Deadlock in DBMS

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.
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.

Deadlock Avoidance

When a database is stuck in a deadlock state, then it is better to avoid the database rather than aborting or restating the database. This is a waste of time and resource.
Deadlock avoidance mechanism is used to detect any deadlock situation in advance.
A method like "wait for graph" is used for detecting the deadlock situation but this method is suitable only for the smaller database.
For the larger database, deadlock prevention method can be used.

In a database, when a transaction waits indefinitely to obtain a lock, then the DBMS should detect whether the transaction is involved in a deadlock or not.
The lock manager maintains a Wait for the graph to detect the deadlock cycle in the database.

This is the suitable method for deadlock detection. In this method, a graph is created based on the transaction and their lock. If the created graph has a cycle or closed loop, then there is a deadlock.
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.

Deadlock prevention method is suitable for a large database.
If the resources are allocated in such a way that deadlock never occurs, then the deadlock can be prevented.
The Database management system analyzes the operations of the transaction whether they can create a deadlock situation or not. If they do, then the DBMS never allowed that transaction to be executed.