In database systems, a deadlock occurs when two or more transactions are unable to proceed because each is waiting for the other to release a resource. Deadlocks can happen when transactions acquire locks on resources in a different order, leading to a circular dependency.

To prevent deadlocks, database systems employ various deadlock handling mechanisms. In this blog post, we will discuss how deadlock handling works in the repeatable-read isolation level.

Understanding Repeatable-Read Isolation Level

The repeatable-read isolation level is a level of isolation in which a transaction reads a consistent snapshot of data throughout its lifetime. In this isolation level, a transaction acquires shared locks on the data it reads, preventing other transactions from modifying that data until it is committed or rolled back.

Deadlock Detection and Resolution in Repeatable-Read Isolation

In the repeatable-read isolation level, database systems use the wait-die technique to handle deadlocks. Here’s how it works:

1. When a transaction requests a lock on a resource, the database checks if granting the lock will result in a deadlock.
2. If the requested lock does not create a deadlock, the transaction is granted the lock, and it continues execution.
3. If the requested lock creates a potential deadlock, the database checks the timestamp of the requesting transaction against the timestamp of the transaction holding the conflicting lock.
   • If the requesting transaction has an earlier timestamp (was initiated earlier), it waits for the conflicting transaction to release the lock.
   • If the requesting transaction has a later timestamp (was initiated later), it is rolled back.
4. The rolled back transaction can then be restarted with a new timestamp and may reattempt acquiring the lock.

By using the wait-die technique, the database system ensures that transactions with lower timestamps wait for transactions with higher timestamps, preventing circular dependencies that lead to deadlocks.

Managing Deadlock Situations

When a transaction is rolled back due to a potential deadlock, the application can catch this event and handle it appropriately. Various approaches can be adopted to manage deadlock situations:

1. Retrying the Transaction: The application can retry the transaction after a certain period of time, allowing it to acquire the required locks and proceed. However, this approach should be used judiciously, as excessive retrying can lead to performance issues and increased chances of deadlocks.
2. Backoff and Retry: Instead of immediately retrying the transaction, the application can introduce a backoff period, where it waits for a specified amount of time before retrying. This helps reduce contention and can mitigate the chances of immediate deadlocks.
3. Killing Idle Transactions: If a transaction remains idle for an extended period, the application can choose to kill it. This approach helps prevent resource wastage and reduces the chances of deadlocks.

Conclusion

In the repeatable-read isolation level, deadlock handling is critical to ensure the smooth functioning of database systems. By using the wait-die technique, transactions with lower timestamps are made to wait for transactions with higher timestamps, avoiding circular dependencies that lead to deadlocks. Additionally, applications can implement strategies such as retrying transactions, introducing backoff periods, or killing idle transactions to manage deadlock situations effectively.

#References:

• Database Isolation Levels - Wikipedia
• Understanding Deadlocks - Microsoft Docs