A deadlock in a database can occur when two or more transactions are executing concurrently and each requires a resource that is locked by the other. This can happen in several ways:
1. Two transactions may attempt to acquire locks on different resources in a different order. For example, transaction A may lock resource X and then attempt to lock resource Y, while transaction B may lock resource Y and then attempt to lock resource X.2. A single transaction may acquire multiple locks on different resources, and another transaction may attempt to acquire a lock on one of those resources that is already locked by the first transaction.
3. A transaction may acquire a lock on a resource and then attempt to acquire another lock on the same resource, which can cause a deadlock if another transaction has already acquired a lock on that resource.
4. A transaction may acquire a lock on a resource and then go into a wait state, while another transaction attempts to acquire a lock on the same resource.
In all these cases, both transactions will be blocked and unable to proceed until one of them is rolled back or the deadlock is resolved by the database management system.
There are several ways to fix a deadlock in a database:
- Deadlock Prevention: Deadlock can be prevented by acquiring locks on resources in a specific order, also known as lock ordering. This will ensure that two transactions will never attempt to acquire locks on the same resources in a different order, which can cause a deadlock.- Deadlock Detection: Deadlock detection can be implemented in the database management system by periodically checking for waiting transactions. If a deadlock is detected, the database management system can roll back one of the transactions to release the lock and allow the other transaction to proceed.
- Timeout: Transactions can be programmed with a timeout, so that if it is waiting for a lock for too long, it will automatically be rolled back, releasing the lock and allowing the other transaction to proceed.
- Deadlock Resolution: Deadlock resolution can be implemented in the database management system by using a timeout-based or a priority-based algorithm. In a timeout-based algorithm, the transaction that has been waiting for the longest period of time is rolled back. In a priority-based algorithm, the transaction with the lowest priority is rolled back.
- Application-level solution: One can also implement application-level solution in the sense that the application should be designed in such a way that the chances of deadlock are minimized. This can be done by reducing the number of locks held by a transaction, or by releasing locks as soon as they are no longer needed.
It's worth noting that, the solution will vary based on the nature of the deadlock and the database management system being used.
