What are Wait-Die and Wound-Wait?

Let’s understand “Wait-Die” and “Wound-Wait” schemes in DBMS in simple terms, along with examples.

What are Wait-Die and Wound-Wait?

These are deadlock prevention techniques used in database transaction management. They decide how to handle a situation where two transactions want to lock the same resource, potentially causing a deadlock.

The two schemes use the timestamp of a transaction to make decisions. The timestamp (TS) is a unique number that represents the time when the transaction started.

• Older transaction: The one with the smaller timestamp (TS).
• Younger transaction: The one with the larger timestamp (TS).

1. Wait-Die Scheme

• Rule:
  • If an older transaction requests a resource held by a younger transaction, it is allowed to wait.
  • If a younger transaction requests a resource held by an older transaction, it is aborted (dies) and restarted with the same timestamp.

Purpose: Prevents deadlock by ensuring that younger transactions are forced to die when they conflict with older ones.

Example (Wait-Die):

• T1 (TS = 10) (older) and T2 (TS = 20) (younger):
  1. T1 wants a lock held by T2:
     • Since T1 is older, T1 waits.
  2. T2 wants a lock held by T1:
     • Since T2 is younger, T2 dies (aborted) and is restarted.

2. Wound-Wait Scheme

• Rule:
  • If an older transaction requests a resource held by a younger transaction, it forces the younger transaction to release the resource (wound). The younger transaction is aborted.
  • If a younger transaction requests a resource held by an older transaction, it is allowed to wait.

Purpose: Prevents deadlock by ensuring that older transactions take precedence over younger ones.

Example (Wound-Wait):

• T1 (TS = 10) (older) and T2 (TS = 20) (younger):
  1. T1 wants a lock held by T2:
     • Since T1 is older, T2 is wounded (aborted) and restarted.
  2. T2 wants a lock held by T1:
     • Since T2 is younger, T2 waits.

Key Differences Between Wait-Die and Wound-Wait:

Simplified Example (Lock Conflict):

Conclusion:

Both Wait-Die and Wound-Wait are effective in preventing deadlocks by using timestamps.

• Wait-Die is more conservative (younger transactions die often).
• Wound-Wait is more aggressive (older transactions wound younger ones).