3. Deadlock and It's Handling

Deadlock:

Definition: Deadlock in computer science occurs when two or more processes or transactions are unable to proceed because each is waiting for the other to release resources (such as locks) that it needs to continue. As a result, the processes or transactions are indefinitely blocked, leading to a state where none of them can progress.

Causes: Deadlock typically arises due to the presence of four necessary conditions:

1. Mutual Exclusion: Each process or transaction holds exclusive access to a resource.
2. Hold and Wait: Processes or transactions hold resources while waiting for additional resources.
3. No Preemption: Resources cannot be forcibly taken away from a process or transaction.
4. Circular Wait: There exists a circular chain of processes or transactions, each waiting for a resource held by the next.

Handling Deadlock:

1. Wait-Die Method:

Overview: In the Wait-Die method, a younger transaction (one that started later) waits if it attempts to acquire a resource held by an older transaction. If the younger transaction attempts to acquire a resource held by an older transaction, it waits. If the older transaction attempts to acquire a resource held by a younger transaction, it aborts.

Process:

1. If a younger transaction requests a resource held by an older transaction:
   • The younger transaction is placed in a waiting state (it waits).
2. If an older transaction requests a resource held by a younger transaction:
   • The older transaction is aborted.
3. If a transaction is aborted, it is restarted later.

Advantages:

• Prevents deadlock by allowing younger transactions to wait for resources held by older transactions.
• Older transactions are not unnecessarily penalized.

Disadvantages:

• May lead to starvation if older transactions frequently abort younger transactions.

2. Wound-Wait Method:

Overview: In the Wound-Wait method, a younger transaction (one that started later) is allowed to proceed if it attempts to acquire a resource held by an older transaction. If the younger transaction attempts to acquire a resource held by an older transaction, the older transaction is aborted.

Process:

1. If a younger transaction requests a resource held by an older transaction:
   • The older transaction is aborted.
2. If an older transaction requests a resource held by a younger transaction:
   • The older transaction waits for the younger transaction to release the resource.

Advantages:

• Prevents deadlock by allowing younger transactions to proceed.
• Avoids unnecessary waiting by older transactions.

Disadvantages:

• May lead to potential data inconsistency if older transactions are aborted frequently.

Conclusion:

The Wait-Die and Wound-Wait methods are deadlock prevention techniques used in concurrency control to avoid deadlock situations in computer systems. These methods ensure that transactions can proceed without being indefinitely blocked by others, thereby maintaining system liveness and preventing resource starvation. The choice between Wait-Die and Wound-Wait depends on factors such as system requirements, performance considerations, and the desired behavior in case of conflicts between transactions.