 Thanks to Jan Algermissen, Mike Amundsen, Bill Burke, Graham Klyne, 638 Leif Hedstrom, Jeni Tennison, Erik Wilde and Jorge Williams for their 639 suggestions and feedback. 641 Appendix B. Frequently Asked Questions 643 B.1. Why not Microformats? 645 Browser-centric Web applications use HTML as their representation 646 format of choice. While it is possible to augment HTML for non- 647 browser clients (using techniques like Microformats [MICROFORMATS]), 648 a few issues become evident when doing so: 650 o HTML has a very forgiving syntax. While this is appropriate for 651 browsers (especially considering that there are many million HTML 652 authors in the world), it makes for a less-than-precise language 653 for machines, resulting in both overhead (parsing and 654 transmission) as well as lack of precision. 656 o HTML is presentation-centric, making it tempting to reformat it 657 from time to time, to improve the "look and feel" of a page. 658 However, doing so can cause comparatively brittle non-browser 659 clients to lose their understanding of the content's semantics, 660 unless very careful controls are in place. 662 Because of this, it's most practical to define a separate format, and 663 JSON is easily machine-readable, precise, and has a better chance of 664 being managed for stability. 666 B.2. Why doesn't the format allow references or inheritance? 668 Adding inheritance or references would allow more modularity in the 669 format and make it more compact, at the cost of considerable 670 complexity and the associated potential for errors (both in the 671 specification and by its users). 673 Since good tools and compression are effective ways to achieve the 674 same ends, this specification doesn't attempt them. 676 B.3. What about authentication? 678 In HTTP, authentication is discoverable by interacting with the 679 resource (usually, by getting a 401 Unauthorized response status 680 code, along with one or more challenges). While the home document 681 could hint it, this isn't yet done, to avoid possible security 682 complications. 684 B.4. What about "Faults" (i.e., errors)? 686 In HTTP, errors are conveyed by HTTP status codes. While this 687 specification could (and even may) allow enumeration of possible 688 error conditions, there's a concern that this will encourage 689 applications to define many such "faults", leading to tight coupling 690 between the application and its clients. 692 B.5. How Do I find the schema for a format? 694 That isn't addressed by home documents. Ultimately, it's up to the 695 media type accepted and generated by resources to define and 696 constrain (or not) their syntax. 698 B.6. How do I express complex query arguments? 700 Complex queries - i.e., those that exceed the expressive power of 701 Link Templates or would require ambiguous properties of a "resources" 702 object - aren't intended to be defined by a home document. The 703 appropriate way to do this is with a "form" language, much as HTML 704 defines. 706 Note that it is possible to support multiple query syntaxes on the 707 same base URL, using more than one link relation type; see the 708 example at the start of the document. 710 Appendix C. Open Issues 712 The following is a list of placeholders for open issues. 714 o top-level object(s) 715 * contact details 716 * overall documentation 717 * release info? 718 * ToS 719 * rate limiting (per-resource?) 720 o Resource Hints 721 * indicate a POST to 201 Created pattern 722 * indicate an "action" POST 723 * outbound links 724 * forms? 725 o Representation Hints 726 * format profiles 727 * deprecation 728 o Defining new top-level and resource object properties - how new 729 ones are minted, registry, etc. 730 o Discovery (e.g., conneg, .well-known, etc.) 731 o LIMITED include function? 733 Author's Address 735 Mark Nottingham 737 Email: mnot@mnot.net 738 URI: http://www.mnot.net/