CODEWITHSUNDEEP
mongodbspring-bootspring-dataasyncresttemplate
uylmz
uylmz

Reputation: 1552

Spring Boot controller preventing multiple inserts upon quick successive requests in mongodb

I have a REST API to calculate something upon a request, and if the same request is made again, return the result from the cache, which consist of documents saved in MongoDB. To know if two request is the same, I am hashing some relevant fields in the request. But when same request is made in a quick succession, duplicate documents occur in MongoDB, which later results in "IncorrectResultSizeDataAccessException" when I try to read them.

To solve it I tried to synchronize on hash value in following controller method (tried to cut out non relevant parts):

@PostMapping(
        path = "/{myPath}",
        consumes = {MediaType.APPLICATION_JSON_UTF8_VALUE},
        produces = {MediaType.APPLICATION_JSON_UTF8_VALUE})
@Async("asyncExecutor")
public CompletableFuture<ResponseEntity<?>> retrieveAndCache( ... a,b,c,d various request parameters) {
    
    //perform some validations on request...
    
    //hash relevant equest parameters
    
    int hash = Objects.hash(a, b, c, d);
    
    synchronized (Integer.toString(hash).intern()) {

        Optional<Result> resultOpt = cacheService.findByHash(hash);
        
        if (resultOpt.isPresent()) {
            return CompletableFuture.completedFuture(ResponseEntity.status(HttpStatus.OK).body(opt.get().getResult()));
        } else {
            Result result = ...//perform requests to external services and do some calculations...
            cacheService.save(result);
            
            return CompletableFuture.completedFuture(ResponseEntity.status(HttpStatus.OK).body(result));
        }
        
    }
}



//cacheService methods
@Transactional
public Optional<Result> findByHash(int hash) {
    return repository.findByHash(hash); //this is the part that throws the error
}

I am sure that no hash collision is occuring, its just when the same request is performed in a quick succession duplicate records occur. To my understanding, it shouldn't occur as long as I have only 1 running instance of my spring boot application. Do you see any other reason than there are multiple instances running in production?

Upvotes: 0

Views: 553

Answers (1)

GeertPt
GeertPt

Reputation: 17846

You should check the settings of your MongoDB client.

If one thread calls the cacheService.save(result) method, and after that method returns, releases the lock, then another thread calls cacheService.findByHash(hash), it's still possible that it will not find the record that you just saved.

It's possible that e.g. the save method returns as soon as the saved object is in the transaction log, but not fully processed yet. Or the save is processed on the primary node, but the findByHash is executed on the secondary node, where it's not replicated yet.

You could use WriteConcern.MAJORITY, but I'm not 100% sure if it covers everything.

Even better is to let MongoDB do the locking by using findAndModify with FindAndModifyOptions.upsert(true), and forget about the lock in your java code.

Upvotes: 1

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