How are task executor of tasklet and partition handler different in spring batch? - spring-batch

Question

I am running spring batch with partitioning. i.e. Partition/Read/Process/Write , where process step spends more time in I/O than a CPU intensive processing.
The problem with current partitioning scheme is that , it is creating some hot partitions, that is - Some of the partitions have more data to process as compared to others. So The job processes most of the records very fast, but last few records are processed slow because a few partitioned threads have more data and they are executing the records one by one, causing the job to complete slower.
Conceptually, I want to be able to spawn each partitioned step in to parallel threads (say 4 threads) , so that hot partitions are also processed in parallel , rather than 1 record at a time.
Here is the batch config I am currently using (before optimisation) for running partition/read/process logic for a job.
<batch:job id="Scheduler">
<batch:step id="Scheduler">
<batch:partition step="ScheduleStep" partitioner="Partitioner">
<batch:handler grid-size="30" task-executor="asyncTaskExecutor"/>
</batch:partition>
</batch:step>
</batch:job>
<batch:step id="ScheduleStep">
<batch:tasklet>
<batch:chunk reader="tsV2Reader" processor="tsV2Processor"
writer="tsV2Writer" commit-interval="1"/>
</batch:tasklet>
</batch:step>
<bean id="asyncTaskExecutor" class="org.springframework.core.task.SimpleAsyncTaskExecutor">
<property name="concurrencyLimit" value="30"/>
</bean>
I want to decrease partitions from 30 to say 10 and let each partition thread spawn its 4 threads, so that even if hot partition exists it still executes 4 records in parallel instead of 1. i.e. Overall 10 x 4 = 40 threads
So I changed the above config by adding task-executor at tasklet level with concurrencyLimit of 4 and changed gridSize and concurrencyLimit in config above to 10. i.e. -
<batch:job id="Scheduler">
<batch:step id="Scheduler">
<batch:partition step="ScheduleStep" partitioner="Partitioner">
<batch:handler **grid-size="10"** task-executor="asyncTaskExecutor"/>
</batch:partition>
</batch:step>
</batch:job>
<batch:step id="ScheduleStep">
<batch:tasklet **task-executor="taskletTaskExecutor" throttle-limit="4"**>
<batch:chunk reader="tsV2Reader" processor="tsV2Processor"
writer="tsV2Writer" commit-interval="1"/>
</batch:tasklet>
</batch:step>
<bean id="asyncTaskExecutor" class="org.springframework.core.task.SimpleAsyncTaskExecutor">
<property name="concurrencyLimit" **value="10"** />
</bean>
**<bean id="taskletTaskExecutor" class="org.springframework.core.task.SimpleAsyncTaskExecutor">
<property name="concurrencyLimit" value="4"/>
</bean>**
It seems to slow down the processing instead of speeding it up. May be the concurrenyLimit (4) of tasklet is being applied to all the threads. That I am not sure. I tried looking up the documentation on how the taskExecutor config for tasklet and handler is interpreted together under the hood, but not able to find relevant information. If someone can help with internals/understanding of these configs in question and how to achieve what I am looking for, will be of great help.