Deadlock handling in MongoDB

24 Oct 2023

References

Deadlocks can occur in any multi-threaded system, including databases like MongoDB. A deadlock happens when two or more transactions or processes are waiting to acquire resources that are being held by each other, resulting in a stalemate and causing the system to halt.

To handle deadlocks in MongoDB, there are a few strategies you can implement:

1. Detecting Deadlocks

To detect deadlocks in MongoDB, you can enable the deadlock detection mechanism by setting the setParameter option transactionLifetimeLimitSeconds to a non-zero value. This option defines the maximum time a transaction can be active before being considered in a potential deadlock. The default value is 60 seconds.

use admin
db.runCommand({ setParameter: 1, transactionLifetimeLimitSeconds: 30 })

2. Retry Mechanism

When a deadlock is detected, the built-in retry mechanism in MongoDB can automatically retry the affected transactions. By default, MongoDB automatically retries a transaction up to three times (with a small delay between retries) before returning an error. The number of retries can be adjusted by setting the maxTransactionLockRequestTimeoutMillis option.

use admin
db.runCommand({ setParameter: 1, maxTransactionLockRequestTimeoutMillis: 60000 })

3. Retry Logic in Application Code

Another approach to handling deadlocks is to implement retry logic in your application code. When a deadlock error is encountered, you can catch it and retry the transaction after a short delay.

from pymongo.errors import AutoReconnect, OperationFailure
import time

def execute_transaction_with_retry(session, operations, retries=3, delay=1):
    for i in range(retries):
        try:
            with session.start_transaction():
                for operation in operations:
                    operation()
                session.commit_transaction()
                return True
        except (AutoReconnect, OperationFailure) as e:
            print("Deadlock detected, retrying...")
            time.sleep(delay)
    return False

In the above example, the execute_transaction_with_retry function retries the given operations in a transaction up to retries times, with a delay of delay seconds between each retry.

Conclusion

Deadlocks can occur in any database system, and MongoDB is no exception. By enabling deadlock detection, adjusting transaction timeout limits, and implementing retry logic in both MongoDB and application code, you can effectively handle deadlocks and ensure the smooth operation of your MongoDB database.

Remember to monitor your application and database performance to identify any recurring deadlock scenarios and optimize your handling strategies accordingly.

#References