Distributed computing

TWO-PHASE COMMIT: PHASE 1

• Transaction manager asks all servers whether they can commit.
• Upon receipt, each able server saves all updates to stable storage and responds yes.
• If server cannot say yes (e.g., because of a concurrency control problem), then it says no. In that case, it can immediately forget the transaction. Transaction manager will abort the transaction at all sites.

• TM

• Servers

• Prepare

• Prepare

• Prepare

• Yes

• Yes

• Yes

TWO-PHASE COMMIT: PHASE 2

• If all servers say yes, then transaction manager writes a commit record to stable storage and tells them all to commit, but if some say no or don’t respond, transaction manager tells them all to abort.
• Upon receipt, the server writes the commit record and then sends an acknowledgement. The transaction manager is done when it receives all acknowledgements.
• If a database server fails during first step, all abort.
• If a database server fails during second step, it can consult the transaction manager to see whether it should commit.

• TM

• Commit

• Commit

• Ack

• Ack

• Ack

• Commit

ALL OF TWO-PHASE COMMIT

• Transaction Manager

• Server

• Prepare

• Yes

• Commit

• Done

• Active

• Ready to Commit

• Committed

• Aborted

• Prepare

• Commit

• Cannot prepare

• States of server