8.8.2.0 | Services | Atom Feed Consumption Service

The Atom Feed Consumption Service is a service in Repose which enables simple reading of Atom feeds. By centralizing this function in a service, Repose developers may share resources (and reduce overhead) when multiple Repose components need to read the same feed.

General Service information

Default Configuration: atom-feed-service.cfg.xml
Released: v7.3.2.0
Schema

The Atom Feed Consumption Service can be enabled in a Repose deployment by adding it to the services list of the System Model like this:

system-model.cfg.xml (partial)

<?xml version="1.0" encoding="UTF-8"?>
<system-model xmlns="http://docs.openrepose.org/repose/system-model/v2.0">
  <repose-cluster id="repose">
    ...
    <services>
      <service name="atom-feed-service"/>
    </services>
    ...
  </repose-cluster>
</system-model>

Examples

Basic Configuration

This is the most basic configuration there is to a non-authenticating feed:

atom-feed-service.cfg.xml

<?xml version="1.1" encoding="UTF-8"?>
<atom-feed-service xmlns="http://docs.openrepose.org/repose/atom-feed-service/v1.0"
                   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <feed id="non-authenticating-feed"      (1)
          uri="http://public.atom.com/feed" (2)
    />
</atom-feed-service>

1	This is a required attribute that defines a unique ID which may be used to identify this feed.
2	This is a required attribute that defines the URI at which this feed can be read.

Full Configuration

atom-feed-service.cfg.xml

<?xml version="1.1" encoding="UTF-8"?>
<atom-feed-service xmlns="http://docs.openrepose.org/repose/atom-feed-service/v1.0"
                   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <feed id="authenticating-feed"           (1)
          uri="http://private.atom.com/feed" (2)
          polling-frequency="5"              (3)
          entry-order="read"                 (4)
          connection-pool-id="default"       (5)
    >
        <authentication xsi:type="OpenStackIdentityV2AuthenticationType" (6)
                        uri="http://identity.jware.com"                  (7)
                        username="zombiekid"                             (8)
                        password="iliketurtles"                          (9)
                        timeout="5000"                                   (10)
        />
    </feed>
</atom-feed-service>

1	This is a required attribute that defines a unique ID which may be used to identify this feed.
2	This is a required attribute that defines the URI at which this feed can be read.
3	This is an optional attribute that defines how often, in seconds, the Atom Feed service checks this feed for updates. Default: `5`
4	This is an optional attribute that defines the order in which new Atom events will be delivered to any listeners registered on this feed. Default: `read` The legal values are: read - a feed listener will receive events in the order in which entries are read from the feed, from top to bottom. reverse-read - a feed listener will receive events in the order in which entries are put on the feed, from bottom to top.
5	This is an optional attribute that defines the HTTP Connection pool ID to use when communicating with this feed.
6	This is an optional element that must be present in order to perform authentication for a secure feed.
7	This is a required attribute that defines the URI at which the OpenStack Identity service can be reached.
8	This is a required attribute that defines the username of the user which will query the authenticated feed. Note that the user must have, at a minimum, read permissions on the associated feed.
9	This is a required attribute that defines the password of the user which will query the authenticated feed.
10	This is an optional attribute that defines the amount of time, in milliseconds, to wait before considering an authentication attempt to be a failure. A value of '0' indicates an infinite wait time. Default: `5000`

Usage in a Custom Filter

Configure an Atom Feed in the atom-feed-service.cfg.xml configuration file described above. Make sure to note the exact value of the "id" attribute for your feed.
Add a dependency to the component under development for the following Maven coordinate (of the form groupId:artifactId:version):
```
org.openrepose:atom-feed-service-api:{repose-version}
```
Wire in the Atom Feed Consumption service spring bean.
Write one or more Atom Feed listener(s) which conform(s) to the AtomFeedListener interface.
Create an instance of your listener, like so:
```
MyAtomFeedListener myListener = ...;
```
Register your listener(s) with the Atom Feed Consumption service using the configured Atom Feed "id" from above, like so:
```
String myListenerId = atomFeedService.registerListener(myFeedId, myListener);
```
You’re done! Your listener will get callbacks for all new entries in your configured Atom Feed.
To unregister your listener, use the listenerId returned from the .registerListener(…) method, like so:
```
atomFeedService.unregisterListener(myListenerId);
```

Architecture

The Atom Feed Consumption service is built on top of the Akka framework. It utilizes the Akka actor system to scale horizontally as the number of Atom Feeds being monitored grows. In addition, most processing done by the Atom Feed Consumption service is asynchronous and non-blocking.

Due to the nature of the Akka actor system, if the Atom Feed Consumption Service is configured to poll a feed more quickly than that feed can be read, the Akka actor queue for that Feed will grow perpetually. This will result in high memory usage, and eventually the JVM will run out of memory and crash.