Home Documents for HTTP APIsmnot@mnot.nethttps://www.mnot.net/
General
Internet-DraftThis document proposes a “home document” format for non-browser HTTP clients.The issues list for this draft can be found at https://github.com/mnot/I-D/labels/json-home.The most recent (often, unpublished) draft is at https://mnot.github.io/I-D/json-home/.Recent changes are listed at https://github.com/mnot/I-D/commits/gh-pages/json-home.For information about implementations, see https://github.com/mnot/I-D/wiki/json-home.It is becoming increasingly common to use HTTP for applications other than traditional
Web browsing. Such “HTTP APIs” are used to integrate processes on disparate systems, make
information available to machines across the Internet, and as part of the implementation of
“micro-services.”By using HTTP, these applications realise a number of benefits, from message framing to caching,
and well-defined semantics that are broadly understood and useful.Often, these applications of HTTP are defined by documenting static URLs that clients need to
know and servers need to implement. Any interaction outside of these bounds is uncharted territory.For some applications, this approach brings issues, especially when the interface changes, either
due to evolution, extension or drift between implementations. Furthermore, implementing more than
one instance of interface can bring further issues, as different environments have different
requirements.The Web itself offers one way to address these issues, using links to navigate between
states. A link-driven application discovers relevant resources at run time, using a shared
vocabulary of link relations and internet media types to support a “follow
your nose” style of interaction – just as a Web browser does to navigate the Web.A client can then decide which resources to interact with “on the fly” based upon its
capabilities (as described by link relations), and the server can safely add new resources and
formats without disturbing clients that are not yet aware of them.Doing so can provide any of a number of benefits, including:Extensibility - Because new server capabilities can be expressed as link relations, new features can be layered in without introducing a new API version; clients will discover them in the home document. This promotes loose coupling between clients and servers.Evolvability - Likewise, interfaces can change gradually by introducing a new link relation and/or format while still supporting the old ones.Customisation - Home documents can be tailored for the client, allowing diffrent classes of service or different client permissions to be exposed naturally.Flexible deployment - Since URLs aren’t baked into documentation, the server can choose what URLs to use for a given service.API mixing - Likewise, more than one API can be deployed on a given server, without fear of collisions.Whether an application ought to use links in this fashion depends on how it is deployed; generally,
the most benefit will be received when multiple instances of the service are deployed, possibly
with different versions, and they are consumed by clients with different capabilities. In
particular, Internet Standards that use HTTP as a substrate are likely to require the attributes
described above.This document defines a “home page” format using the JSON format for APIs to use as a
launching point for the interactions they offer, using links. Having a well-defined format for this
purpose promotes good practice and tooling.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
.An API Home Document (or, interchangeably, “home document”) uses the format described in
and has the media type “application/json-home”.Note: this media type is not final, and will change before final publication.Its content consists of a root object with:A “resources” member, whose value is an object that describes the resources associated with the
API. Its member names are link relation types (as defined by ), and their values are
Resource Objects ().Optionally, a “api” member, whose value is an API Object () that contains
information about the API as a whole.For example:Here, we have a home document for the API “Example API”, whose author can be contacted at the
e-mail address “api-admin@example.com”, and whose documentation is at
“https://example.com/api-docs/”.It links to a resource “/widgets/” with the relation “tag:me@example.com,2016:widgets”. It also
links to an unknown number of resources with the relation type “tag:me@example.com,2016:widget”
using a URI Template , along with a mapping of identifiers to a variable for use in that
template.It also gives several hints about interacting with the latter “widget” resources, including the
HTTP methods usable with them, the PATCH and POST formats they accept, and the fact that they
support partial requests using the “bytes” range-specifier.It gives no such hints about the “widgets” resource. This does not mean that it (for example)
doesn’t support any HTTP methods; it means that the client will need to discover this by
interacting with the resource, and/or examining the documentation for its link relation type.Effectively, this names a set of behaviors, as described by a resource object, with a link relation
type. This means that several link relations might apply to a common base URL; e.g.:Note that the examples above use both tag and https URIs; any URI scheme
can be used to identify link relations and other artefacts in home documents. Typically, these are
not links to be followed; they are only used to identify things.An API Object contains links to information about the API itself.Two optional members are defined:“title” has a string value indicating the name of the API;“links” has an object value, whose member names are link relation types , and values
are URLs . The context of these links is the API home document as a whole.No links are required to be conveyed, but APIs might benefit from setting the following:author - a suitable URL (e.g., mailto: or https:) for the author(s) of the APIdescribedBy - a link to documentation for the APIlicense - a link to the legal terms for using the APIFuture members of the API Object MAY be defined by specifications that update this document.A Resource Object links to resources of the type indicated in their name using one of two
mechanisms; either a direct link (in which case there is exactly one resource of that relation type
associated with the API), or a templated link, in which case there are zero to many such resources.Direct links are indicated with an “href” property, whose value is a URI .Templated links are indicated with an “hrefTemplate” property, whose value is a URI Template
. When “hrefTemplate” is present, the Resource Object MUST have a “hrefVars” property;
see “Resolving Templated Links”.Resource Objects MUST have exactly one of the “href” or “href-vars” properties.In both forms, the links that “href” and “hrefTemplate” refer to are URI-references
whose base URI is that of the API Home Document itself.Resource Objects MAY also have a “hints” property, whose value is an object that uses named
Resource Hints (see ) as its properties.A URI can be derived from a Templated Link by treating the “hrefTemplate” value as a Level 3 URI
Template , using the “hrefVars” property to fill the template.The “hrefVars” property, in turn, is an object that acts as a mapping between variable names
available to the template and absolute URIs that are used as global identifiers for the semantics
and syntax of those variables.For example, given the following Resource Object:If you understand that “https://example.org/param/widget” is an numeric identifier for a widget, you
can then find the resource corresponding to widget number 12345 at
“https://example.org/widgets/12345” (assuming that the Home Document is located at
“https://example.org/”).Resource hints allow clients to find relevant information about interacting with a resource
beforehand, as a means of optimizing communications, as well as advertising available behaviors
(e.g., to aid in laying out a user interface for consuming the API).Hints are just that – they are not a “contract”, and are to only be taken as advisory. The runtime
behavior of the resource always overrides hinted information.For example, a resource might hint that the PUT method is allowed on all “widget” resources. This
means that generally, the user has the ability to PUT to a particular resource, but a specific
resource might reject a PUT based upon access control or other considerations. More fine-grained
information might be gathered by interacting with the resource (e.g., via a GET), or by another
resource “containing” it (such as a “widgets” collection) or describing it (e.g., one linked to it
with a “describedBy” link relation).This specification defines a set of common hints, based upon information that’s discoverable by
directly interacting with resources. See for information on defining new
hints.Resource Hint Name: allowDescription: Hints the HTTP methods that the current client will be able to use to interact with
the resource; equivalent to the Allow HTTP response header.Specification: [this document]Content MUST be an array of strings, containing HTTP methods. As per HTTP, when GET is supported, a
client MAY assume that HEAD is supported.Resource Hint Name: formatsDescription: Hints the representation types that the resource makes available, using the
GET method.Specification: [this document]Content MUST be an object, whose keys are media types, and values are objects, currently empty.Resource Hint Name: accept-PatchDescription: Hints the PATCH request formats accepted by the resource for this
client; equivalent to the Accept-Patch HTTP response header.Specification: [this document]Content MUST be an array of strings, containing media types.When this hint is present, “PATCH” SHOULD be listed in the “allow” hint.Resource Hint Name: acceptPostDescription: Hints the POST request formats accepted by the resource for this client.Specification: [this document]Content MUST be an array of strings, containing media types.When this hint is present, “POST” SHOULD be listed in the “allow” hint.Resource Hint Name: acceptPutDescription: Hints the PUT request formats accepted by the resource for this client.Specification: [this document]Content MUST be an array of strings, containing media types.When this hint is present, “PUT” SHOULD be listed in the “allow” hint.Resource Hint Name: acceptRangesDescription: Hints the range-specifiers available to the client for this resource; equivalent to
the Accept-Ranges HTTP response header .Specification: [this document]Content MUST be an array of strings, containing HTTP range-specifiers (typically, “bytes”).Resource Hint Name: acceptPreferDescription: Hints the preferences supported by the resource. Note that, as per that
specifications, a preference can be ignored by the server.Specification: [this document]Content MUST be an array of strings, containing preferences.Resource Hint Name: docsDescription: Hints the location for human-readable documentation for the relation type of the
resource.Specification: [this document]Content MUST be a string containing an absolute-URI referring to documentation that
SHOULD be in HTML format.Resource Hint Name: preconditionRequiredDescription: Hints that the resource requires state-changing requests (e.g., PUT, PATCH) to
include a precondition, as per , to avoid conflicts due to concurrent updates.Specification: [this document]Content MUST be an array of strings, with possible values “etag” and “last-modified” indicating
type of precondition expected.Resource Hint Name: authSchemesDescription: Hints that the resource requires authentication using the HTTP Authentication
Framework .Specification: [this document]Content MUST be an array of objects, each with a “scheme” property containing a string that
corresponds to a HTTP authentication scheme, and optionally a “realms” property containing an array
of zero to many strings that identify protection spaces that the resource is a member of.For example, a Resource Object might contain the following hint:Resource Hint Name: statusDescription: Hints the status of the resource.Specification: [this document]Content MUST be a string; possible values are:“deprecated” - indicates that use of the resource is not recommended, but it is still available.“gone” - indicates that the resource is no longer available; i.e., it will return a 404 (Not
Found) or 410 (Gone) HTTP status code if accessed.Clients need to exercise care when using hints. For example, a naive client might send credentials
to a server that uses the auth-req hint, without checking to see if those credentials are
appropriate for that server.This specification defines the HTTP Resource Hint Registry. See for a general
description of the function of resource hints.In particular, resource hints are generic; that is, they are potentially applicable to any
resource, not specific to one application of HTTP, nor to one particular format. Generally, they
ought to be information that would otherwise be discoverable by interacting with the resource.Hint names MUST be composed of the lowercase letters (a-z), digits (0-9), underscores (“_”) and
hyphens (“-“), and MUST begin with a lowercase letter.Hint content SHOULD be described in terms of JSON constructs.New hints are registered using the Expert Review process described in to enforce the
criteria above. Requests for registration of new resource hints are to use the following template:Resource Hint Name: [hint name]Description: [a short description of the hint’s semantics]Specification: [reference to specification document]Initial registrations are enumerated in .TBDKey words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Uniform Resource Identifier (URI): Generic SyntaxA Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK]Guidelines for Writing an IANA Considerations Section in RFCsMany protocols make use of identifiers consisting of constants and other well-known values. Even after a protocol has been defined and deployment has begun, new values may need to be assigned (e.g., for a new option type in DHCP, or a new encryption or authentication transform for IPsec). To ensure that such quantities have consistent values and interpretations across all implementations, their assignment must be administered by a central authority. For IETF protocols, that role is provided by the Internet Assigned Numbers Authority (IANA).In order for IANA to manage a given namespace prudently, it needs guidelines describing the conditions under which new values can be assigned or when modifications to existing values can be made. If IANA is expected to play a role in the management of a namespace, IANA must be given clear and concise instructions describing that role. This document discusses issues that should be considered in formulating a policy for assigning values to a namespace and provides guidelines for authors on the specific text that must be included in documents that place demands on IANA.This document obsoletes RFC 2434. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Web LinkingThis document specifies relation types for Web links, and defines a registry for them. It also defines the use of such links in HTTP headers with the Link header field. [STANDARDS-TRACK]URI TemplateA URI Template is a compact sequence of characters for describing a range of Uniform Resource Identifiers through variable expansion. This specification defines the URI Template syntax and the process for expanding a URI Template into a URI reference, along with guidelines for the use of URI Templates on the Internet. [STANDARDS-TRACK]The JavaScript Object Notation (JSON) Data Interchange FormatJavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data.This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance.Hypertext Transfer Protocol (HTTP/1.1): CachingThe Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.PATCH Method for HTTPSeveral applications extending the Hypertext Transfer Protocol (HTTP) require a feature to do partial resource modification. The existing HTTP PUT method only allows a complete replacement of a document. This proposal adds a new HTTP method, PATCH, to modify an existing HTTP resource. [STANDARDS-TRACK]Media Type Specifications and Registration ProceduresThis document defines procedures for the specification and registration of media types for use in HTTP, MIME, and other Internet protocols. This memo documents an Internet Best Current Practice.Hypertext Transfer Protocol (HTTP/1.1): Conditional RequestsThe Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP/1.1 conditional requests, including metadata header fields for indicating state changes, request header fields for making preconditions on such state, and rules for constructing the responses to a conditional request when one or more preconditions evaluate to false.Hypertext Transfer Protocol (HTTP/1.1): Range RequestsThe Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document defines range requests and the rules for constructing and combining responses to those requests.Hypertext Transfer Protocol (HTTP/1.1): AuthenticationThe Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypermedia information systems. This document defines the HTTP Authentication framework.Prefer Header for HTTPThis specification defines an HTTP header field that can be used by a client to request that certain behaviors be employed by a server while processing a request.The 'tag' URI SchemeThis document describes the "tag" Uniform Resource Identifier (URI) scheme. Tag URIs (also known as "tags") are designed to be unique across space and time while being tractable to humans. They are distinct from most other URIs in that they have no authoritative resolution mechanism. A tag may be used purely as an entity identifier. Furthermore, using tags has some advantages over the common practice of using "http" URIs as identifiers for non-HTTP-accessible resources. This memo provides information for the Internet community.Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and RoutingThe Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document provides an overview of HTTP architecture and its associated terminology, defines the "http" and "https" Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes related security concerns for implementations.Problem Details for HTTP APIsThis document defines a "problem detail" as a way to carry machine- readable details of errors in a HTTP response to avoid the need to define new error response formats for HTTP APIs.Thanks to Jan Algermissen, Mike Amundsen, Bill Burke, Sven Dietze, Graham Klyne, Leif Hedstrom,
Joe Hildebrand, Jeni Tennison, Erik Wilde and Jorge Williams for their suggestions and feedback.When making an API home document available, there are a few things to keep in mind:A home document is best located at a memorable URI, because its URI will effectively become the
URI for the API itself to clients.Home documents can be personalized, just as “normal” home pages can. For example, you might
advertise different URIs, and/or different kinds of link relations, depending on the client’s
identity.Home documents ought to be assigned a freshness lifetime (e.g., “Cache-Control: max-age=3600”) so
that clients can cache them, to avoid having to fetch it every time the client interacts with the
service.Custom link relation types, as well as the URIs for variables, should lead to documentation for
those constructs.The URIs used in API home documents MAY change over time. However, changing them can cause issues
for clients that are relying on cached home documents containing old links.To mitigate the impact of such changes, servers ought to consider:Reducing the freshness lifetime of home documents before a link change, so that clients are less
likely to refer to an “old” document.Regarding the “old” and “new” URIs as equally valid references for an “overlap” period.After that period, handling requests for the “old” URIs appropriately; e.g., with a 404 Not
Found, or by redirecting the client to the new URI.Using home documents affords the opportunity to change the “shape” of the API over time, without
breaking old clients.This includes introducing new functions alongside the old ones – by adding new link relation types
with corresponding resource objects – as well as adding new template variables, media types, and
so on.It’s important to realise that a home document can serve more than one “API” at a time; by listing
all relevant relation types, it can effectively “mix” different APIs, allowing clients to work with
different resources as they see fit.Clients might use home documents in a variety of ways.In the most common case – actually consuming the API – the client will scan the Resources Object
for the link relation(s) that it is interested in, and then to interact with the resource(s)
referred to. Resource Hints can be used to optimize communication with the client, as well as to
inform as to the permissible actions (e.g., whether PUT is likely to be supported).Note that the home document is a “living” document; it does not represent a “contract”, but rather
is expected to be inspected before each interaction. In particular, links from the home document
MUST NOT be assumed to be valid beyond the freshness lifetime of the home document, as per HTTP’s
caching model .As a result, clients ought to cache the home document (as per ), to avoid fetching it
before every interaction (which would otherwise be required).Likewise, a client encountering a 404 (Not Found) on a link is encouraged obtain a fresh copy of
the home document, to assure that it is up-to-date.There are a fair number of existing service description formats, including those that specialise in
“RESTful” interactions. However, these formats generally are optimised for pairwise integration, or
one-server-to-many-client integration, and less capable of describing protocols where both the
server and client can evolve and be extended.Adding inheritance or references would allow more modularity in the format and make it more
compact, at the cost of considerable complexity and the associated potential for errors (both in
the specification and by its users).Since good tools and compression are effective ways to achieve the same ends, this specification
doesn’t attempt them.In HTTP, errors are conveyed by HTTP status codes. While this specification could (and even may)
allow enumeration of possible error conditions, there’s a concern that this will encourage
applications to define many such “faults”, leading to tight coupling between the application and
its clients. See for further considerations.That isn’t addressed by home documents. Ultimately, it’s up to the media type accepted and
generated by resources to define and constrain (or not) their syntax.Complex queries – i.e., those that exceed the expressive power of Link Templates or would require
ambiguous properties of a “resources” object – aren’t intended to be defined by a home document.
The appropriate way to do this is with a “form” language, much as HTML defines.Note that it is possible to support multiple query syntaxes on the same base URL, using more than
one link relation type; see the example at the start of the document.