A deadlock is a situation in a SQL database where two or more transactions are unable to proceed because each transaction is waiting for a resource that is held by another transaction. This creates a cyclic dependency where none of the transactions can make progress, resulting in a deadlock.
How does a deadlock occur?
Deadlocks can occur due to the following conditions known as the “deadlock cycle”:
- Mutual Exclusion: Each transaction holds exclusive access to a resource that cannot be shared.
- Hold and Wait: A transaction holds resources while also waiting for additional resources held by another transaction.
- No Preemption: Resources cannot be forcibly taken away from a transaction; they can only be released voluntarily.
- Circular Wait: A cycle is formed when each transaction in a set is waiting for a resource held by another transaction in the set.
Example scenario:
Let’s consider a scenario involving two transactions, A and B, where each transaction is accessing two resources, X and Y:
- Transaction A acquires resource X.
- Transaction B acquires resource Y.
- Both transactions now need an additional resource that is held by the other transaction. For example, A needs resource Y, and B needs resource X.
- Since neither transaction can proceed without the required resource, they enter a deadlock state.
In this scenario, neither transaction A nor B can continue processing, resulting in a deadlock.
Dealing with deadlocks:
To handle deadlocks, SQL databases often employ deadlock detection and resolution mechanisms. These mechanisms include:
- Deadlock detection: The database periodically checks for deadlocks and identifies the transactions involved. Once a deadlock is detected, the database can take appropriate action to resolve it.
- Deadlock prevention: By carefully designing an application and database, deadlocks can be prevented to some extent. This includes avoiding situations that could result in deadlock cycles, such as ordering resource acquisition or using timeouts.
- Deadlock avoidance: Some databases use algorithms that analyze transaction requests and resource availability to determine if a particular request will lead to a deadlock. If a deadlock is predicted, the request may be delayed or denied to avoid the deadlock.
It’s important to note that preventing deadlocks entirely can be challenging, especially in complex systems with numerous transactions and resources. Thus, a combination of prevention, detection, and resolution mechanisms is often employed to minimize the occurrence and impact of deadlocks.
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