Atom Feed
Simplefeed
What is Atom?
[Atom](http://en.wikipedia.org/wiki/Atom_(standard\)) is an XML-based syndication format which represents time-ordered series of events. In Atom terminology, each event is an entry and the series of events is a feed.
Both feeds and entries have metadata associated with them, for example a title and a unique identifier. They can also have links with a rel attribute describing the relationship the link represents, for example a link with rel "self" that points back to the selfsame entry or feed.
<?xml version="1.0">
<feed xmlns="http://www.w3.org/2005/Atom">
<id>urn:uuid:ff31a040-75bc-11e2-bcfd-0800200c9a66</id>
<title type="text">Notifications</title>
<updated>2013-01-02T10:03:00Z</updated>
<generator>Foo CMS</generator>
<link rel="self" href="http://example.com/documents/2" />
<entry>
<id>urn:uuid:eb3ee5a0-75be-11e2-bcfd-0800200c9a66</id>
<title type="text">Edit</title>
<updated>2013-01-02T10:01:00Z</updated>
<author>
<name>jsmith</name>
</author>
<link rel="self" href="http://example.com/entry/33" />
<content type="application/vnd.example.document+xml">
<title>HR policies</title>
<link href="http://example.com/cms/pdf/456" />
</content>
</entry>
<entry>
<id>urn:uuid:1d3334a0-75bd-11e2-bcfd-0800200c9a66</id>
<title type="text">User created</title>
<updated>2013-01-02T07:31:00Z</updated>
<author>
<name>dgonzales</name>
</author>
<link rel="self" href="http://example.com/entry/32" />
<content type="application/vnd.example.user+xml">
<username>jsmith</username>
<role>editor</role>
<email>jsmith@example.com</email>
</content>
</entry>
</feed>
Atom is optimised and designed for RESTful systems that communicate over HTTP.
Atom is a general-purpose format that can be extended to fit a particular domain. By using a well-understood general-purpose format as a basis, we can reuse tools, infrastructure and semantics. The Atom syndication format was formalised in RFC 4287.
Event-driven systems
Simplefeed
"Atom" can also sometimes refer to AtomPub, a protocol for editing and publishing web resources built on top of the Atom XML format. If you need your feed to be editable over HTTP, AtomPub is a good place to start.
As well as conforming to the Atom XML format, our Simplefeed protocol is based on the following rules:
- Entries are ordered by the time they were added to the feed, from newest at the top of the feed to oldest at the bottom.
- An entry, once published, never changes.
- The series of entries is paginated over many Atom documents.
- All documents except the newest will have a link with rel "next-archive" pointing to the next document in the feed.
- All documents except the oldest will have a link with rel "prev-archive" pointing to the previous document in the feed.
- Only the newest document, known as the recent document, may change, and that can only change by newer events being prepended to it.
- When the feed provider decides the recent document is finished, it archives that document and creates a new document.
- There is a published URL that always points to the recent document and serves as an entry point to the feed.
Though it is good practice to use intuitive URLs, the protocol in no way depends on the structure of the URLs. Consumers should follow links, and not attempt to construct URLs themselves.
Paginating feeds
The recent document is the published entry point to the feed. Consumers of the event feed will always be able to retrievehttp://example.com/recent to get the most recent entries.
Documents older than the recent one are known as archived documents. Archived documents do not change after they have been created, but the recent document will continue to have entries prepended to it until the feed provider archives it. This means that caching can be much more agressive for archived documents than the recent document.
<?xml version="1.0">
<feed xmlns="http://www.w3.org/2005/Atom">
<id>urn:uuid:ff31a040-75bc-11e2-bcfd-0800200c9a66</id>
<title type="text">Recent notifications</title>
<link rel="self" href="http://example.com/recent" />
<link rel="prev-archive" href="http://example.com/documents/3" />
<!-- There is no "next-archive" link as this is the most recent document. -->
<link rel="via" href="http://example.com/documents/4" />
<entry>..</entry>
<entry>..</entry>
<entry>..</entry>
</feed>
The recent document can also be accessed through its permanent URL, which is represented with a "via" link. Once the feed provider has archived the current recent document, it will create a new one and leave the via link intact.
<?xml version="1.0">
<feed xmlns="http://www.w3.org/2005/Atom">
<id>urn:uuid:ff31a040-75bc-11e2-bcfd-0800200c9a66</id>
<title type="text">Notifications</title>
<link rel="self" href="http://example.com/documents/4" />
<link rel="prev-archive" href="http://example.com/documents/3" />
<!-- There is no "next-archive" link as this is the most recent document. -->
<entry>..</entry>
<entry>..</entry>
<entry>..</entry>
</feed>
Consumers who are interested in older entries can follow the "prev-archive" link to the previous document.
<?xml version="1.0">
<feed xmlns="http://www.w3.org/2005/Atom">
<id>urn:uuid:ff31a040-75bc-11e2-bcfd-0800200c9a66</id>
<title type="text">Notifications</title>
<link rel="self" href="http://example.com/documents/3" />
<link rel="next-archive" href="http://example.com/documents/4" />
<link rel="prev-archive" href="http://example.com/documents/2" />
<entry>..</entry>
<entry>..</entry>
<entry>..</entry>
</feed>
This document has been archived, and should therefore never change. Consumers wishing to find older or newer entries than the ones in this document can find them by following the "prev-archive" and "next-archive" links respectively.
<?xml version="1.0">
<feed xmlns="http://www.w3.org/2005/Atom">
<id>urn:uuid:ff31a040-75bc-11e2-bcfd-0800200c9a66</id>
<title type="text">Notifications</title>
<link rel="self" href="http://example.com/documents/2" />
<link rel="next-archive" href="http://example.com/documents/3" />
<link rel="prev-archive" href="http://example.com/documents/1" />
<entry>..</entry>
<entry>..</entry>
<entry>..</entry>
</feed>
This is another archived document, and can also be heavily cached. Finding older or newer entries is again a matter of following the "prev-archive" or "next-archive" links. Consumers who wish to read the entire history can continue to follow "prev-archive" links until they come across a document with no such link, which will be the first.
<?xml version="1.0">
<feed xmlns="http://www.w3.org/2005/Atom">
<id>urn:uuid:ff31a040-75bc-11e2-bcfd-0800200c9a66</id>
<title type="text">Notifications</title>
<link rel="self" href="http://example.com/documents/1" />
<link rel="next-archive" href="http://example.com/documents/2" />
<!-- There is no "prev-archive" link as this is the first document. -->
<entry>..</entry>
<entry>..</entry>
<entry>..</entry>
</feed>
Consuming feeds
This consumer wants to check if there are any more recent entries, so it issues a GET request on http://example.com/recent, which is the published entry-point of the feed.
<?xml version="1.0">
<feed xmlns="http://www.w3.org/2005/Atom">
<link rel="self" href="http://example.com/recent" />
<link rel="prev-archive" href="http://example.com/documents/3" />
<link rel="via" href="http://example.com/documents/4" />
<entry>
<id>urn:uuid:e2089090-75c7-11e2-bcfd-0800200c9a66</id>
</entry>
<entry>
<id>urn:uuid:d765c950-75c7-11e2-bcfd-0800200c9a66</id>
</entry>
</feed>
The entry with id "urn:uuid:fc374b00-75c7-11e2-bcfd-0800200c9a66" is not present. This is because since the consumer last checked the feed, the provider has added new entries to the front of the feed, archiving a document and starting a new one in the process.
The consumer therefore issues a GET request for the previous document, which has the URL "http://example.com/documents/3".
<?xml version="1.0">
<feed xmlns="http://www.w3.org/2005/Atom">
<link rel="self" href="http://example.com/documents/3" />
<link rel="next-archive" href="http://example.com/documents/4" />
<link rel="prev-archive" href="http://example.com/documents/2" />
<entry>
<id>urn:uuid:f37a81d0-75c7-11e2-bcfd-0800200c9a66</id>
</entry>
<entry>
<id>urn:uuid:fc374b00-75c7-11e2-bcfd-0800200c9a66</id>
</entry>
</feed>
This time, the consumer does find the entry it last processed. The consumer can now start to work its way back to the front of the feed, by processing the one new entry in document 3, and then going through the entries in the recent document.
As the consumer goes through the entries, it keeps updating its record of the most recent entry processed. Note that because the series of entries is time-ordered, the client need only keep track of the identifier of the most recent entry it has processed.
Notice that the feed provider does not have to keep track of who the consumers are or which entry they are each up to. The guarantee that new events are always added to the front of the list allows consumers to do that for themselves.
Implementation considerations
References
A great reference for understanding Atom's use in RESTful event-driven systems is REST in Practice by Jim Webber, Savas Parastatidis and Ian Robinson. We based our description of Simplefeed on Chapter 7.
Atomfeed is a library for using Simplefeed from Java or Java Spring web applications. Currently a work-in-progress.