Two-Phase Commit Protocol Quiz
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Questions and Answers

The canCommit? operation is used by a coordinator to determine if a participant is ready to abort a transaction.

False (B)

Participants in a two-phase commit protocol respond to the coordinator with a yes vote if they are willing to commit the transaction.

True (A)

The doCommit operation is used by a participant to inform the coordinator that the participant has successfully completed the transaction.

False (B)

A haveCommitted operation is called by the coordinator to the participants in order to commit the transaction.

<p>False (B)</p> Signup and view all the answers

In the second phase of the two-phase commit protocol, if the coordinator receives a no vote or a failure from any participant, it immediately aborts all parts of the transaction.

<p>True (A)</p> Signup and view all the answers

If a participant votes 'yes' during the first phase of the protocol and then receives a doAbort message from the coordinator, the participant can ignore this message and commit the transaction anyway.

<p>False (B)</p> Signup and view all the answers

The two-phase commit protocol ensures that all participants will commit the transaction if they receive a doCommit message from the coordinator.

<p>True (A)</p> Signup and view all the answers

The getDecision operation is used by the coordinator to query all participants for their votes on a transaction.

<p>False (B)</p> Signup and view all the answers

At Step 2, it is not acceptable to abort if no commit decision is made.

<p>False (B)</p> Signup and view all the answers

A cohort must block until communication is re-established at Step 4.

<p>True (A)</p> Signup and view all the answers

A coordinator can collect all commit votes before deciding on an abort.

<p>False (B)</p> Signup and view all the answers

Blocking can occur if a coordinator crashes.

<p>True (A)</p> Signup and view all the answers

A subtransaction can only report its status to the top-level transaction.

<p>False (B)</p> Signup and view all the answers

If a parent transaction aborts, its subtransactions are required to abort.

<p>True (A)</p> Signup and view all the answers

The interface of a coordinator allows it to inquire about the status of its parent transaction.

<p>False (B)</p> Signup and view all the answers

A parent transaction must commit even if all of its subtransactions have provisionally committed.

<p>False (B)</p> Signup and view all the answers

Flashcards

Two-Phase Commit Protocol

A method for achieving consensus in distributed systems through two phases: voting and committing.

canCommit?()

A function called by the coordinator asking if a participant can commit a transaction.

doCommit()

A command sent by the coordinator to have participants commit their transactions.

doAbort()

A command sent by the coordinator to abort the transaction for participants who voted yes when a no is present.

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haveCommitted()

A confirmation call from participants to the coordinator verifying that they have committed the transaction.

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getDecision()

A function where participants ask the coordinator for transaction decision after voting.

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Phase 1: Voting Phase

The initial phase of the two-phase commit where participants vote on whether to commit the transaction.

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Phase 2: Completion Phase

The second phase where the coordinator acts based on the collected votes and sends commit or abort requests.

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TimeOut Protocol

A system to manage transaction commits and handle failures in distributed databases.

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Blocking

A situation where a cohort is unable to proceed due to lack of communication with the coordinator.

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Cohort Communication

Cohorts block until they can communicate with the coordinator again after a disruption.

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Nested Transactions

Transactions that can open subtransactions which depend on a parent transaction.

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Provisional Commit

A temporary state where a transaction indicates it may be committed but is still dependent on its parent.

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Status Inquiry

A request to check if a transaction is committed, aborted, or provisional.

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Parent-Child Relationship

In nested transactions, the parent's commitment status affects its subtransactions.

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Coordinator Role

The entity managing transactions and their statuses within a distributed system.

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Study Notes

Two-Phase Commit Protocol

  • The two-phase commit protocol is used for coordinating transactions across multiple participants (e.g., databases) ensuring that all participants either commit or abort the transaction.

  • Phase 1 (Voting Phase): The coordinator sends a canCommit? request to each participant to determine if it can commit the transaction. A participant votes Yes or No. If a participant votes No, the transaction immediately aborts. Participants preparing to vote Yes must first save the transaction's steps in permanent storage

  • Phase 2 (Completion Based on Vote): The coordinator gathers votes.

    • If all votes are Yes, the coordinator sends a doCommit request to each participant to commit their part of the transaction.

    • If any vote is No, or if the coordinator fails to gather all votes in a certain amount of time, the coordinator sends a doAbort request to participating nodes who voted Yes.

  • Participant Actions: Participants that voted Yes wait for a doCommit or doAbort request. Upon receiving it, they follow the instruction. To mark commit completion, they perform a haveCommitted call to the coordinator.

Timeout Protocol

  • This protocol addresses situations where a participant or coordinator might fail or become unresponsive.
  • If, after Phase 1 (voting phase), the coordinator hasn't received all the necessary votes before the timeout period expires, the coordinator proceeds to abort the transaction without completing Phase 2. This avoids indefinite blocking.

Coordinator Interface

  • haveCommitted(trans, participant): The participant calls this to confirm it successfully committed the transaction to the coordinator.

  • getDecision(trans): If a participant needs a response from the coordinator, but no reply is received after a reasonable time, it can call this method to get the coordinator's decision (Commit or Abort).

Blocking

  • Coordinator crashes: If there's a communications failure, or the coordinator crashes during the process, the participant will be blocked.

  • Communication problems among participants.

  • Participant failure.

Nested Transactions

  • A top-level transaction can contain subtransactions.

  • Subtransactions use a unique ID derived from their parent transaction's ID.

  • openSubTransaction(trans): Creates a new sub-transaction with an identifier.

  • getStatus(trans): Queries the status of a subtransaction (committed, aborted, provisional).

  • A subtransaction has the same life cycle (commit or abort) as its parent transaction.

  • A parent is allowed to commit even if some subtransactions abort. If a parent aborts, all its subtransactions must also abort.

Information Held by Coordinators

  • Coordinator keeps track of its subtransactions.
  • When a subtransaction is provisionally committed, it sends its status and its descendants' statuses to the parent coordinator.
  • If a subtransaction aborts, it informs its parent coordinator.

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Related Documents

Atomic Commit Protocols PDF

Description

Test your understanding of the Two-Phase Commit Protocol, a crucial method for coordinating transactions across multiple databases. The quiz covers details about the voting phase, completion based on votes, and participant actions. Validate your knowledge of how transactions are committed or aborted.

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