Detection and recovery from deadlock in dbms

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                             Deadlock detaction 

When a transaction waits indefinately to obtain a lock, The database managememt system should detect whether the transaction is involved in a deadlock or not.

Wait for graph 


  • Wait-for-graph is one of the methods for detecting the deadlock situation. 
  • This method is suitable for smaller database. In this method a graph is drawn based on the transaction and their lock on the resource.
  •  If the graph created has a closed loop or a cycle, then there is a deadlock.

 For the above mentioned scenario the Wait-For graph is drawn below.

Recovery from Deadlock 

When a detection algorithm determines that a deadlock exists, the system must recover from the deadlock. The most common solution is to roll back one or more transactions to break the deadlock. Choosing which transaction to abort is known as Victim Selection.

Choice of Deadlock victim

In below wait-for graph transactions T26, T28 and T27 are deadlocked. In order to remove deadlock one of the transaction out of these three transactions must be roll backed.
We should roll back those transactions that will incur the minimum cost. When a deadlock is detected, the choice of which transaction to abort can be made using following criteria:
                           
• The transaction which have the fewest locks
• The transaction that has done the least work
• The transaction that is farthest from completion

Rollback 

Once we have decided that a particular transaction must be rolled back, we must determine how far this transaction should be rolled back. The simplest solution is a total rollback; Abort the transaction and then restart it. However it is more effective to roll back the transaction only as far as necessary to break the deadlock. But this method requires the system to maintain additional information about the state of all the running system.

 Thank you viewers..
This is published by soumy Sinha..

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