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State Replication is the simpler of Talon's 2 two High Availability models. With State Replication, Talon replicates changes to state and the outbound messages emitted by your application's message handlers. In the event of a failover to a backup or a cold start recovery from a transaction log your application's state is available at the same point where processing left off, and the engine will retransmit any outbound messages that were left in doubt as a result of the failure. This sections discusses |
Coding For State Replication
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- Modeling Messages and State
- Declaring a main class annotated for with HAPolicy of StateReplication
- Providing Talon with a state factory for creating your application state
- Writing message handlers to perform business logic.
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@AppHAPolicy(value=AepEngine.HAPolicy.StateReplication)
public class Application {
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} |
Provide a StateFactory
An AEPEngine AepEngine is application state agnostic: it deals with your application an 's state as a collection of plain old java object graph objects organized into a state tree with a single object root. Given the root object for your application's state, the underlying store will track changes made to fields on the root (and its descendants) and replicate or persist those changes. As such, an AepEngine needs to be able to create a new application state during when your application is initialized. This is done by finding a method on your main application class anotated annotated with @AppStateFactoryAccessor. The state factory accessor returns a newly initialized set of state for the engine to manage. As messages are processed the engine will pass the relevant state root back into the application to be operated upon.
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@AppStateFactoryAccessor
final public IAepApplicationStateFactory getStateFactory() {
return new IAepApplicationStateFactory() {
@Override
final public Repository createState(MessageView view) {
return Repository.create();
}
};
} |
As your application makes changes to this root object (setting fields etc), the engine will monitor the root and replicate deltas to backup members or disk.
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The state factory should return an empty, uninitialized object. The platform will invoke the state factory during application initialization with a null argument to determine the type of the application's root state, and will subsequently invoke the state factory on receipt of a MessageView when state has not already been created for the application. The application should not store a reference to the application state that it returns ... it is the job of the platform to manage the state tree once it has been created by either passing the state root into an EventHandler or returning it via a call to |
Inject Message Sender
If your application will send messages, it can add an injection point for the underlying AEPEngine AepEngine to inject a message sender.
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To actually achieve high availability storage must be configured for the application. The primary means of storage is for Talon apps is through clustered replication to a backup instance. Talon also logs state changes to a disk-based transaction log as a fallback mechanism. Storage and persistence can be enabled in the application's configuration xmlXML.
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<app name="processor" mainClass="com.sample.Application">
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<storage enabled="true">
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<clustering enabled="true"/>
<persistence enabled="true">
<!--
When using Xbuf encoded entities,
detached persist is not supported.
-->
<detachedPersist enabled="false"/>
</persistence>
</storage>
</app> |
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Enabling persistence causes the replication stream that is sent to backup instances to also be logged locally on disk to a transaction log file. The transaction log file can be used to recover application state from a cold start , and is also used to initialize new cluster members that connect to it so when clustering is enabled, persistence must be enabled as well.
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Limitations and Upcoming Enhancements
See State Graph Tree Limitations for a discussion of current state graph tree limitations.