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Batch

Batch is a concept to offload workload from the current execution to be processed in the background. This allows to run a process engine command asynchronously on a large set of instances without blocking. It also decouples the separate command invocations from each other.

For example the process instance migration command can be executed using a batch. This allows to migrate process instances asynchronously. In a synchronous process instance migration, all migrations are executed in a single transaction. First of all, this requires all of them to succeed to commit the transaction. For a large set of process instances, the transaction can also become too large to even be committed to the database. With batch migration both of these traits change. A batch executes the migration in smaller chunks, each using a single transaction.

Benefits:

  • asynchronous (non-blocking) execution
  • execution can utilize multiple threads and job executors
  • decoupling of execution, i.e., every batch execution job uses its own transaction

Disadvantages:

  • manual polling for completion of the batch
  • contention with other jobs executed by the process engine
  • a batch can fail partially while a subset was already executed, e.g., some process instances were migrated where others failed

Technically, a batch represents a set of jobs which execute a command in the context of the process engine.

The batch utilizes the job executor of the process engine to execute the batch jobs. A single batch consists of three job types:

  • Seed job: creates all batch execution jobs required to complete the batch
  • Execution jobs: the actual execution of the batch command, e.g., the process instance migration
  • Monitor job: after the seed job finished, it monitors the progress of the batch execution and completion

API

The following gives an overview of the Java API for batches.

Creating a Batch

A batch is created by executing a process engine command asynchronously. You can find a list of currently supported batch types in the Batch operations. The Java API can be used to create Batch command. Refer to specific commands for exact usage examples.

Query a Batch

You can query a running batch by the id and the type, for example to query for all running process instance migration batches.

List<Batch> migrationBatches = processEngine.getManagementService()
  .createBatchQuery()
  .type(Batch.TYPE_PROCESS_INSTANCE_MIGRATION)
  .list();

Batch Statistics

You can query for statistics of batches by using the management service. The batch statistics will contain information about the remaining, completed and failed batch execution jobs.

List<BatchStatistics> migrationBatches = processEngine.getManagementService()
  .createBatchStatisticsQuery()
  .type(Batch.TYPE_PROCESS_INSTANCE_MIGRATION)
  .list();

History of a Batch

For the history level FULL a historic batch entry is created. You can query it using the history service.

HistoricBatch historicBatch = processEngine.getHistoryService()
  .createHistoricBatchQuery()
  .batchId(batch.getId())
  .singleResult();

The history also contains job log entries for the seed, monitor and execution jobs. You can query the corresponding job log entries by the specific job definition id.

HistoricBatch historicBatch = ...

List<HistoricJobLog> batchExecutionJobLogs = processEngine.getHistoryService()
  .createHistoricJobLogQuery()
  .jobDefinitionId(historicBatch.getBatchJobDefinitionId())
  .orderByTimestamp()
  .list();

You can make a configuration for history cleanup of the finished historic batch operations.

Suspend a Batch

To pause the execution of a batch and all corresponding jobs, a batch can be suspended using the management service.

processEngine.getManagementService()
  .suspendBatchById("myBatch");

A suspended batch can then be activated again, also using the management service.

processEngine.getManagementService()
  .activateBatchById("myBatch");

Delete a Batch

A running batch can be deleted using the management service.

// Delete a batch preserving the history of the batch
processEngine.getManagementService()
  .deleteBatch("myBatch", false);

// Delete a batch include history of the batch
processEngine.getManagementService()
  .deleteBatch("myBatch", true);

A historic batch can be deleted using the history service.

processEngine.getHistoryService()
  .deleteHistoricBatch("myBatch");

For a running batch which still executes jobs it is recommended to suspend the batch before deleting it. See the Suspend a Batch section for details.

Priority of a Batch

As all batch jobs are executed using the job executor, it is possible to use the job prioritization feature to adjust the priority of batch jobs. The default batch job priority is set by the process engine configuration batchJobPriority.

It is possible to adjust the priority of a specific batch job definition or even a single batch job using the management service.

Batch batch = ...;

String batchJobDefinitionId = batch.getBatchJobDefinitionId();

processEngine.getManagementService()
  .setOverridingJobPriorityForJobDefinition(batchJobDefinitionId, 100, true);

Operation log

Please note that a user operation log is written for Batch creation itself only, execution of the seed job as well as individual jobs that perform operations are performed by Job Executor and therefore are not considered to be user operations.

Job Definitions

Seed Job

A batch initially creates a seed job. This seed will be repeatedly executed to create all batch execution jobs. For example if a user starts a process instance migration batch for 1000 process instances. With the default process engine configuration the seed job will create 10 batch execution jobs on every invocation. Every execution job will migrate 1 process instance. In sum the seed job will be invoked 100 times, until it has created the 1000 execution jobs required to complete the batch.

The Java API can be used to get the job definition for the seed job of a batch:

Batch batch = ...;

JobDefinition seedJobDefinition = processEngine.getManagementService()
  .createJobDefinitionQuery()
  .jobDefinitionId(batch.getSeedJobDefinitionId())
  .singleResult();

To pause the creation of further batch execution jobs, the seed job definition can be suspended with the management service:

processEngine.getManagementService()
  .suspendJobByJobDefinitionId(seedJobDefinition.getId());

Execution Jobs

The execution of a batch is split into several execution jobs. The specific number of jobs depends on the total jobs of the batch and the process engine configuration (see seed job). Every execution job executes the actual batch command for a given number of invocations, e.g., migrate a number of process instances. The execution jobs will be executed by the job executor. They behave like other jobs which means they can fail and the job executor will retry failed batch execution jobs. Also, there will be incidents for failed batch execution jobs with no retries left.

The Java API can be used to get the job definition for the execution jobs of a batch, e.g., for a process instance migration batch:

Batch batch = ...;

JobDefinition executionJobDefinition = processEngine.getManagementService()
  .createJobDefinitionQuery()
  .jobDefinitionId(batch.getBatchJobDefinitionId())
  .singleResult();

To pause the execution of further batch execution jobs, the batch job definition can be suspended with the management service:

processEngine.getManagementService()
  .suspendJobByJobDefinitionId(executionJobDefinition.getId());

Monitor Job

After all batch execution jobs were created by the seed job a monitor job is created for the batch. This job regularly polls if the batch has been completed, i.e., all batch execution jobs were completed. The polling interval can be configured by the batchPollTime (default: 30 seconds) property of the process engine configuration.

The Java API can be used to get the job definition for the monitor job of a batch:

Batch batch = ...;

JobDefinition monitorJobDefinition = processEngine.getManagementService()
  .createJobDefinitionQuery()
  .jobDefinitionId(batch.getMonitorJobDefinitionId())
  .singleResult();

To prevent the completion of the batch, i.e., deletion of the runtime data, the monitor job definition can be suspended with the management service:

processEngine.getManagementService()
  .suspendJobByJobDefinitionId(monitorJobDefinition.getId());

Configuration

You can configure the number of jobs created by every seed job invocation batchJobsPerSeed (default: 100) and the number of invocations per batch execution job invocationsPerBatchJob (default: 1) in the process engine configuration.

The number of invocations per batch execution job can be changed for each batch operation type individually with the help of the process engine configuration property invocationsPerBatchJobByBatchType. In case you haven’t specified the invocations per batch job by type, the configuration falls back to the global configuration specified via invocationsPerBatchJob.

You can configure the property in three ways:

  1. Programmatically with the help of a Process Engine Plugin
  2. In Spring-based environments via Spring XML Configuration 
    <bean id="processEngineConfiguration" 
      class="org.camunda.bpm.engine.impl.cfg.StandaloneInMemProcessEngineConfiguration">

  <!-- ... -->

  <property name="invocationsPerBatchJobByBatchType">
    <map>
      <entry key="process-set-removal-time" value="10" />
      <entry key="historic-instance-deletion" value="3" />

      <!-- in case of custom batch operations -->
      <entry key="my-custom-operation" value="7" />
    </map>
  </property>
</bean>
  3. In Spring Boot environment via the application.yaml file 
    camunda.bpm.generic-properties.properties:
  invocations-per-batch-job-by-batch-type:
    process-set-removal-time:     10
    historic-instance-deletion:   3
    my-custom-operation:          7  # in case of custom batch operations

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