8/1/2023 0 Comments Database deadlock resolutionThere are two algorithms for this purpose, namely wait-die and wound-wait. Accordingly, the algorithm decides whether the transaction can wait or one of the transactions should be aborted. However, if the item is locked by some other transaction in incompatible mode, the lock manager runs an algorithm to test whether keeping the transaction in waiting state will cause a deadlock or not. If it is available, the lock manager allocates the data item and the transaction acquires the lock. The lock manager checks whether the lock is available. Transactions start executing and request data items that they need to lock. The method can be briefly stated as follows. It analyzes the transactions and the locks to determine whether or not waiting leads to a deadlock. The deadlock avoidance approach handles deadlocks before they occur. Using this approach, the system is prevented from being deadlocked since none of the waiting transactions are holding any lock. If another transaction needs any of the already acquired locks, it has to wait until all the locks it needs are available. In this method, a transaction acquires all the locks before starting to execute and retains the locks for the entire duration of transaction. One of the most popular deadlock prevention methods is pre-acquisition of all the locks. The convention is that when more than one transactions request for locking the same data item, only one of them is granted the lock. The deadlock prevention approach does not allow any transaction to acquire locks that will lead to deadlocks. There are three classical approaches for deadlock handling, namely −Īll of the three approaches can be incorporated in both a centralized and a distributed database system. Hence, a waiting cycle is formed, and none of the transactions can proceed executing. ![]() T3 is waiting for Y which is locked by T2 and T2 is waiting for Z which is locked by T1. This is a directed graph in which the vertices denote transactions and the edges denote waits for data items.įor example, in the following wait-for-graph, transaction T1 is waiting for data item X which is locked by T3. A deadlock can be indicated by a cycle in the wait-for-graph. What are Deadlocks?ĭeadlock is a state of a database system having two or more transactions, when each transaction is waiting for a data item that is being locked by some other transaction. We’ll study the deadlock handling mechanisms in both centralized and distributed database system. ![]() This chapter overviews deadlock handling mechanisms in database systems.
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