Spring batch job with multi level chunk - spring-boot

Question

I need to write a batch jobs, which reads from DB a list of users and then for each user get data from external system which will return data in batch of 50 to 500 and then insert this data to DB.
I have wrote a steps as below
Tasklet load all users and pass to Reader (step 1) .
Reader takes all users and then pass one by one to processor (step-2) .
Processor calls external system and get full data for one user.
Writer gets this data for each user and insert to DB.
Using SimpleAsyncTaskExecutor , I am running this in paraller but , I want to have a steps as below
Tasklet load all users and pass to Reader (step-1 ,stepTasklet).
Reader reads list of all user (step-2,parallelstep).
Another reader/processor get once chunk of data(50 records) for that user.
Writer gets 50 records data for each user and insert to DB.
This step need to be in parallel for multiple users and also chunk should be at data level not like currently as user level
Can someone help.
#Bean
public Job getParallelJob() {
JobParametersIncrementer jobParametersIncrementers = new JobParametersIncrementer() {
long count = new Date().getTime();
#Override
public JobParameters getNext( JobParameters parameters) {
return new JobParameters(Collections.singletonMap("count", new JobParameter(count)));
}
};
return this.jobBuilderFactory.get("paralleljob").incrementer(jobParametersIncrementers)
.start(stepTasklet())
.next(parallelStep(taskExecutor())).build();
}