7. Scaling and Parallel Processing
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## 7. Scaling and Parallel Processing

Many batch processing problems can be solved with single threaded, single process jobs, so it is always a good idea to properly check if that meets your needs before thinking about more complex implementations. Measure the performance of a realistic job and see if the simplest implementation meets your needs first: you can read and write a file of several hundred megabytes in well under a minute, even with standard hardware.

When you are ready to start implementing a job with some parallel processing, Spring Batch offers a range of options, which are described in this chapter, although some features are covered elsewhere. At a high level there are two modes of parallel processing: single process, multi-threaded; and multi-process. These break down into categories as well, as follows:

- Multi-threaded Step (single process)

- Parallel Steps (single process)

- Remote Chunking of Step (multi process)

- Partitioning a Step (single or multi process)

Next we review the single-process options first, and then the multi-process options.

## 7.1 Multi-threaded Step

The simplest way to start parallel processing is to add a `TaskExecutor` to your Step configuration, e.g. as an attribute of the `tasklet`:

    <step id="loading">
        <tasklet task-executor="taskExecutor">...</tasklet>
    </step>

In this example the taskExecutor is a reference to another bean definition, implementing the `TaskExecutor` interface. `TaskExecutor` is a standard Spring interface, so consult the Spring User Guide for details of available implementations. The simplest multi-threaded `TaskExecutor` is a `SimpleAsyncTaskExecutor`.

The result of the above configuration will be that the Step executes by reading, processing and writing each chunk of items (each commit interval) in a separate thread of execution. Note that this means there is no fixed order for the items to be processed, and a chunk might contain items that are non-consecutive compared to the single-threaded case. In addition to any limits placed by the task executor (e.g. if it is backed by a thread pool), there is a throttle limit in the tasklet configuration which defaults to 4. You may need to increase this to ensure that a thread pool is fully utilised, e.g.

    <step id="loading"> <tasklet
        task-executor="taskExecutor"
        throttle-limit="20">...</tasklet>
    </step>

Note also that there may be limits placed on concurrency by any pooled resources used in your step, such as a `DataSource`. Be sure to make the pool in those resources at least as large as the desired number of concurrent threads in the step.

There are some practical limitations of using multi-threaded Steps for some common Batch use cases. Many participants in a Step (e.g. readers and writers) are stateful, and if the state is not segregated by thread, then those components are not usable in a multi-threaded Step. In particular most of the off-the-shelf readers and writers from Spring Batch are not designed for multi-threaded use. It is, however, possible to work with stateless or thread safe readers and writers, and there is a sample (parallelJob) in the Spring Batch Samples that show the use of a process indicator (see Section 6.12, “Preventing State Persistence”) to keep track of items that have been processed in a database input table.

Spring Batch provides some implementations of `ItemWriter` and `ItemReader`. Usually they say in the Javadocs if they are thread safe or not, or what you have to do to avoid problems in a concurrent environment. If there is no information in Javadocs, you can check the implementation to see if there is any state. If a reader is not thread safe, it may still be efficient to use it in your own synchronizing delegator. You can synchronize the call to `read()` and as long as the processing and writing is the most expensive part of the chunk your step may still complete much faster than in a single threaded configuration.

## 7.2 Parallel Steps

As long as the application logic that needs to be parallelized can be split into distinct responsibilities, and assigned to individual steps then it can be parallelized in a single process. Parallel Step execution is easy to configure and use, for example, to execute steps `(step1,step2)` in parallel with `step3`, you could configure a flow like this:

    <job id="job1">
        <split id="split1" task-executor="taskExecutor" next="step4">
            <flow>
                <step id="step1" parent="s1" next="step2"/>
                <step id="step2" parent="s2"/>
            </flow>
            <flow>
                <step id="step3" parent="s3"/>
            </flow>
        </split>
        <step id="step4" parent="s4"/>
    </job>

    <beans:bean id="taskExecutor" class="org.spr...SimpleAsyncTaskExecutor"/>

The configurable "task-executor" attribute is used to specify which TaskExecutor implementation should be used to execute the individual flows. The default is `SyncTaskExecutor`, but an asynchronous TaskExecutor is required to run the steps in parallel. Note that the job will ensure that every flow in the split completes before aggregating the exit statuses and transitioning.

See the section on Section 5.3.5, “Split Flows” for more detail.

## 7.3 Remote Chunking

In Remote Chunking the Step processing is split across multiple processes, communicating with each other through some middleware. Here is a picture of the pattern in action:

The Master component is a single process, and the Slaves are multiple remote processes. Clearly this pattern works best if the Master is not a bottleneck, so the processing must be more expensive than the reading of items (this is often the case in practice).

The Master is just an implementation of a Spring Batch `Step`, with the ItemWriter replaced with a generic version that knows how to send chunks of items to the middleware as messages. The Slaves are standard listeners for whatever middleware is being used (e.g. with JMS they would be `MesssageListeners`), and their role is to process the chunks of items using a standard `ItemWriter` or `ItemProcessor` plus `ItemWriter`, through the `ChunkProcessor` interface. One of the advantages of using this pattern is that the reader, processor and writer components are off-the-shelf (the same as would be used for a local execution of the step). The items are divided up dynamically and work is shared through the middleware, so if the listeners are all eager consumers, then load balancing is automatic.

The middleware has to be durable, with guaranteed delivery and single consumer for each message. JMS is the obvious candidate, but other options exist in the grid computing and shared memory product space (e.g. Java Spaces).

## 7.4 Partitioning

Spring Batch also provides an SPI for partitioning a Step execution and executing it remotely. In this case the remote participants are simply Step instances that could just as easily have been configured and used for local processing. Here is a picture of the pattern in action:

The Job is executing on the left hand side as a sequence of Steps, and one of the Steps is labelled as a Master. The Slaves in this picture are all identical instances of a Step, which could in fact take the place of the Master resulting in the same outcome for the Job. The Slaves are typically going to be remote services, but could also be local threads of execution. The messages sent by the Master to the Slaves in this pattern do not need to be durable, or have guaranteed delivery: Spring Batch meta-data in the `JobRepository` will ensure that each Slave is executed once and only once for each Job execution.

The SPI in Spring Batch consists of a special implementation of Step (the `PartitionStep`), and two strategy interfaces that need to be implemented for the specific environment. The strategy interfaces are `PartitionHandler` and `StepExecutionSplitter`, and their role is show in the sequence diagram below:

The Step on the right in this case is the "remote" Slave, so potentially there are many objects and or processes playing this role, and the PartitionStep is shown driving the execution. The PartitionStep configuration looks like this:

    <step id="step1.master">
        <partition step="step1" partitioner="partitioner">
            <handler grid-size="10" task-executor="taskExecutor"/>
        </partition>
    </step>

Similar to the multi-threaded step's throttle-limit attribute, the grid-size attribute prevents the task executor from being saturated with requests from a single step.

There is a simple example which can be copied and extended in the unit test suite for Spring Batch Samples (see `*PartitionJob.xml` configuration).

Spring Batch creates step executions for the partitions called "step1:partition0", etc., so many people prefer to call the master step "step1:master" for consistency. With Spring 3.0 you can do this using an alias for the step (specifying the `name` attribute instead of the `id`).

### 7.4.1 PartitionHandler

The `PartitionHandler` is the component that knows about the fabric of the remoting or grid environment. It is able to send `StepExecution` requests to the remote Steps, wrapped in some fabric-specific format, like a DTO. It does not have to know how to split up the input data, or how to aggregate the result of multiple Step executions. Generally speaking it probably also doesn't need to know about resilience or failover, since those are features of the fabric in many cases, and anyway Spring Batch always provides restartability independent of the fabric: a failed Job can always be restarted and only the failed Steps will be re-executed.

`The PartitionHandler` interface can have specialized implementations for a variety of fabric types: e.g. simple RMI remoting, EJB remoting, custom web service, JMS, Java Spaces, shared memory grids (like Terracotta or Coherence), grid execution fabrics (like GridGain). Spring Batch does not contain implementations for any proprietary grid or remoting fabrics.

Spring Batch does however provide a useful implementation of `PartitionHandler` that executes Steps locally in separate threads of execution, using the `TaskExecutor` strategy from Spring. The implementation is called `TaskExecutorPartitionHandler`, and it is the default for a step configured with the XML namespace as above. It can also be configured explicitly like this:

    <step id="step1.master">
        <partition step="step1" handler="handler"/>
    </step>

    <bean class="org.spr...TaskExecutorPartitionHandler">
        <property name="taskExecutor" ref="taskExecutor"/>
        <property name="step" ref="step1" />
        <property name="gridSize" value="10" />
    </bean>

The `gridSize` determines the number of separate step executions to create, so it can be matched to the size of the thread pool in the `TaskExecutor`, or else it can be set to be larger than the number of threads available, in which case the blocks of work are smaller.

The `TaskExecutorPartitionHandler` is quite useful for IO intensive Steps, like copying large numbers of files or replicating filesystems into content management systems. It can also be used for remote execution by providing a Step implementation that is a proxy for a remote invocation (e.g. using Spring Remoting).

### 7.4.2 Partitioner

The Partitioner has a simpler responsibility: to generate execution contexts as input parameters for new step executions only (no need to worry about restarts). It has a single method:

    public interface Partitioner {
        Map<String, ExecutionContext> partition(int gridSize);
    }

The return value from this method associates a unique name for each step execution (the `String`), with input parameters in the form of an `ExecutionContext`. The names show up later in the Batch meta data as the step name in the partitioned `StepExecutions`. The `ExecutionContext` is just a bag of name-value pairs, so it might contain a range of primary keys, or line numbers, or the location of an input file. The remote `Step` then normally binds to the context input using `#{...}` placeholders (late binding in step scope), as illustrated in the next section.

The names of the step executions (the keys in the `Map` returned by `Partitioner`) need to be unique amongst the step executions of a Job, but do not have any other specific requirements. The easiest way to do this, and to make the names meaningful for users, is to use a prefix+suffix naming convention, where the prefix is the name of the step that is being executed (which itself is unique in the `Job`), and the suffix is just a counter. There is a `SimplePartitioner` in the framework that uses this convention.

An optional interface `PartitioneNameProvider` can be used to provide the partition names separately from the partitions themselves. If a `Partitioner` implements this interface then on a restart only the names will be queried. If partitioning is expensive this can be a useful optimisation. Obviously the names provided by the `PartitioneNameProvider` must match those provided by the `Partitioner`.

### 7.4.3 Binding Input Data to Steps

It is very efficient for the steps that are executed by the PartitionHandler to have identical configuration, and for their input parameters to be bound at runtime from the ExecutionContext. This is easy to do with the StepScope feature of Spring Batch (covered in more detail in the section on Late Binding). For example if the `Partitioner` creates `ExecutionContext` instances with an attribute key `fileName`, pointing to a different file (or directory) for each step invocation, the `Partitioner` output might look like this:

Table 7.1. Example step execution name to execution context provided by Partitioner targeting directory processing

Step Execution Name (key) | ExecutionContext (value)
filecopy:partition0 | fileName=/home/data/one
filecopy:partition1 | fileName=/home/data/two
filecopy:partition2 | fileName=/home/data/three

Then the file name can be bound to a step using late binding to the execution context:

    <bean id="itemReader" scope="step"
          class="org.spr...MultiResourceItemReader">
        <property name="resource" value="#{stepExecutionContext[fileName]}/*"/>
    </bean>

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