Distributed Transactions

• Transaction may access data at several sites.

• Each site hasalocal transaction manager responsible for:

  • Maintaining a log for recovery purposes

  • Participating in coordinating the concurrent execution of the transactions executing at that site.

• Each site has a transaction coordinator, which is responsible for:

  • Starting the execution of transactions that originate at the site.
  • Distributing subtransactions at appropriate sites for execution.
  • Coordinating the termination of each transaction that originates at the site, which may result in the transaction being committed at all sites or aborted at all sites.

System Failure Modes

• Failures unique to distributed systems:
  • Failure ofasite.
  • Loss of massages
    • Handled by network transmission control protocols such as TCP-IP
  • Failure ofacommunication link
  • Handled by network protocols, by routing messages via alternative links
  • Network partition
    • A network is said to be partitioned when it has been split into two or more subsystems that lack any connection between them
      • Note:asubsystem may consist ofasingle node
• Network partitioning and site failures are generally indistinguishable.

Commit Protocols

• Commit protocols are used to ensure atomicity across sites

  • A transaction which executes at multiple sites must either be committed at all the sites, or aborted at all the sites.

  • not acceptable to haveatransaction committed at one site and aborted at another

• The two-phase commit (2PC) protocol is widely used

• The three-phase commit (3PC) protocol is more complicated and more expensive, but avoids some drawbacks of two-phase commit protocol. This protocol is not used in practice.

Three Phase Commit (3PC)

• Assumptions:

  • No network partitioning
  • At any point, at least one site must be up.
  • At most K sites (participants as well as coordinator) can fail

• Phase 1: Obtaining Preliminary Decision: Identical to 2PC Phase 1.

  • Every site is ready to commit if instructed to do so.

• Phase2 of 2PC is split into 2 phases, Phase 2 and Phase 3 of 3PC

  • In phase 2 coordinator makes a decision as in 2PC (called the pre-commit decision) and records it in multiple (at least K) sites
    • In phase 3, coordinator sends commit/abort message to all participating sites,
• Under 3PC, knowledge of pre-commit decision can be used to commit despite coordinator failure
  • Avoids blocking problem as long as<Ksites fail</li>
• Drawbacks:
  • higher overheads
  • assumptions may not be satisfied in practice