Deadlock handling in snapshot isolation level

24 Oct 2023

In database transactions, deadlocks can occur when two or more transactions wait indefinitely for each other to release resources. This deadlock situation can lead to delays and even database system failures. Snapshot Isolation Level is a technique used to prevent dirty reads and allows transactions to see consistent results during their execution. In this blog post, we will discuss how deadlocks are handled when using the Snapshot Isolation Level in a database.

Understanding Snapshot Isolation Level

Snapshot Isolation Level ensures that each transaction works with a consistent snapshot of the database as of the beginning of the transaction. This means that a transaction will not see any changes made by other concurrent transactions during its execution. Instead, it reads data from a snapshot, which remains unaffected by other transactions.

Deadlock Detection and Handling

In Snapshot Isolation Level, concurrent transactions can still encounter deadlock situations. When a deadlock occurs, the database system must detect and resolve it to allow transactions to proceed. Here are a few approaches to handle deadlocks in Snapshot Isolation Level:

1. Deadlock Detection

To handle deadlocks, the database system needs to detect them first. Most modern database systems use resource wait-for graphs to detect deadlocks. These graphs represent the relationships between transactions and the resources they are waiting for. Once a cycle is detected in the graph, it confirms the occurrence of a deadlock.

2. Deadlock Resolution

Once a deadlock is detected, the database system needs to resolve it. In Snapshot Isolation Level, the most common approach is to choose one transaction as a victim and roll it back. The rollback process releases the resources held by the victim transaction and allows other transactions involved in the deadlock to proceed.

3. Retry Mechanism

After a transaction is rolled back due to a deadlock, it can be retried. The retry mechanism allows the transaction to start again and attempt to execute in a way that avoids deadlocks. This can be achieved by incorporating delay techniques or by introducing a back-off strategy between retries to give other transactions a chance to proceed.

Benefits of Deadlock Handling in Snapshot Isolation Level

Ensures database consistency by preventing dirty reads.
Avoids transaction failures and delays caused by deadlocks.
Provides a mechanism to resolve deadlocks and allow transactions to proceed.
Allows for retry mechanisms to avoid future deadlocks.

Conclusion

Snapshot Isolation Level is a powerful technique for ensuring database consistency and preventing dirty reads. While it can reduce the chances of deadlocks occurring, they can still happen in concurrent transactions. Through the detection and resolution of deadlocks, the database system can ensure the smooth execution of transactions and maintain database integrity. Incorporating deadlock handling mechanisms is crucial when working with Snapshot Isolation Level to guarantee the reliable operation of database systems.

References: