What happens if one of the services crashes during the Commit phase in 2PC?

In the two-phase commit (2PC), the Commit phase is the second phase where the coordinator sends a command to participants to confirm the transaction after successful voting (Prepare phase).

If one of the services crashes during the Commit phase:

• That service will not receive the Commit command or will not be able to process it.
• The coordinator will wait for confirmations from all participants but will not receive a response from the crashed service.
• Depending on the implementation, the coordinator may:
  • Retry sending the Commit command.
  • Wait for the service to recover.
  • In case of timeout, consider the transaction failed and initiate a rollback with other participants.

The problem is that 2PC locks resources of participants until confirmation is received, so a crash during the Commit phase can lead to blocking and delays.

To address these issues, the following are used:

• Timeouts and retry mechanisms.
• Use of more resilient protocols (e.g., three-phase commit).
• Replication and fault tolerance of services.

Thus, if a service crashes during Commit, the transaction may hang or roll back, depending on the coordinator's logic and recovery mechanisms.