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INT-3148: Add MetadataStore Chapter

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JIRA: https://jira.springsource.org/browse/INT-3148

* Add a general chapter about `MetadataStore`
* Add cross-links between chapters on the matter of `MetadataStore`

Doc Polishing
This commit is contained in:
Artem Bilan
2013-12-12 13:56:09 +02:00
committed by Gary Russell
parent ef756bfd6d
commit 66f714e33a
8 changed files with 109 additions and 102 deletions
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src/reference/docbook/meta-data-store.xml Normal file
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<?xml version="1.0" encoding="UTF-8"?>
<section xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="metadata-store"
xmlns:xlink="http://www.w3.org/1999/xlink">
<title>Metadata Store</title>
<para>
Many external systems, services or resources aren't transactional (Twitter, RSS, file system etc.)
and there is no any ability to mark the data as read. Or there is just need to implement the
Enterprise Integration Pattern <ulink url="http://eaipatterns.com/IdempotentReceiver.html">Idempotent Receiver</ulink>
in some integration solutions. To achieve this goal and store some previous state of the Endpoint before the next
interaction with external system, or deal with the next Message, Spring Integration provides the <emphasis>Metadata Store</emphasis>
component being an implementation of the <interfacename>org.springframework.integration.metadata.MetadataStore</interfacename>
interface with a general <emphasis>key-value</emphasis> contract.
</para>
<para>
The <emphasis>Metadata Store</emphasis> is designed to store various types of generic meta-data
(e.g., published date of the last feed entry that has been processed) to help components such as the Feed adapter deal with duplicates.
If a component is not directly provided with a reference to a <interfacename>MetadataStore</interfacename>,
the algorithm for locating a metadata store is as follows: First, look for a bean with id
<code>metadataStore</code> in the ApplicationContext. If one is found then it will be used, otherwise
it will create a new instance of <classname>SimpleMetadataStore</classname> which is an in-memory implementation
that will only persist metadata within the lifecycle of the currently running Application Context. This means
that upon restart you may end up with duplicate entries.
</para>
<para>
If you need to persist metadata between Application Context restarts, two
persistent <interfacename>MetadataStores</interfacename> are provided by the framework:
</para>
<itemizedlist>
<listitem>PropertiesPersistingMetadataStore</listitem>
<listitem><xref linkend="redis-metadata-store"/></listitem>
</itemizedlist>
<para>
The <classname>PropertiesPersistingMetadataStore</classname> is backed by a properties file and a
<interfacename><ulink url="http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/util/PropertiesPersister.html">PropertiesPersister</ulink></interfacename>.
</para>
<programlisting language="xml"><![CDATA[<bean id="metadataStore"
class="org.springframework.integration.store.PropertiesPersistingMetadataStore"/>]]></programlisting>
<para>
Alternatively, you can provide your own implementation of the
<interfacename>MetadataStore</interfacename> interface (e.g. JdbcMetadataStore)
and configure it as a bean in the Application Context.
</para>
<section id="idempotent-receiver">
<title>Idempotent Receiver</title>
<para>
The <emphasis>Metadata Store</emphasis> is useful for implementating the
EIP <ulink url="http://eaipatterns.com/IdempotentReceiver.html">Idempotent Receiver</ulink> pattern, when
there is need to <emphasis>filter</emphasis> an incoming Message if it has already been processed, and just discard
it or perform some other logic on discarding. The following configuration is an example of how to do this:
</para>
<programlisting language="xml"><![CDATA[<int:filter input-channel="serviceChannel"
output-channel="idempotentServiceChannel"
discard-channel="discardChannel"
expression="@metadataStore.get(headers.businessKey) == null"/>
<int:publish-subscribe-channel id="idempotentServiceChannel"/>
<int:outbound-channel-adapter channel="idempotentServiceChannel"
expression="@metadataStore.put(headers.businessKey, '')"/>
<int:service-activator input-channel="idempotentServiceChannel" ref="service"/>]]></programlisting>
<para>
The <code>value</code> of the idempotent entry may be some expiration date, after which that entry should
be removed from <emphasis>Metadata Store</emphasis> by some scheduled reaper.
</para>
</section>
</section>