Deadlocks in DBMS
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Deadlocks in DBMS
A deadlock is a situation that can occur in a database management system (DBMS) where two or more transactions are blocked, unable to proceed because each is waiting for a resource held by the other. In other words, the transactions are stuck in a cycle of waiting for each other, and as a result, the system becomes unresponsive and unable to make progress.
Deadlocks can occur when transactions access resources in different orders. For example, transaction A might acquire a lock on resource X and then wait for a lock on resource Y, while transaction B has already acquired a lock on resource Y and is waiting for a lock on resource X. In this scenario, neither transaction can proceed, and the system is deadlocked.
DBMSs typically use a concurrency control mechanism to manage access to resources and ensure that transactions can execute concurrently without conflicts. Lock-based concurrency control is a common mechanism used to ensure data consistency and prevent data corruption. Locks are used to protect resources from concurrent access by multiple transactions. Transactions request locks on resources they need, and the DBMS grants the lock if the resource is not currently locked by another transaction.
However, lock-based concurrency control can also cause deadlocks. When two or more transactions request locks on resources that are already locked by other transactions, they may end up in a situation where they are all waiting for each other to release their locks, resulting in a deadlock.
DBMSs use various techniques to detect and resolve deadlocks. One common technique is to use a timeout mechanism, where transactions are allowed to wait for a resource for a limited amount of time. If the resource is not granted within the timeout period, the transaction is aborted, and the resources it holds are released. This technique can prevent the system from becoming deadlocked, but it can also result in wasted work if transactions are repeatedly aborted.
Another technique is to use a wait-for graph, which is a directed graph that represents the transactions and resources involved in a deadlock. Each node in the graph represents a transaction or a resource, and each edge represents a wait-for relationship. A transaction node points to a resource node if the transaction is waiting for that resource, and a resource node points to a transaction node if the resource is held by that transaction. By analyzing the wait-for graph, the DBMS can detect deadlocks and take appropriate actions to resolve them, such as aborting one or more transactions to break the deadlock.
In conclusion, deadlocks are a common issue in DBMSs that can arise when transactions access resources in different orders. Lock-based concurrency control is a common mechanism used to manage access to resources, but it can also cause deadlocks. DBMSs use various techniques, such as timeout mechanisms and wait-for graphs, to detect and resolve deadlocks and prevent them from causing system-wide failures. It is important for DBMS developers and administrators to be aware of deadlocks and to implement appropriate measures to prevent and resolve them.
Deadlocks in DBMS
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