Wait for graph method and its usage with example.

Wait for Graph is a method used for Deadlock detection. A deadlock is a condition where two or more
transactions are waiting indefinitely for one another to give up locks. Deadlock is said to be one of the
most feared complications in DBMS as no task ever gets finished and is in waiting state forever.

It is an enhanced version of the Resource Allocation Graph in which we do not show the resources.

The wait for the graph is maintained by the system for every transaction which is waiting for some
data held by the others. The system keeps checking the graph if there is any cycle in the graph.

If all the resources have single instances and the cycle is being formed then the system is in Deadlock.
If the cycle is there but resource have more than one instance then the deadlock may or may not be
present.

Example:  In the student table, transaction T1 holds a lock on some rows and needs to update some
rows in the grade table. Simultaneously, transaction T2 holds locks on some rows in the grade table
and needs to update the rows in the Student table held by Transaction T1.

The wait for a graph for the above scenario is shown below:

The wait-for-graph scheme is not applicable to a resource allocation system with multiple instances of each resource type.