Unexpected in Spring partition when using synchronized

问题

I am using Spring Batch and Partition to do parallel processing. Hibernate and Spring Data Jpa for db. For the partition step, the reader, processor and writer have stepscope and so I can inject partition key and range(from-to) to them. Now in processor, I have one synchronized method and expected this method to be ran once at time, but it is not the case.

I set it to have 10 partitions , all 10 Item reader read the right partitioned range. The problem comes with item processor. Blow code has the same logic I use.

public class accountProcessor implementes ItemProcessor{
    @override
    public Custom process(item) {
        createAccount(item);
        return item;
    }

    //account has unique constraints username, gender, and email
    /*
        When 1 thread execute that method, it will create 1 account 
        and save it. If next thread comes in and  try to save the  same  account, 
        it  should find the account created by first thread and do one update. 
        But now it doesn't happen, instead findIfExist return null 
        and it  try to do another insert of duplicate data
    */
    private synchronized void createAccount(item) {
        Account account = accountRepo.findIfExist(item.getUsername(),  item.getGender(),  item.getEmail());
        if(account  == null) {
            //account  doesn't  exist
            account = new Account();
            account.setUsername(item.getUsername());
            account.setGender(item.getGender());
            account.setEmail(item.getEmail());
            account.setMoney(10000);
        } else {
            account.setMoney(account.getMoney()-10);
        }
        accountRepo.save(account);
    }
}

The expected output is that only 1 thread will run this method at any given time and so that there will be no duplicate inserttion in db as well as avoid DataintegrityViolationexception.

Actually result is that second thread can't find the first account and try to create a duplicate account and save to db, which will cause DataintegrityViolationexception, unique constraints error.

Since I synchronized the method, thread should execute it in order, second thread should wait for first thread to finish and then run, which mean it should be able to find the first account.

I tried with many approaches, like a volatile set to contains all unique accounts, do saveAndFlush to make commits asap, using threadlocal whatsoever, no of these works.

Need some help.

回答1:

Since you made the item processor step-scoped, you don't really need synchronization as each step will have its own instance of the processor.

But it looks like you have a design problem rather than an implementation issue. You are trying to sychronize threads to act in a certain order in a parallel setup. When you decide to go parallel and divide the data into partitions and give each worker (either local or remote) a partition to work on, you must admit that these partitions will be processed in an undefined order and that there should be no relation between records of each partition or between the work done by each worker.

When 1 thread execute that method, it will create 1 account and save it. If next thread comes in and try to save the same account, it should find the account created by first thread and do one update. But now it doesn't happen, instead findIfExist return null and it try to do another insert of duplicate data

That's because the transaction of thread1 may not be committed yet, hence thread2 won't find the record you think have been inserted by thread1.

It looks like you are trying to create or update some accounts with a partitioned setup. I'm not sure if this setup is suitable for the problem at hand.

As a side note, I would not call accountRepo.save(account); in an item processor but rather do that in an item writer.

Hope this helps.

来源：https://stackoverflow.com/questions/54660694/unexpected-in-spring-partition-when-using-synchronized