CS 603 Distributed Transactions February 18, 2002 What is a Transaction? Atomic The external view is that either everything in the transaction happened, or nothing did Consistent If the start state is valid, the end state is valid Isolated A transaction in process is not affected by and does not affect any other transaction
Durable Once complete, the effects of a transaction are permanent, even in the event of failure. Distributed Transactions What is the difference with Distributed Transactions? NOTHING! Must still support ACID properties Important for all the same reasons Why do we study Distributed Transactions? Some properties harder to implement Basic single-system techniques not sufficient What is a Distributed Transaction?
Data Data Transaction component Transaction component Transaction component Transaction component Data Data
Why are Distributed Transactions Hard? Atomic Different parts of a transaction may be at different sites How do we ensure all or none committed? Consistent Failure may affect only part of transaction Isolated Commitment must occur simultaneously at all sites Durable
Not much different when other problems solved Makes delayed commit difficult Key Issues Commitment Standard techniques preserve properties when commit occurs Distributed systems need commit protocols so we know when commit has occurred Failures Standard techniques support durability for commit/abort What happens if a site fails during commitment? Committing a Distributed Transaction Data
component log Data Data Two-Phase Commit (Lamport 76, Gray 79) Assumes central coordinator Coordinator initiates protocol Participants: entities with actions to be committed/aborted Phase 1: Coordinator asks if participants can commit Participants respond yes/no
Phase 2: If all votes yes, coordinator sends Commit Otherwise send Abort Participants send Have Committed / Have Aborted Two-Phase Commit Data log Transaction CanCommit? CanCommit? Transaction component component Commit Commit Done Done
Yes Yes Coordinator Yes log Data Done Commit Transaction component CanCommit? Data Yes Done Commit
Transaction CanCommit? component Data log log Two-Phase Commit: Fault Tolerance Participant fails in Phase 1: Coordinator doesnt get unanimous yes Abort Participant fails in Phase 2: On reawakening, can ask coordinator if it should commit or abort Requires that both commit and abort states be
durable before sending yes vote Two-Phase Commit: Problems Blocks on failure Timeout before abort if participant fails All participants must wait for recovery if coordinator fails While blocked, transaction must remain Isolated Hold locks on data items touched Prevents other transactions from completing Solution: Wednesday
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