Write Conflict how it actually works

[Nishant_Kumar4 (Nishant Kumar)]

I read that write conflict occurs when MongoDB detects that the document has been modified by another operation since it was last read. So I wrote the following code to see if it throws the write conflict or not

public void test() throws InterruptedException {
        Thread t1 = new Thread(()->{
            try {
                processor.performTransaction(LIVE,0);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        });
        Thread t2 = new Thread(()->{
            try {
                MDC.put(OrgMongoDBFactory.SHARD_KEY, "1");
                processor.performTransaction(COMPLETED,10000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        });

        Data d1 = DataFixture.getData();
        d1.setRoll(0l);
        d1.setName("jm1");
        d1.setAge(0);
        d1.save(journeyMetaVersion);
        t1.start();
        t2.start();
        Thread.sleep(80000);
        List<Data> versions = dataDao.findAll();
        log.info("the version is: {}", versions);
    }

Inside Processor following method is there

public void performTransaction(Data.dataStatus status,int waitTime) throws InterruptedException {
        Data d1 = Data.findByRollNameAndAge(0l,"jm1",0);
        Thread.sleep(waitTime);
        log.info("got data as: {}", d1);
        d1.setStatus(status);
        Data d2 = DataDao.save(d1);
        log.info("the data is: {}", d2);
        Thread.sleep(waitTime);
        log.info("completed waiting");
    }

Here two threads are created which read from the Data collection and fetches the value. After that thread2 goes to sleep and thread1 goes ahead and updates the value. Now once the sleeptime is over for thread2 it tries to update the value. So here the update call should fail based on the above definition of Write conflict as post the last read there changes to the document.
But when I run the following code it doesn’t give any error
Please explain the possible reason for the above

[MaBeuLux88_xxx (Maxime Beugnet)]

Hi @Nishant_Kumar4,

Here are a couple of other posts in the forum that can help maybe:

Write conflict in transaction

I have an application that tries to update the same row in a collection in 2 separate threads at the same time. Each thread has its own connection the MongoDB server and has started a transaction. Thread 1 has altered the row and the 2nd thread tries to update the same row when MongoDB returns the write conflict error. The 2nd thread waits 5 millisecs and then tries to update the row again, when MongoDB returns transaction aborted. Each thread is updating and inserting into multiple documents …

Understanding locking within transactions (and how it deals with high concurrency)

TLDR: Do the locks created by operations that occur within mongodb transactions unlock immediately after the operation is complete or after the transaction is complete (committed/aborted)? The background: I’m using mongo to model the lobby state of a game round that clients are able to join and I would just like some clarification on how the use of transactions and correct data modeling can allow me to handle a high amount of concurrent requests while eliminating the risk of race conditions an…

Now back to your issue: in your code, I don’t see you use the API for transactions.

Transactions — MongoDB Manual

Without a transaction, there is no reason for a write conflict as all the operations would then be considered atomic (and transactional on their own).

I hope this helps.
Cheers,
Maxime.