Oracle Enterprise Scheduler Services (ESS) provides a powerful and useful mechanism to process data in parallel - "Subrequests". The common use case is to employ ESS Subrequest feature for parallel execution of child jobs. For example, parallel processing of payroll for a large number of employees can be achieved by executing subjobs or child jobs i.e one job for each letter of the alphabet. This feature combined with ESS support for handling job dependencies (and incompatibilities - a ESS feature) will ensure that a payroll job does not run at the same time as a salary increase job.
In simple terms, an ESS job request is considered as a subrequest when a running job submits a new request, passing its own execution context. Passing the execution context ties the request being submitted to the currently running request.
Once a parent request submits a SubRequest, that parent must return control back to Oracle Enterprise Scheduler, in the manner appropriate for its job type, indicating that it has paused execution. Oracle Enterprise Scheduler then sets the parent state to PAUSED and starts processing the sub-request. Once the sub-request finishes, Oracle Enterprise Scheduler places the parent request on the ready queue, where it remains PAUSED, until it is picked up by an appropriate request processor. The parent is then set to RUNNING state and re-run as a resumed request.
In general, SubRequests are treated like any other request in regard to their processing. ESS will not automatically update the parent request status to ERROR on failure of a sub-request. It is up to the parent job to determine its own tolerance for sub-request failures and complete with the appropriate request status based on the outcome of the sub-requests.
Some important points to remember:
- ESS does not keep any kind of "ordering" in regard to subrequests submitted by a given execution of the parent request. Once the parent pauses, the processing of subrequests is like any other request; i.e., it is subject to workassignments, incompatibilities, async throttling, available worker threads, and so on
- There is no internal restriction imposed by ESS that limits how many sub-requests could be executing at a given time
- ESS WebService does not support sub-request submission
- Cross-cluster sub-requests are not possible
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