The Atom Publishing Protocol

The Atom Publishing Protocol is an application-level protocol for publishing and editing Web Resources using HTTP and XML 1.0 . The protocol supports the creation of Web Resources and provides facilities for: Collections: Sets of Resources, which can be retrieved in whole or in part. Services: Discovery and description of Collections. Editing: Creating, editing, and deleting Resources. The Atom Publishing Protocol is different from many contemporary protocols in that the server is given wide latitude in processing requests from clients. See for more details.

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 .

Atom Protocol Document formats are specified in terms of the XML Information Set , serialized as XML 1.0 . The Infoset terms "Element Information Item" and "Attribute Information Item" are shortened to "element" and "attribute" respectively. Therefore, when this specification uses the term "element", it is referring to an Element Information Item, and when it uses the term "attribute", it is referring to an Attribute Information Item.

Some sections of this specification are illustrated with fragments of a non-normative RELAX NG Compact schema . However, the text of this specification provides the definition of conformance. Complete schemas appear in .

XML elements defined by this specification MAY have an xml:base attribute . When xml:base is used, it serves the function described in Section 5.1.1 of URI Generic Syntax , by establishing the base URI (or IRI) for resolving relative references found within the scope of the xml:base attribute. Any element defined by this specification MAY have an xml:lang attribute, whose content indicates the natural language for the element and its descendents. Requirements regarding the content and interpretation of xml:lang are specified in Section 2.12 of XML 1.0 .

For convenience, this protocol can be referred to as the "Atom Protocol" or "APP". The following terminology is used by this specification: URI - A Uniform Resource Identifier as defined in . In this specification the phrase "the URI of a document" is shorthand for "a URI which, when dereferenced, is expected to produce that document as a representation". IRI - An Internationalized Resource Identifier as defined in . Before an IRI found in a document is used by HTTP, the IRI is first converted to a URI. See . Resource - A network-accessible data object or service identified by an IRI, as defined in . See for further discussion on Resources. relation (or "relation of") - Refers to the "rel" attribute value of an atom:link element. Representation - An entity included with a request or response as defined in . Collection - A Resource that contains a set of Member Resources. Collections are represented as Atom Feeds. See . Member (or Member Resource) - A Resource whose IRI is listed in a Collection by an atom:link element with a relation of "edit" or "edit-media". See . The protocol defines two kinds of Members: Entry Resource - Members of a Collection that are represented as Atom Entry Documents, as defined in . Media Resource - Members of a Collection that have representations other than Atom Entry Documents. Media Link Entry - an Entry Resource that contains metadata about a Media Resource. See . Workspace - A named group of Collections. See . Service Document - A document that describes the location and capabilities of one or more Collections, grouped into Workspaces. See . Category Document - A document that describes the categories allowed in a Collection. See .

The Atom Protocol specifies operations for publishing and editing Resources using HTTP. It uses Atom-formatted representations to describe the state and metadata of those Resources. It defines how Collections of Resources can be organized, and specifies formats to support their discovery, grouping and categorization.

Atom Protocol documents allow the use of IRIs , as well as URIs to identify Resources. Before an IRI in a document is used by HTTP, the IRI is first converted to a URI according to the procedure defined in Section 3.1 of . In accordance with that specification, the conversion SHOULD be applied as late as possible. Conversion does not imply Resource creation - the IRI and the URI into which it is converted identify the same Resource. While the Atom Protocol specifies the formats of the representations that are exchanged and the actions that can be performed on the IRIs embedded in those representations, it does not constrain the form of the URIs that are used. HTTP specifies that the URI space of each server is controlled by that server, and this protocol imposes no further constraints on that control.

A Resource whose IRI is listed in a Collection is called a Member Resource. The protocol defines two kinds of Member Resources - Entry Resources and Media Resources. Entry Resources are represented as Atom Entry Documents . Media Resources can have representations in any media type. A Media Resource is described within a Collection using an Entry called a Media Link Entry. This diagram shows the classification of Resources within the Atom Protocol:

Member Resources | ----------------- | | Entry Resources Media Resources | Media Link Entry The Atom Protocol defines Collection Resources for managing and organizing both kinds of Member Resource. A Collection is represented by an Atom Feed Document. A Collection Feed's Entries contain the IRIs of, and metadata about, the Collection's Member Resources. A Collection Feed can contain any number of Entries, which might represent all the Members of the Collection, or an ordered subset of them (see ). In the diagram of a Collection below, there are two Entries. The first contains the IRI of an Entry Resource. The second contains the IRIs of both a Media Resource and a Media Link Entry Resource, which contains the metadata for that Media Resource:

Collection | o- Entry | | | o- Member Entry IRI (Entry Resource) | o- Entry | o- Member Entry IRI (Media Link Entry) | o- Media IRI (Media Resource) The Atom Protocol does not make a distinction between Feeds used for Collections and other Atom Feeds. The only mechanism that this specification supplies for indicating a Feed is a Collection Feed is the presence of its IRI in a Service Document. Service Documents represent server-defined groups of Collections, and are used to initialize the process of creating and editing Resources. These groups of Collections are called Workspaces. Workspaces have names, but no IRIs, and no specified processing model. The Service Document can indicate which media types, and which categories, a Collection will accept. In the diagram below, there are two Workspaces each describing the IRIs, acceptable media types, and categories for a Collection:

The Atom Publishing Protocol uses HTTP methods to author Member Resources as follows: GET is used to retrieve a representation of a known Resource. POST is used to create a new, dynamically-named, Resource. When the client submits non-Atom-Entry representations to a Collection for creation, two Resources are always created - a Media Entry for the requested Resource, and a Media Link Entry for metadata about the Resource that will appear in the Collection. PUT is used to edit a known Resource. It is not used for Resource creation. DELETE is used to remove a known Resource. The Atom Protocol only covers the creating, editing, and deleting of Entry and Media Resources. Other Resources could be created, edited and deleted as the result of manipulating a Collection, but the number of those Resources, their media-types, and effects of Atom Protocol operations on them are outside the scope of this specification. Since all aspects of client-server interaction are defined in terms of HTTP, should be consulted for any areas not covered in this specification.

The Atom Protocol imposes few restrictions on the actions of servers. Unless a constraint is specified here, servers can be expected to vary in behavior, in particular around the manipulation of Atom Entries sent by clients. For example, although this specification only defines the expected behavior of Collections with respect to GET and POST, this does not imply that PUT, DELETE, PROPPATCH and others are forbidden on Collection Resources - only that this specification does not define what the server's response would be to those methods. Similarly while some HTTP status codes are mentioned explicitly, clients ought to be prepared to handle any status code from a server. Servers can choose to accept, reject, delay, moderate, censor, reformat, translate, relocate or re-categorize the content submitted to them. Only some of these choices are immediately relayed back to the client in responses to client requests; other choices may only become apparent later, in the feed or published entries. The same series of requests to two different publishing sites can result in a different series of HTTP responses, different resulting feeds or different entry contents. As a result, client software has to be written flexibly to accept what the server decides are the results of its submissions. Any server response or server content modification not explicitly forbidden by this specification or HTTP is therefore allowed.

While specific HTTP status codes are shown in the interaction diagrams below, an APP client should be prepared to handle any status code. For example, a PUT to a Member URI could result in the return of a "204 No Content" status code, which still indicates success.

Client Server | | | 1.) GET to Service Document URI | |------------------------------------------>| | | | 2.) 200 Ok | | Service Document | |<------------------------------------------| | | The client sends a GET request to the URI of the Service Document. The server responds with a Service Document enumerating the IRIs of a group of Collections and the capabilities of those Collections supported by the server. The content of this document can vary based on aspects of the client request, including, but not limited to, authentication credentials.

To list the members of a Collection, the client sends a GET request to the URI of a Collection. An Atom Feed Document is returned whose Entries contain the IRIs of Member Resources. The returned Feed may describe all, or only a partial list, of the Members in a Collection (see ).

Client Server | | | 1.) GET to Collection URI | |------------------------------->| | | | 2.) 200 Ok | | Atom Feed | |<-------------------------------| | | The client sends a GET request to the URI of the Collection. The server responds with an Atom Feed Document containing the IRIs of the Collection Members.

Client Server | | | 1.) POST to Collection URI | | Member Representation | |------------------------------------------>| | | | 2.) 201 Created | | Location: Member Entry URI | |<------------------------------------------| | | The client POSTs a representation of the Member to the URI of the Collection. If the Member Resource was created successfully, the server responds with a status code of 201 and a Location: header that contains the IRI of the newly created Entry Resource. Media Resources could have also been created and their IRIs can be found through the Entry Resource. See for more details.

Once a Resource has been created and its Member URI is known, that URI can be used to retrieve, edit, and delete the Resource. describes extensions to the Atom Syndication Format used in the Atom Protocol for editing purposes.

Client Server | | | 1.) GET to Member URI | |------------------------------------------>| | | | 2.) 200 Ok | | Member Representation | |<------------------------------------------| | | The client sends a GET request to the URI of a Member Resource to retrieve its representation. The server responds with the representation of the Member Resource.

Client Server | | | 1.) PUT to Member URI | | Member Representation | |------------------------------------------>| | | | 2.) 200 OK | |<------------------------------------------| The client sends a PUT request to store a representation of a Member Resource. If the request is successful, the server responds with a status code of 200.

Client Server | | | 1.) DELETE to Member URI | |------------------------------------------>| | | | 2.) 200 OK | |<------------------------------------------| | | The client sends a DELETE request to the URI of a Member Resource. If the deletion is successful the server responds with a status code of 200. A different approach is taken for deleting Media Resources; see for details.

The Atom Protocol uses the response status codes defined in HTTP to indicate the success or failure of an operation. Consult the HTTP specification for detailed definitions of each status code. Implementers are asked to note that according to the HTTP specification, HTTP 4xx and 5xx response entities SHOULD include a human-readable explanation of the error.

This specification defines two kinds of documents - Category Documents and Service Documents. A Category Document () contains lists of categories specified using the "atom:category" element from the Atom Syndication Format (see Section 4.2.2 of ). A Service Document () groups available Collections into Workspaces. The namespace name for either kind of document is:

http://purl.org/atom/app# The namespace name 'http://purl.org/atom/app#' needs to be updated throughout the document with the final URI upon publication Atom Publishing Protocol XML Documents MUST be "namespace-well-formed" as specified in Section 7 of . This specification uses the prefix "app:" for the namespace name. The prefix "atom:" is used for "http://www.w3.org/2005/Atom", the namespace name of the Atom Syndication Format . These namespace prefixes are not semantically significant. This specification does not define any DTDs for Atom Protocol formats, and hence does not require them to be "valid" in the sense used by .

Unrecognized markup in an Atom Publishing Protocol document is considered "foreign markup" as defined in Section 6 of the Atom Syndication Format . Foreign markup can be used anywhere within a Category or Service Document unless it is explicitly forbidden. Processors that encounter foreign markup MUST NOT stop processing and MUST NOT signal an error. Clients SHOULD preserve foreign markup when transmitting such documents. The namespace name "http://purl.org/atom/app#" is reserved for forward compatible revisions of the Category and Service Document types - this does not exclude the addition of elements and attributes that might not be recognized by processors conformant to this specification. Such unrecognized markup from the "http://purl.org/atom/app#" namespace MUST be treated as foreign markup.

Category Documents contain lists of categories described using the "atom:category" element from the Atom Syndication Format . Categories can also appear in Service Documents, where they indicate the categories allowed in a Collection (see ). Category Documents are identified with the "application/atomcat+xml" media type (see ).

<?xml version="1.0" ?> <app:categories xmlns:app="http://purl.org/atom/app#" xmlns:atom="http://www.w3.org/2005/Atom" fixed="yes" scheme="http://example.com/cats/big3"> <atom:category term="animal" /> <atom:category term="vegetable" /> <atom:category term="mineral" /> </app:categories> This Category Document contains atom:category elements, with the terms 'animal', 'vegetable', and 'mineral'. None of the categories use the "label" attribute defined in . They all inherit the "http://example.com/cats/big3" "scheme" attribute declared on the app:categories element. Therefore if the 'mineral' category were to appear in an Atom Entry or Feed Document, it would appear as:

<atom:category scheme="http://example.com/cats/big3" term="mineral"/>

The root of a Category Document is the "app:categories" element. An app:categories element can contain zero or more "atom:category" elements from the Atom Syndication Format namespace ("http://www.w3.org/2005/Atom"). An atom:category child element that has no "scheme" attribute inherits the attribute from its app:categories parent. An atom:category child element with an existing "scheme" attribute does not inherit the "scheme" value of its "app:categories" parent element.

atomCategory = element atom:category { atomCommonAttributes, attribute term { text }, attribute scheme { atomURI }?, attribute label { text }?, undefinedContent } appInlineCategories = element app:categories { attribute fixed { "yes" | "no" }?, attribute scheme { atomURI }?, (atomCategory*, undefinedContent) } appOutOfLineCategories = element app:categories { attribute href { atomURI }, undefinedContent } appCategories = appInlineCategories | appOutOfLineCategories

The app:categories element can contain a "fixed" attribute, with a value of either "yes" or "no", indicating whether the list of categories is a fixed or an open set. The absence of the "fixed" attribute is equivalent to the presence of a "fixed" attribute with a value of "no". Alternatively, the app:categories element MAY contain an "href" attribute, whose value MUST be an IRI reference identifying a Category Document. If the "href" attribute is provided, the app:categories element MUST be empty and MUST NOT have the "fixed" or "scheme" attributes.

For authoring to commence, a client needs to discover the capabilities and locations of the available Collections. Service Documents are designed to support this discovery process. How Service Documents are discovered is not defined in this specification. Service Documents are identified with the "application/atomsvc+xml" media type (see ).

A Service Document groups Collections into Workspaces. Operations on Workspaces, such as creation or deletion, are not defined by this specification. This specification assigns no meaning to Workspaces; that is, a Workspace does not imply any specific processing assumptions. There is no requirement that a server support multiple Workspaces. In addition, a Collection MAY appear in more than one Workspace.

<?xml version="1.0" encoding='utf-8'?> <service xmlns="http://purl.org/atom/app#" xmlns:atom="http://www.w3.org/2005/Atom"> <workspace> <atom:title>Main Site</atom:title> <collection href="http://example.org/blog/main" > <atom:title>My Blog Entries</atom:title> <categories href="http://example.com/cats/forMain.cats" /> </collection> <collection href="http://example.org/blog/pic" > <atom:title>Pictures</atom:title> <accept>image/png</accept> <accept>image/jpeg</accept> <accept>image/gif</accept> </collection> </workspace> <workspace> <atom:title>Sidebar Blog</atom:title> <collection href="http://example.org/sidebar/list" > <atom:title>Remaindered Links</atom:title> <accept>application/atom+xml;type=entry</accept> <categories fixed="yes"> <atom:category scheme="http://example.org/extra-cats/" term="joke" /> <atom:category scheme="http://example.org/extra-cats/" term="serious" /> </categories> </collection> </workspace> </service> The Service Document above describes two Workspaces. The first Workspace is called "Main Site", and has two Collections called "My Blog Entries" and "Pictures", whose IRIs are "http://example.org/blog/main" and "http://example.org/blog/pic" respectively. The "Pictures" Collection includes three "accept" elements indicating the types of image files the client can send to the Collection to create new Media Resources (entries associated with Media Resources are discussed in ). The second Workspace is called "Sidebar Blog" and has a single Collection called "Remaindered Links" whose IRI is "http://example.org/sidebar/list". The Collection has an "accept" element whose content is "application/atom+xml;type=entry", indicating it will accept Atom Entries from a client. Within each of the two Entry Collections, the "categories" element provides a list of available categories for Member Entries. In the "My Blog Entries" Collection, the list of available categories is available through the "href" attribute. The "Sidebar Blog" Collection provides a category list within the Service Document, but states the list is fixed, signaling a request from the server that Entries be POSTed using only those two categories.

The root of a Service Document is the "app:service" element. The app:service element is the container for service information associated with one or more Workspaces. An app:service element MUST contain one or more app:workspace elements.

namespace app = "http://purl.org/atom/app#" start = appService

appService = element app:service { appCommonAttributes, ( appWorkspace+ & extensionElement* ) }

Workspaces are server-defined groups of Collections. The "app:workspace" element contains zero or more app:collection elements describing the Collections of Resources available for editing.

appWorkspace = element app:workspace { appCommonAttributes, ( atomTitle & appCollection* & extensionSansTitleElement* ) } atomTitle = element atom:title { atomTextConstruct }

The app:workspace element MUST contain one "atom:title" element (as defined in ), giving a human-readable title for the Workspace.

The "app:collection" element describes a Collection. The app:collection Element MUST contain one "atom:title" element. The app:collection element MAY contain any number of app:accept elements, indicating the types of representations accepted by the Collection. The order of such elements is not significant. The app:collection element MAY contain any number of app:categories elements.

appCollection = element app:collection { appCommonAttributes, attribute href { atomURI }, ( atomTitle & appAccept* & appCategories* & extensionSansTitleElement* ) }

The app:collection element MUST contain an "href" attribute, whose value gives the IRI of the Collection.

The "atom:title" element is defined in , and gives a human-readable title for the Collection.

The content of an "app:accept" element value is a media-range as defined in . The media range specifies a type of representation that can be POSTed to a Collection. The app:accept element is similar to the HTTP Accept request-header . Media type parameters are allowed within app:accept, but app:accept has no notion of preference - "accept-params" or "q" arguments, as specified in Section 14.1 of are not significant. White space (as defined in ) around the app:accept element's media-range is insignificant and MUST be ignored. A value of "application/atom+xml;type=entry" MAY appear in any app:accept list of media-ranges and indicates that Atom Entry Documents can be POSTed to the Collection. If no app:accept element is present, clients SHOULD treat this as equivalent to an app:accept element with the content "application/atom+xml;type=entry". If one accept element exists and is empty, clients SHOULD assume that the Collection does not support the creation of new Entries.

appAccept = element app:accept { appCommonAttributes, ( text? ) }

The app:collection element MAY appear as a child of an atom:feed or atom:source element in an Atom Feed Document. Its content identifies a Collection by which new Entries can be added to appear in the feed. When it appears in an atom:feed or atom:source element, the app:collection element is considered foreign markup as defined in Section 6 of .

The "app:categories" element provides a list of the categories that can be applied to the members of a Collection. See for the detailed definition of app:categories. The server MAY reject attempts to create or store members whose categories are not present in it's categories list. Collections that indicate the category set is open SHOULD NOT reject otherwise acceptable members whose categories are not in its categories list. The absence of an "app:categories" element means that the category handling of the Collection is unspecified. A "fixed" category list that contains zero categories indicates the Collection does not accept category data.

The Member URI allows clients to retrieve, edit and delete a Member Resource using HTTP's GET, PUT and DELETE methods. Entry Resources are represented as Atom Entry documents. Member URIs appear in two places. They are returned in a Location header after successful Resource creation using POST, as described in below. They can also appear in a Collection Feed's entries, as atom:link elements with a link relation of "edit". A Member Entry SHOULD contain such an atom:link element with a link relation of "edit", which indicates the Member URI.

To add members to a Collection, clients send POST requests to the URI of the Collection. Successful member creation is indicated with a 201 ("Created") response code. When the Collection responds with a status code of 201, it SHOULD also return a response body, which MUST be an Atom Entry Document representing the newly-created Resource. Since the server is free to alter the POSTed Entry, for example by changing the content of the atom:id element, returning the Entry can be useful to the client, enabling it to correlate the client and server views of the new Entry. When a Member Resource is created, its Member Entry URI MUST be returned in a Location header in the Collection's response. If the creation request contained an Atom Entry Document, and the subsequent response from the server contains a Content-Location header that matches the Location header character-for-character, then the client is authorized to interpret the response entity as being a complete representation of the newly created Entry. Without a matching Content-Location header, the client MUST NOT assume the returned entity is a complete representation of the created Resource. The request body sent with the POST need not be an Atom Entry. For example, it might be a picture, or a movie. Collections MAY return a response with a status code of 415 ("Unsupported Media Type") to indicate that the media-type of the POSTed entity is not allowed or supported by the Collection. For a discussion of the issues in creating such content, see .

Below, the client sends a POST request containing an Atom Entry representation using the URI of the Collection:

POST /edit/ HTTP/1.1 Host: example.org User-Agent: Thingio/1.0 Authorization: Basic ZGFmZnk6c2VjZXJldA== Content-Type: application/atom+xml;type=entry Content-Length: nnn Slug: First Post <?xml version="1.0"?> <entry xmlns="http://www.w3.org/2005/Atom"> <title>Atom-Powered Robots Run Amok</title> <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id> <updated>2003-12-13T18:30:02Z</updated> <author><name>John Doe</name></author> <content>Some text.</content> </entry> The server signals a successful creation with a status code of 201. The response includes a Location: header indicating the Member Entry URI of the Atom Entry, and a representation of that Entry in the body of the response.

HTTP/1.1 201 Created Date: Fri, 7 Oct 2005 17:17:11 GMT Content-Length: nnn Content-Type: application/atom+xml;type=entry;charset="utf-8" Location: http://example.org/edit/first-post.atom ETag: "c180de84f991g8" <?xml version="1.0"?> <entry xmlns="http://www.w3.org/2005/Atom"> <title>Atom-Powered Robots Run Amok</title> <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id> <updated>2003-12-13T18:30:02Z</updated> <author><name>John Doe</name></author> <content>Some text.</content> <link rel="edit" href="http://example.org/edit/first-post.atom"/> </entry> The Entry created and returned by the Collection might not match the Entry POSTed by the client. A server MAY change the values of various elements in the Entry, such as the atom:id, atom:updated and atom:author values, and MAY choose to remove or add other elements and attributes, or change element content and attribute values.

To edit a Member Resource, clients send PUT requests to its Member URI, as specified in . To avoid unintentional loss of data when editing Member Entries or Media Link Entries, Atom Protocol clients SHOULD preserve all metadata that has not been intentionally modified, including unknown foreign markup as defined in Section 6 of .

To delete a Member Resource, clients send a DELETE request to its Member URI, as specified in . The deletion of a Media Link Entry SHOULD result in the deletion of the corresponding Media Resource.

Implementers are advised to pay attention to cache controls, and to make use of the mechanisms available in HTTP when editing Resources, in particular entity-tags as outlined in . Clients are not assured to receive the most recent representations of Collection Members using GET if the server is authorizing intermediaries to cache them.

Below, the client creates a Member Entry using POST:

POST /myblog/entries HTTP/1.1 Host: example.org Authorization: Basic ZGFmZnk6c2VjZXJldA== Content-Type: application/atom+xml;type=entry Content-Length: nnn Slug: First Post <?xml version="1.0" ?> <entry xmlns="http://www.w3.org/2005/Atom"> <title>Atom-Powered Robots Run Amok</title> <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id> <updated>2007-02-123T17:09:02Z</updated> <author><name>Captain Lansing</name></author> <content>It's something moving... solid metal</content> </entry> The server signals a successful creation with a status code of 201, and returns an ETag header in the response. Because, in this case, the server returned a Content-Location and Location header with the same value, the returned Entry representation can be understood to be a complete representation of the newly created Entry (see ).

HTTP/1.1 201 Created Date: Fri, 23 Feb 2007 21:17:11 GMT Content-Length: nnn Content-Type: application/atom+xml;type=entry Location: http://example.org/edit/first-post.atom Content-Location: http://example.org/edit/first-post.atom ETag: "e180ee84f0671b1" <?xml version="1.0" ?> <entry xmlns="http://www.w3.org/2005/Atom"> <title>Atom-Powered Robots Run Amok</title> <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id> <updated>2007-02-123T17:09:02Z</updated> <author><name>Captain Lansing</name></author> <content>It's something moving... solid metal</content> </entry> The client can, if it wishes, use the returned ETag value to later construct a "Conditional GET" as defined in . In this case, prior to editing, the client sends the ETag value for the Member using the If-None-Match: header.

GET /edit/first-post.atom HTTP/1.1 Host: example.org Authorization: Basic ZGFmZnk6c2VjZXJldA== If-None-Match: "e180ee84f0671b1" If the Entry has not been modified, the response will be a status code of 304 (Not Modified). This allows the client to determine it still has the most recent representation of the Entry at the time of editing.

HTTP/1.1 304 Not Modified Date: Sat, 24 Feb 2007 13:17:11 GMT After editing, the client can PUT the Entry and send the ETag entity value in an If-Match header, informing the server to accept the entry on the condition the entity value sent still matches the server's.

PUT /edit/first-post.atom HTTP/1.1 Host: example.org Authorization: Basic ZGFmZnk6c2VjZXJldA== Content-Type: application/atom+xml;type=entry Content-Length: nnn If-Match: "e180ee84f0671b1" <?xml version="1.0" ?> <entry xmlns="http://www.w3.org/2005/Atom"> <title>Atom-Powered Robots Run Amok</title> <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id> <updated>2007-02-24T16:34:06Z</updated> <author><name>Captain Lansing</name></author> <content>Update: it's a hoax!</content> </entry> The server however has since received a more recent copy than the client's, and responds with a status code of 412 (Precondition Failed).

HTTP/1.1 412 Precondition Failed Date: Sat, 24 Feb 2007 16:34:11 GMT This informs the client that the server has a more recent version of the Entry and will not allow the sent entity to be stored.

A client can POST Media Resources as well as Entry Resources to a Collection. If a server accepts such a request, then it MUST create two new Resources - one that corresponds to the entity sent in the request, called the Media Resource, and an associated Member Entry, called the Media Link Entry. Media Link Entries are represented as Atom Entries, and appear in the Collection. The Media Link Entry contains the metadata and IRI of the (perhaps non-textual) Media Resource. The Media Link Entry thus makes the metadata about the Media Resource separately available for retrieval and alteration. The server can signal the media types it will accept using the app:accept element in the Service Document, as specified in . Successful responses to creation requests MUST include the URI of the Media Link Entry in the Location header. The Media Link Entry SHOULD contain an atom:link element with a link relation of "edit-media" that contains the Media Resource IRI. The Media Link Entry MUST have an atom:content element with a "src" attribute. The value of the "src" attribute is an IRI for the newly created Media Resource. It is OPTIONAL that the IRI of the "src" attribute on the atom:content element be the same as the Media Resource IRI. For example, the "src" attribute value might instead be a link into a static cache or content distribution network and not the Media Resource IRI. Implementers are asked to note that specifies that Atom Entries MUST contain an atom:summary element. Thus, upon successful creation of a Media Link Entry, a server MAY choose to populate the atom:summary element (as well as any other required elements such as atom:id, atom:author and atom:title) with content derived from the POSTed entity or from any other source. A server might not allow a client to modify the server selected values for these elements. For Resource creation this specification only defines cases where the POST body has an Atom Entry entity declared as an Atom media type ("application/atom+xml"), or a non-Atom entity declared as a non-Atom media type. When a client is POSTing an Atom Entry to a collection, it may use a media-type of either "application/atom+xml" or "application/atom +xml;type=entry". This specification does not specify any request semantics or server behavior in the case where the POSTed media-type is "application/atom+xml" but the body is something other than an Atom Entry. In particular, what happens on POSTing an Atom Feed Document to a Collection using the "application/atom+xml" media type is undefined. The Atom Protocol does not specify a means to create multiple representations of the same Resource (for example a PNG and a JPG of the same image) either on creation or editing.

Below, the client sends a POST request containing a PNG image to the URI of a Collection that accepts PNG images:

POST /edit/ HTTP/1.1 Host: media.example.org Content-Type: image/png Slug: The Beach Authorization: Basic ZGFmZnk6c2VjZXJldA== Content-Length: nnn ...binary data... The server signals a successful creation with a status code of 201. The response includes a Location header indicating the Member URI of the Media Link Entry and a representation of that entry in the body of the response. The Media Link Entry includes a content element with a src attribute. It also contains a link with a link relation of "edit-media", specifying the IRI to be used for modifying the Media Resource.

HTTP/1.1 201 Created Date: Fri, 7 Oct 2005 17:17:11 GMT Content-Length: nnn Content-Type: application/atom+xml;type=entry;charset="utf-8" Location: http://example.org/media/edit/the_beach.atom <?xml version="1.0"?> <entry xmlns="http://www.w3.org/2005/Atom"> <title>The Beach</title> <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id> <updated>2005-10-07T17:17:08Z</updated> <author><name>Daffy</name></author> <summary type="text" /> <content type="image/png" src="http://media.example.org/the_beach.png"/> <link rel="edit-media" href="http://media.example.org/edit/the_beach.png" /> <link rel="edit" href="http://example.org/media/edit/the_beach.atom" /> </entry> Later, the client sends a PUT request containing the new PNG using the URI indicated in the Media Link Entry's "edit-media" link:

PUT /edit/the_beach.png HTTP/1.1 Host: media.example.org Content-Type: image/png Authorization: Basic ZGFmZnk6c2VjZXJldA== Content-Length: nnn ...binary data... The server signals a successful edit with a status code of 200.

HTTP/1.1 200 Ok Date: Fri, 8 Oct 2006 17:17:11 GMT The client can edit the metadata for the picture. First GET the Media Link Entry:

GET /media/edit/the_beach.atom HTTP/1.1 Host: example.org Authorization: Basic ZGFmZnk6c2VjZXJldA== The Media Link Entry is returned.

HTTP/1.1 200 Ok Date: Fri, 7 Oct 2005 17:18:11 GMT Content-Length: nnn Content-Type: application/atom+xml;type=entry;charset="utf-8" ETag: "c181bb840673b5" <?xml version="1.0"?> <entry xmlns="http://www.w3.org/2005/Atom"> <title>The Beach</title> <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id> <updated>2005-10-07T17:17:08Z</updated> <author><name>Daffy</name></author> <summary type="text" /> <content type="image/png" src="http://media.example.org/the_beach.png"/> <link rel="edit-media" href="http://media.example.org/edit/the_beach.png" /> <link rel="edit" href="http://example.org/media/edit/the_beach.atom" /> </entry> The metadata can be updated, in this case to add a summary, and then PUT back to the server.

PUT /media/edit/the_beach.atom HTTP/1.1 Host: example.org Authorization: Basic ZGFmZnk6c2VjZXJldA== Content-Type: application/atom+xml;type=entry Content-Length: nnn If-Match: "c181bb840673b5" <?xml version="1.0"?> <entry xmlns="http://www.w3.org/2005/Atom"> <title>The Beach</title> <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id> <updated>2005-10-07T17:17:08Z</updated> <author><name>Daffy</name></author> <summary type="text"> A nice sunset picture over the water. </summary> <content type="image/png" src="http://media.example.org/the_beach.png"/> <link rel="edit-media" href="http://media.example.org/edit/the_beach.png" /> <link rel="edit" href="http://example.org/media/edit/the_beach.atom" /> </entry> The update was successful.

HTTP/1.1 200 Ok Date: Fri, 7 Oct 2005 17:19:11 GMT Content-Length: 0 Multiple media Resources can be added to the Collection.

POST /edit/ HTTP/1.1 Host: media.example.org Content-Type: image/png Slug: The Pier Authorization: Basic ZGFmZnk6c2VjZXJldA== Content-Length: nnn ...binary data... The Resource is created successfully.

HTTP/1.1 201 Created Date: Fri, 7 Oct 2005 17:17:11 GMT Content-Length: nnn Content-Type: application/atom+xml;type=entry;charset="utf-8" Location: http://example.org/media/edit/the_pier.atom <?xml version="1.0"?> <entry xmlns="http://www.w3.org/2005/Atom"> <title>The Pier</title> <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efe6b</id> <updated>2005-10-07T17:26:43Z</updated> <author><name>Daffy</name></author> <summary type="text" /> <content type="image/png" src="http://media.example.org/the_pier.png"/> <link rel="edit-media" href="http://media.example.org/edit/the_pier.png" /> <link rel="edit" href="http://example.org/media/edit/the_pier.atom" /> </entry> The client can now create a new Atom Entry in the blog Entry Collection that references the two newly created Media Resources.

POST /blog/ HTTP/1.1 Host: example.org Content-Type: application/atom+xml;type=entry Slug: A day at the beach Authorization: Basic ZGFmZnk6c2VjZXJldA== Content-Length: nnn <?xml version="1.0"?> <entry xmlns="http://www.w3.org/2005/Atom"> <title>A fun day at the beach</title> <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6b</id> <updated>2005-10-07T17:40:02Z</updated> <author><name>Daffy</name></author> <content type="xhtml"> <xhtml:div xmlns:xhtml="http://www.w3.org/1999/xhtml"> <xhtml:p>We had a good day at the beach. <xhtml:img alt="the beach" src="http://media.example.org/the_beach.png"/> </xhtml:p> <xhtml:p>Later we walked down to the pier. <xhtml:img alt="the pier" src="http://media.example.org/the_pier.png"/> </xhtml:p> </xhtml:div> </content> </entry> The Resource is created successfully.

HTTP/1.1 200 Ok Date: Fri, 7 Oct 2005 17:20:11 GMT Content-Length: nnn Content-Type: application/atom+xml;type=entry;charset="utf-8" Location: http://example.org/blog/atom/a-day-at-the-beach.atom <?xml version="1.0"?> <entry xmlns="http://www.w3.org/2005/Atom"> <title>A fun day at the beach</title> <id>http://example.org/blog/a-day-at-the-beach.xhtml</id> <updated>2005-10-07T17:43:07Z</updated> <author><name>Daffy</name></author> <content type="xhtml"> <xhtml:div xmlns:xhtml="http://www.w3.org/1999/xhtml"> <xhtml:p>We had a good day at the beach. <xhtml:img alt="the beach" src="http://media.example.org/the_beach.png"/> </xhtml:p> <xhtml:p>Later we walked down to the pier. <xhtml:img alt="the pier" src="http://media.example.org/the_pier.png"/> </xhtml:p> </xhtml:div> </content> <link rel="edit" href="http://example.org/blog/edit/a-day-at-the-beach.atom"/> <link rel="alternate" type="text/html" href="http://example.org/blog/a-day-at-the-beach.html"/> </entry> Note that the returned Entry contains a link with a relation of "alternate" that points to the associated HTML page that was created. This is not required by this specification, but is included to show the kinds of changes a server can make to an Entry.

Slug is an HTTP entity-header whose presence in a POST to a Collection constitutes a request by the client to use the header's value as part of any URIs that would normally used to retrieve the to-be-created Entry or Media resources. Servers MAY use the value of the Slug header when creating the Member URI of the newly-created Resource, for instance, by using some or all of the words in the value for the last URI segment. Servers MAY also use the value when creating the atom:id, or as the title of a Media Link Entry (see .). Servers MAY choose to ignore the Slug entity-header. Servers MAY alter the header value before using it. For instance, a server might filter out some characters or replace accented letters with non-accented ones, replace spaces with underscores, change case, and so on.

The syntax of this header MUST conform to the augmented BNF grammar in section 2.1 of the HTTP/1.1 specification . The TEXT rule is described in section 2.2 of the same document.

Slug = "Slug" ":" *TEXT The field-value of the Slug header is a percent-encoded utf-8 Unicode string that does not contain CR or LF, where CR and LF are defined in . All non-ASCII characters in the utf-8 representation MUST be percent-encoded according to the rules in Section 2.1 of .

Here is an example of the Slug: header that uses percent-encoding to represent the Unicode character U+00E8 (LATIN SMALL LETTER E WITH GRAVE):

POST /myblog/entries HTTP/1.1 Host: example.org Content-Type: image/png Slug: The Beach at S%C3%A8te Authorization: Basic ZGFmZnk6c2VjZXJldA== Content-Length: nnn ...binary data... See for an example of the Slug: header applied to the creation of an Entry Resource.

Collection Resources MUST provide representations in the form of Atom Feed documents whose Entries contain the IRIs of the Members in the Collection. No distinction is made between Collection Feeds and other kinds of Feeds - a Feed might act both as a 'public' feed for subscription purposes and as a Collection Feed. Each Entry in the Feed Document SHOULD have an atom:link element with a relation of "edit" (See ). The Entries in the returned Atom Feed SHOULD be ordered by their "app:edited" property, with the most recently edited Entries coming first in the document order. The app:edited value is not equivalent to the HTTP Last-Modified: header and cannot be used to determine the freshness of cached responses. Clients MUST NOT assume that an Atom Entry returned in the Feed is a full representation of an Entry Resource and SHOULD perform a GET on the URI of the Member Entry before editing it. See for a discussion on the implications of cache control directives when obtaining entries.

Collections can contain large numbers of Resources. A client such as a web spider or web browser might be overwhelmed if the response to a GET contained every Entry in a Collection - in turn the server might also waste bandwidth and processing time on generating a response that cannot be handled. For this reason, servers MAY respond to Collection GET requests with a Feed Document containing a partial list of the Collection's members, and a link to the next partial list feed, if it exists. The first such partial list returned MUST contain the most recently edited member Resources and MUST have an atom:link with a "next" relation whose "href" value is the URI of the next partial list of the Collection. This next partial list will contain the next most recently edited set of Member Resources (and an atom:link to the following partial list if it exists). In addition to the "next" relation, partial list feeds MAY contain link elements with "rel" attribute values of "previous", "first", and "last", that can be used to navigate through the complete set of entries in the Collection. For instance, suppose a client is supplied the URI "http://example.org/entries/go" of a Collection of Member entries, where the server as a matter of policy avoids generating feed documents containing more than 10 Entries. The Atom Feed Document for the Collection will then represent the first partial list of a set of 10 linked feed documents. The "first" relation will reference the initial Feed Document in the set and the "last" relation references the final Feed Document in the set. Within each document, the "next" and "previous" link relations reference the preceding and subsequent documents.

<feed xmlns="http://www.w3.org/2005/Atom"> <link rel="first" href="http://example.org/entries/go" /> <link rel="next" href="http://example.org/entries/2" /> <link rel="last" href="http://example.org/entries/10" /> ... </feed> The "next" and "previous" link elements for the partial list feed located at "http://example.org/entries/2" would look like this:

The "app:edited" element is a Date construct (as defined by ), whose content indicates the last time an Entry was edited. If the entry has not been edited yet, the content indicates the time it was created. Atom Entry elements in Collection documents SHOULD contain one "app:edited" element, and MUST NOT contain more than one.

appEdited = element app:edited ( atomDateConstruct ) The server SHOULD change the value of this element every time an Entry Resource or an associated Media Resource has been edited.

This specification adds the value "edit" to the Atom Registry of Link Relations (see section 7.1 of ). The value of "edit" specifies that the value of the href attribute is the IRI of an editable Member Entry. When appearing within an atom:entry, the href IRI can be used to retrieve, update and delete the Resource represented by that Entry. An atom:entry MUST NOT contain more than one "edit" link relation.

This specification adds the value "edit-media" to the Atom Registry of Link Relations (see section 7.1 of ). When appearing within an atom:entry, the value of the href attribute is an IRI that can be used to modify a Media Resource associated with that Entry. An atom:entry element MAY contain zero or more "edit-media" link relations. An atom:entry MUST NOT contain more than one atom:link element with a rel attribute value of "edit-media" that has the same "type" and "hreflang" attribute values. All "edit-media" link relations in the same Entry reference the same Resource. If a client encounters multiple "edit-media" link relations in an Entry then it SHOULD choose a link based on the client preferences for "type" and "hreflang". If a client encounters multiple "edit-media" link relations in an Entry and has no preference based on the "type" and "hreflang" attributes then the client SHOULD pick the first "edit-media" link relation in document order.

The Atom Syndication Format defines the "application/atom+xml" media type to identify both Atom Feed and Atom Entry Documents. Implementation experience has demonstrated that Atom Feed and Entry Documents can have different processing models and that there are situations where they need to be differentiated. This document defines an optional "type" parameter used to differentiate the two types of Atom documents.

This document defines a new "type" parameter for use with the "application/atom+xml" media type. The "type" parameter has a value of "entry" or "feed". Neither the parameter name nor its value are case sensitive. The value "entry" indicates that the media type identifies an Atom Entry Document. The root element of the document MUST be atom:entry. The value "feed" indicates that the media type identifies an Atom Feed Document. The root element of the document MUST be atom:feed. If not specified, the type is assumed to be unspecified, requiring Atom processors to examine the root element to determine the type of Atom document.

New specifications MAY require that the "type" parameter be used to identify the Atom Document type. Producers of Atom Entry Documents SHOULD use the "type" parameter regardless of whether or not it is required. Producers of Atom Feed Documents MAY use the parameter. Atom processors that do not recognize the "type" parameter MUST ignore its value and examine the root element to determine the document type. Atom processors that do recognize the "type" parameter SHOULD detect and report inconsistencies between the parameter's value and the actual type of the document's root element.

This specification defines an Atom Format Structured Extension, as defined in Section 6 of , for publishing control within the "http://purl.org/atom/app#" namespace.

namespace app = "http://purl.org/atom/app#" pubControl = element app:control { atomCommonAttributes, pubDraft? & extensionElement } pubDraft = element app:draft { "yes" | "no" } The "app:control" element MAY appear as a child of an atom:entry that is being created or updated via the Atom Publishing Protocol. The app:control element MUST appear only once in an Entry. The app:control element is considered foreign markup as defined in Section 6 of . The app:control element and its child elements MAY be included in Atom Feed or Entry Documents. The app:control element can contain an optional "app:draft" element as defined below, and can contain extension elements as defined in Section 6 of .

The inclusion of the "app:draft" element represents a request by the client to control the visibility of a Member Resource. Server support is optional and thus the app:draft element MAY be ignored by the server. The number of app:draft elements in app:control MUST be zero or one. The content of an app:draft element MUST be one of "yes" or "no". If the element contains "no" this indicates a client request that the Member Resource be made publicly visible. If the app:draft element is not present then servers that support the extension MUST behave as though an app:draft element containing "no" was sent.

The Atom Publishing Protocol is based on HTTP. Authentication requirements for HTTP are covered in Section 11 of . The use of authentication mechanisms to prevent POSTing or editing by unknown or unauthorized clients is RECOMMENDED but not required. When authentication is not used, clients and servers are vulnerable to trivial spoofing, denial of service, and defacement attacks. However, in some contexts, this is an acceptable risk. The type of authentication deployed is a local decision made by the server operator. Clients are likely to face authentication schemes that vary across server deployments. At a minimum, client and server implementations MUST be capable of being configured to use HTTP Basic Authentication in conjunction with a TLS connection as defined in (but note that has been superseded by ). See for more information on TLS. The choice of authentication mechanism will impact interoperability. The minimum level of security referenced above (Basic Authentication with TLS) is considered good practice for Internet applications at the time of publication of this specification and sufficient for establishing a baseline for interoperability. Implementers are encouraged to investigate and use alternative mechanisms regarded as equivalently good or better at the time of deployment. It is RECOMMENDED that clients be implemented in such a way that new authentication schemes can be deployed. Because this protocol uses HTTP response status codes as the primary means of reporting the result of a request, servers are advised to respond to unauthorized or unauthenticated requests using an appropriate 4xx HTTP response code (e.g. 401 "Unauthorized" or 403 "Forbidden") in accordance with .

The Atom Publishing Protocol is based on HTTP and thus subject to the security considerations found in Section 15 of .

Atom Publishing Protocol server implementations need to take adequate precautions to ensure malicious clients cannot consume excessive server resources (CPU, memory, disk, etc).

Atom Publishing Protocol server implementations are susceptible to replay attacks. Specifically, this specification does not define a means of detecting duplicate requests. Accidentally sent duplicate requests are indistinguishable from intentional and malicious replay attacks.

Atom Publishing Protocol implementations are susceptible to a variety of spoofing attacks. Malicious clients may send Atom Entries containing inaccurate information anywhere in the document.

Atom Feed and Entry documents can contain XML External Entities as defined in Section 4.2.2 of . Atom implementations are not required to load external entities. External entities are subject to the same security concerns as any network operation and can alter the semantics of an Atom document. The same issues exist for Resources linked to by Atom elements such as atom:link and atom:content.

Atom Entry Documents sent to a server might contain XML Digital Signatures and might be encrypted using XML Encryption as specified in Section 5 of . Servers are allowed to modify received Resource representations in ways that can invalidate signatures covering those representations.

Atom Publishing Protocol implementations handle URIs and IRIs. See Section 7 of and Section 8 of for security considerations related to their handling and use.

Atom Feed and Entry documents can contain a broad range of content types including code that might be executable in some contexts. Malicious clients could attempt to attack servers or other clients by injecting code into a Collection Document's Entry or Media Resources. Server implementations are strongly encouraged to verify that client supplied content is safe prior to accepting, processing or publishing it. In the case of HTML, experience indicates that verification based on a white list of acceptable content is more effective than a black list of forbidden content. Additional information about XHTML and HTML content safety can be found in Section 8.1 of

This document uses two new media types that conform to the registry mechanism described in , a new message header that conforms to the registry mechanism described in , and two new link relations that conform to the registry mechanism described in .

An Atom Publishing Protocol Category Document, when serialized as XML 1.0, can be identified with the following media type: application atomcat+xml None. This parameter has identical semantics to the charset parameter of the "application/xml" media type as specified in . Identical to those of "application/xml" as described in , section 3.2. As defined in this specification. update upon publication In addition, as this media type uses the "+xml" convention, it shares the same security considerations as described in , section 10. There are no known interoperability issues. This specification. update upon publication No known applications currently use this media type. Additional information: As specified for "application/xml" in , section 3.2. .atomcat As specified for "application/xml" in , section 5. As specified in , section 6. TEXT Joe Gregorio <joe@bitworking.org> COMMON This specification's author(s). update upon publication

An Atom Publishing Protocol Service Document, when serialized as XML 1.0, can be identified with the following media type: application atomsvc+xml None. This parameter has identical semantics to the charset parameter of the "application/xml" media type as specified in . Identical to those of "application/xml" as described in , section 3.2. As defined in this specification. update upon publication In addition, as this media type uses the "+xml" convention, it shares the same security considerations as described in , section 10. There are no known interoperability issues. This specification. update upon publication No known applications currently use this media type. Additional information: As specified for "application/xml" in , section 3.2. .atomsvc As specified for "application/xml" in , section 5. As specified in , section 6. TEXT Joe Gregorio <joe@bitworking.org> COMMON This specification's author(s). update upon publication

SLUG http standard. IETF (iesg@ietf.org) Internet Engineering Task Force This specification. update on rfc number assignment)

edit An IRI of an editable Member Entry. When appearing within an atom:entry, the href IRI can be used to retrieve, update and delete the Resource represented by that Entry. Undefined; this relation can be used for background processing or to provide extended functionality without displaying its value. Automated agents should take care when this relation crosses administrative domains (e.g., the URI has a different authority than the current document).

edit-media An IRI of an editable Media Resource. When appearing within an atom:entry, the href IRI can be used to retrieve, update and delete the Media Resource associated with that Entry. Undefined; this relation can be used for background processing or to provide extended functionality without displaying its value. Automated agents should take care when this relation crosses administrative domains (e.g., the URI has a different authority than the current document).

IANA is requested to add a reference to this specification in the 'application/atom+xml' media type registration.