Deadlock in database
- 2. Deadlock: Deadlock 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. A deadlock can be indicated by a cycle in the wait-for-graph. This is a directed graph in which the vertices denote transactions and the edges denote waits for data items.
- 3. Example: For example, in the following wait-for-graph, transaction T1 is waiting for data item X which is locked by T3. T3 is waiting for Y which is locked by T2 and T2 is waiting for Z which is locked by T1. Hence, a waiting cycle is formed, and none of the transactions can proceed executing.
- 4. DEADLOCK HANDLING • when a deadlock occurs, different operating systems respond to them in different non- standard manners. • The Major approaches are as follows IGNORING DEADLOCK DETECTION
- 5. IGNORING DEADLOCK In this approach, it is assumed that a deadlock will never occur. This is also an application of the Ostrich algorithm. This approach was initially used by MINIX and UNIX. This is used when the time intervals between occurrences of deadlocks are large and the data loss incurred each time is tolerable
- 6. DETECTION The deadlock detection and removal approach runs a deadlock detection algorithm periodically and removes deadlock in case there is one.
- 7. Prevention: The deadlock prevention approach does not allow any transaction to acquire locks that will lead to deadlocks. The convention is that when more than one transactions request for locking the same data item, only one of them is granted the lock.
- 8. prevention One of the most popular deadlock prevention methods is pre-acquisition of all the locks. In this method, a transaction acquires all the locks before starting to execute and retains the locks for the entire duration of transaction. If another transaction needs any of the already acquired locks, it has to wait until all the locks it needs are available. Using this approach, the system is prevented from being deadlocked since none of the waiting transactions are holding any lock.
- 9. Deadlock Avoidance The deadlock avoidance approach handles deadlocks before they occur. It analyzes the transactions and the locks to determine whether or not waiting leads to a deadlock. The method can be briefly stated as follows. Transactions start executing and request data items that they need to lock. The lock manager checks whether the lock is available. If it is available, the lock manager allocates the data item and the transaction acquires the lock.
- 10. There are two algorithms for this purpose, namely wait- die and wound-wait. Let us assume that there are two transactions, T1 and T2, where T1 tries to lock a data item which is already locked by T2. The algorithms are as follows − Wait-Die − If T1 is older than T2, T1 is allowed to wait. Otherwise, if T1 is younger than T2, T1 is aborted and later restarted. Wound-Wait − If T1 is older than T2, T2 is aborted and later restarted. Otherwise, if T1 is younger than T2, T1 is allowed to wait.
- 11. Deadlock Detection and Removal The deadlock detection and removal approach runs a deadlock detect and removes deadlock in case there is one.
- 12. Detection: If deadlock occur then identifying the prepresses and recourses involved in deadlock.
- 13. Recovery: ProcessTermination First of all get resources back and gain forward progress. Could delete all the resources which occur in deadlock. Delete one at a time until deadlock is broken is a time consuming. Delete the processor which consume the time during execution.
- 14. Failure and Faults of DDBMS Soft Failure Soft failure is the type of failure that causes the loss in volatile memory of the computer and not in the persistent storage. Operating system failure. Main memory crash. Transaction failure Failure of supporting software. Power failure.
- 15. Hard Failure A hard failure is the type of failure that causes loss of data in the persistent or non-volatile storage like disk. Power failure. Faults in media. Read-write malfunction. Corruption of information on the disk. Read/write head crash of disk.
- 16. THE END