Deadlock in Databases

In databases, deadlocks often occur when multiple transactions attempt to acquire locks on the same set of resources in different orders. Database management systems (DBMS) use various techniques to handle deadlocks:

1. Lock Timeout: Transactions are aborted if they cannot acquire a lock within a specified time.
2. Deadlock Detection: DBMS periodically checks for cycles in the wait-for graph and resolves deadlocks by rolling back transactions.
3. Wait-for Graphs: Used to detect deadlocks by tracking which transactions are waiting for which locks.
   

Example in SQL:

A simple SQL example illustrating a deadlock might involve two transactions:

-- Transaction 1 BEGIN TRANSACTION; UPDATE Accounts SET Balance = Balance - 100 WHERE AccountID = 1; -- Transaction 2 BEGIN TRANSACTION; UPDATE Accounts SET Balance = Balance - 100 WHERE AccountID = 2;   -- Now both transactions try to update the other account, leading to a deadlock UPDATE Accounts SET Balance = Balance - 100 WHERE AccountID = 2; UPDATE Accounts SET Balance = Balance - 100 WHERE AccountID = 1;

 

In this scenario, if Transaction 1 holds a lock on AccountID 1 and Transaction 2 holds a lock on AccountID 2, and they both attempt to update the account that the other is holding, a deadlock occurs.

Understanding and managing deadlocks is crucial for ensuring the reliability and efficiency of concurrent systems, especially in complex applications and database environments.