8. Repeat

Batch processing is about repetitive actions - either as a simple optimization, or as part of a job. To strategize and generalize the repetition as well as to provide what amounts to an iterator framework, Spring Batch has the RepeatOperations interface. The RepeatOperations interface looks like this:

public interface RepeatOperations {

    RepeatStatus iterate(RepeatCallback callback) throws RepeatException;

}

The callback is a simple interface that allows you to insert some business logic to be repeated:

public interface RepeatCallback {

    RepeatStatus doInIteration(RepeatContext context) throws Exception;

}

The callback is executed repeatedly until the implementation decides that the iteration should end. The return value in these interfaces is an enumeration that can either be RepeatStatus.CONTINUABLE or RepeatStatus.FINISHED. A RepeatStatus conveys information to the caller of the repeat operations about whether there is any more work to do. Generally speaking, implementations of RepeatOperations should inspect the RepeatStatus and use it as part of the decision to end the iteration. Any callback that wishes to signal to the caller that there is no more work to do can return RepeatStatus.FINISHED.

The simplest general purpose implementation of RepeatOperations is RepeatTemplate. It could be used like this:

RepeatTemplate template = new RepeatTemplate();

template.setCompletionPolicy(new FixedChunkSizeCompletionPolicy(2));

template.iterate(new RepeatCallback() {

    public ExitStatus doInIteration(RepeatContext context) {
        // Do stuff in batch...
        return ExitStatus.CONTINUABLE;
    }

});

In the example we return RepeatStatus.CONTINUABLE to show that there is more work to do. The callback can also return ExitStatus.FINISHED if it wants to signal to the caller that there is no more work to do. Some iterations can be terminated by considerations intrinsic to the work being done in the callback, others are effectively infinite loops as far as the callback is concerned and the completion decision is delegated to an external policy as in the case above.

Inside a RepeatTemplate the termination of the loop in the iterate method is determined by a CompletionPolicy which is also a factory for the RepeatContext. The RepeatTemplate has the responsibility to use the current policy to create a RepeatContext and pass that in to the RepeatCallback at every stage in the iteration. After a callback completes its doInIteration, the RepeatTemplate has to make a call to the CompletionPolicy to ask it to update its state (which will be stored in the RepeatContext). Then it asks the policy if the iteration is complete.

Spring Batch provides some simple general purpose implementations of CompletionPolicy. The SimpleCompletionPolicy just allows an execution up to a fixed number of times (with RepeatStatus.FINISHED forcing early completion at any time).

Users might need to implement their own completion policies for more complicated decisions. For example, a batch processing window that prevents batch jobs from executing once the online systems are in use would require a custom policy.

If there is an exception thrown inside a RepeatCallback, the RepeatTemplate consults an ExceptionHandler which can decide whether or not to re-throw the exception.

public interface ExceptionHandler {

    void handleException(RepeatContext context, Throwable throwable)
        throws RuntimeException;

}

A common use case is to count the number of exceptions of a given type, and fail when a limit is reached. For this purpose Spring Batch provides the SimpleLimitExceptionHandler and slightly more flexible RethrowOnThresholdExceptionHandler. The SimpleLimitExceptionHandler has a limit property and an exception type that should be compared with the current exception - all subclasses of the provided type are also counted. Exceptions of the given type are ignored until the limit is reached, and then rethrown. Those of other types are always rethrown.

An important optional property of the SimpleLimitExceptionHandler is the boolean flag useParent. It is false by default, so the limit is only accounted for in the current RepeatContext. When set to true, the limit is kept across sibling contexts in a nested iteration (e.g. a set of chunks inside a step).

Often it is useful to be able to receive additional callbacks for cross cutting concerns across a number of different iterations. For this purpose Spring Batch provides the RepeatListener interface. The RepeatTemplate allows users to register RepeatListeners, and they will be given callbacks with the RepeatContext and RepeatStatus where available during the iteration.

The interface looks like this:

public interface RepeatListener {
    void before(RepeatContext context);
    void after(RepeatContext context, RepeatStatus result);
    void open(RepeatContext context);
    void onError(RepeatContext context, Throwable e);
    void close(RepeatContext context);
}

The open and close callbacks come before and after the entire iteration. before, after and onError apply to the individual RepeatCallback calls.

Note that when there is more than one listener, they are in a list, so there is an order. In this case open and before are called in the same order while after, onError and close are called in reverse order.

Implementations of RepeatOperations are not restricted to executing the callback sequentially. It is quite important that some implementations are able to execute their callbacks in parallel. To this end, Spring Batch provides the TaskExecutorRepeatTemplate, which uses the Spring TaskExecutor strategy to run the RepeatCallback. The default is to use a SynchronousTaskExecutor, which has the effect of executing the whole iteration in the same thread (the same as a normal RepeatTemplate).

Sometimes there is some business processing that you know you want to repeat every time it happens. The classic example of this is the optimization of a message pipeline - it is more efficient to process a batch of messages, if they are arriving frequently, than to bear the cost of a separate transaction for every message. Spring Batch provides an AOP interceptor that wraps a method call in a RepeatOperations for just this purpose. The RepeatOperationsInterceptor executes the intercepted method and repeats according to the CompletionPolicy in the provided RepeatTemplate.

Here is an example of declarative iteration using the Spring AOP namespace to repeat a service call to a method called processMessage (for more detail on how to configure AOP interceptors see the Spring User Guide):

<aop:config>
    <aop:pointcut id="transactional"
        expression="execution(* com..*Service.processMessage(..))" />
    <aop:advisor pointcut-ref="transactional"
        advice-ref="retryAdvice" order="-1"/>
</aop:config>

<bean id="retryAdvice" class="org.spr...RepeatOperationsInterceptor"/>

The example above uses a default RepeatTemplate inside the interceptor. To change the policies, listeners etc. you only need to inject an instance of RepeatTemplate into the interceptor.

If the intercepted method returns void then the interceptor always returns ExitStatus.CONTINUABLE (so there is a danger of an infinite loop if the CompletionPolicy does not have a finite end point). Otherwise it returns ExitStatus.CONTINUABLE until the return value from the intercepted method is null, at which point it returns ExitStatus.FINISHED. So the business logic inside the target method can signal that there is no more work to do by returning null, or by throwing an exception that is re-thrown by the ExceptionHandler in the provided RepeatTemplate.