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javaspring-batchspring-batch-tasklet

When Tasklet#execute should return CONTINUABLE?

I read that:

When processing is complete in your Tasklet implementation, you return an org.springframework.batch.repeat.RepeatStatus object. There are two options with this: RepeatStatus.CONTINUABLE and RepeatStatus.FINISHED. These two values can be confusing at first glance. If you return RepeatStatus.CONTINUABLE, you aren't saying that the job can continue. You're telling Spring Batch to run the tasklet again. Say, for example, that you wanted to execute a particular tasklet in a loop until a given condition was met, yet you still wanted to use Spring Batch to keep track of how many times the tasklet was executed, transactions, and so on. Your tasklet could return RepeatStatus.CONTINUABLE until the condition was met. If you return RepeatStatus.FINISHED, that means the processing for this tasklet is complete (regardless of success) and to continue with the next piece of processing.

But I can't imagine example of using this feature. Could you explain it for me ? When the next time tasklet will be invoked ?

Solution

  • Let's say that you have a large set of items (for example files), and you need to enrich each one of them in some way, which requires consuming an external service. The external service might provide a chunked mode that can process up to 1000 requests at once instead of making a separate remote call for each single file. That might be the only way you can bring down your overall processing time to the required level.

    However, this is not possible to implement using Spring Batch's Reader/Processor/Writer API in a nice way, because the Processor is fed item by item and not entire chunks of them. Only the Writer actually sees chunks of items.

    You could implement this using a Tasklet that reads the next up to 1000 unprocessed files, sends a chunked request to the service, processes the results, writes output files and deletes or moves the processed files. Finally it checks if there are more unprocessed files left. Depending on that it returns either FINISHED or CONTINUABLE, in which case the framework would invoke the Tasklet again to process the next up to 1000 files. This is actually a quite realistic scenario, so I hope that illustrates the purpose of the feature.