This document discusses distributed transactions and the challenges of ensuring they satisfy the ACID properties of atomicity, consistency, isolation and durability even when transactions span multiple systems. It introduces the two-phase commit protocol, where a coordinator first polls participants if they can commit and then tells them to either commit or abort, addressing failures through durable logs and timeouts. While two-phase commit ensures all or nothing completion, it risks long blocks if the coordinator or participants fail.

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Distributed Transaction

• 1.
  CS 603 DistributedTransactions February 18, 2002
• 2.
  What is aTransaction? • 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.
• 3.
  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
• 4.
  What is aDistributed Transaction? Data Transaction component Transaction component Data Data Transaction component Transaction component Data
• 5.
  Why are DistributedTransactions 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
• 6.
  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?
• 7.
  Committing a DistributedTransaction Data Transaction component log Transaction component Data log Data Transaction component log Transaction component Data log
• 8.
  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
• 9.
  Two-Phase Commit DataTransaction component log Done Done Done Transaction component Data log Data Transaction component log Done Transaction component Data log CanCommit? CanCommit? Coordinator Commit CanCommit? CanCommit? Yes Yes Yes Yes Commit Commit Commit
• 10.
  Two-Phase Commit: FaultTolerance • Participant fails in Phase 1: – Coordinator doesn’t 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
• 11.
  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