Distributed computing

QUESTIONS AND ANSWERS

• Q: What happens if the transaction manager fails?
• A: A database server who said yes to the first phase but has received neither a commit nor abort instruction must wait until the transaction manager recovers. It is said to be blocked.
• Q: How does a recovering transaction manager know whether it committed a given transaction before failing?
• A: The transaction manager must write a commit T record to stable storage after it receives yes’s from all data base servers on behalf of T and before it sends any commit messages to them.
• Q: Is there any way to avoid having a data base server block when the transaction manager fails?
• A: A database server may consult other database servers who have participated in the transaction, if it knows who they are.

OPTIMIZATION FOR READ-ONLY TRANSACTIONS

• Read-only transactions.
• Suppose a given server has done only reads (no updates) for a transaction.
• Instead of responding to the transaction manager that it can commit, it responds READ-only
• The transaction manager can thereby avoid sending that server a commit message

THREE-PHASE COMMIT

• A non-blocking protocol, assuming that:
• A process fail-stops and does not recover during the protocol
• The network delivers messages from A to B in the order they were sent
• Live processes respond within the timeout period
• Non-blocking = surviving servers can decide what to do.

PROTOCOL

• Transaction Manager
• (Initiator)

• Server
• (Agent)

• Willing?

• Willing-Yes
• Prepare

• OK

• Committed

• Done

STATES OF SERVER ASSUMING FIRST TM DOES NOT FAIL

• Active

• Willing

• Ready to
• Commit

• Abort

• Send
• Willing-Yes

• Receive
• Prepare

• No

• Committed

• Receive
• Committed