9. Feed Adapter

Spring Integration provides support for Syndication via Feed Adapters

9.1 Introduction

As we know Web syndication is a form of syndication where material such as news items, press releases that is available to any website is also made available via we feeds such as RSS, ATOM etc.

Spring integration provides support for Web Syndication via FEED adapter which comes with a convenient namespace-based configuration. To configure FEED namespace include the following elements into the headers of your XML configuration file:


9.2 Feed Inbound Channel Adapter

The only adapter that is really needed to provide support for retrieving feeds is an inbound channel adapter which allows you to subscribe to a particular URL. Below is the configuration for such adapter:

<int-feed:inbound-channel-adapter id="feedAdapter" 
	<int:poller fixed-rate="10000" max-messages-per-poll="100" />

In the above configuration we are subscribing to a URL identified by url attribute.

As news items are retrieved they will be converted to a Message and sent to a channel identified by channel attribute. The payload of such message will be com.sun.syndication.feed.synd.SyndEntry which encapsulates various data (i.e., content, dates, authors etc.) about a news item.

You can also see that Inbound Feed Channel Adapter is a Polling consumer which means you have to provide a poller configuration. However, one important thing you must understand with regard to Feed sinc its inner-workings are slightly different then any other poling consumer. When Inbound Feed adapter is started it does the first poll and receives com.sun.syndication.feed.synd.SyndEntryyFeed which is an object that contains multiple SyndEntry objects. Each entry is stored in the local entry queue and is released based on the value in the max-messages-per-poll attribute where each Message will contain a single entry. If during retrieval of the entries from the entry queue the queue had become empty the adapter will attempt to update the Feed populating the queue with more entries (SyndEntry) if available, otherwise the next attempt to poll for a feed will be determined by the trigger of the poller (e.g., every 10 seconds in the above configuration).

Duplicate Entries

Polling for a Feed might result in the entries that have already been processed ("I already read that news item, why are you showing it to me again?"). Spring Integration provides a convenient mechanism to eliminate the need to worry about duplicate entries. Each feed entry will have publish date field. Every time the new Message is generated and sent, Spring Integration will store the value of the publish date in the instance of the org.springframework.integration.store.MetadataStore which is a strategy interface designed to store various types of meta-data (e.g., publish date of the last feed entry that has been processed) to help components such as Feed to deal with duplicates.

The default rule for locating this meta-data store is as follows; Spring Integration will look for a bean of type org.springframework.integration.store.MetadataStore in the ApplicationContext. If one found then it will be used, otherwise it will create a new instance of SimpleMetadataStore which is a simple in-memory implementation that will only persist meta-data within the life-cycle of the application context. This means that upon restart you may end up with duplicate entries. If you need to persist meta-data between Application Context restarts, you may use PropertiesPersistingMetadataStore which is a property file based persister or provide your own implementation of the MetedataStore interface (e.g.,JdbcMetadatStore) and configure it as bean in the Application Context.

<bean class="org.springframework.integration.store.PropertiesPersistingMetadataStore"/>