Two-Phase Commit Protocol Overview

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Questions and Answers

If a cohort cannot communicate with the coordinator at Step 4 of TimeOut Protocol, what should it do?

  • Block until communication is re-established. (correct)
  • Request a new coordinator from other cohorts.
  • Commit the transaction and proceed to the next step.
  • Immediately abort the transaction.

What is the purpose of the openSubTransaction(trans) operation in nested transactions?

  • To abort the parent transaction.
  • To check the status of a subtransaction.
  • To open a new subtransaction whose parent is `trans` and return a unique identifier. (correct)
  • To create a new subtransaction that is independent of the parent.

What are the possible outcomes of a transaction using the getStatus(trans) operation?

  • Completed, running, stopped.
  • Successful, failed, pending.
  • Committed, aborted, provisional. (correct)
  • Committed, aborted, provisional, in progress.

How does the OpenTransaction operation relate to the openSubTransaction operation in nested transactions?

<p>The <code>OpenTransaction</code> operation creates a top-level transaction, while <code>openSubTransaction</code> creates a subtransaction under the top-level transaction. (C)</p> Signup and view all the answers

Which of the following is a reason for blocking in a TimeOut Protocol?

<p>A cohort cannot communicate with the coordinator. (C)</p> Signup and view all the answers

What is the role of the coordinator in nested transactions?

<p>To manage the commit and abort decisions for all subtransactions. (B)</p> Signup and view all the answers

If a subtransaction aborts, what happens to its parent transaction?

<p>The parent transaction can still commit if it's not dependent on the subtransaction. (C)</p> Signup and view all the answers

How does a subtransaction inform its parent about its commit or abort status?

<p>By sending a message to the parent coordinator. (C)</p> Signup and view all the answers

Which of the following actions does a participant take after receiving a 'canCommit?' request?

<p>The participant prepares to commit by saving objects in permanent storage. (B)</p> Signup and view all the answers

What is the purpose of the 'getDecision' call in the Two-Phase Commit Protocol?

<p>To allow a participant to request the decision on a transaction after it has voted 'Yes' but has not received a response from the coordinator. (D)</p> Signup and view all the answers

In the Two-Phase Commit Protocol, what does the coordinator do if it receives at least one 'No' vote from the participants?

<p>The coordinator sends a 'doAbort' request to all participants that voted 'Yes'. (B)</p> Signup and view all the answers

What is the primary function of the 'haveCommitted' call in the Two-Phase Commit Protocol?

<p>To confirm to the coordinator that a participant has successfully committed its part of the transaction. (C)</p> Signup and view all the answers

What is the purpose of Phase 1 in the Two-Phase Commit Protocol?

<p>To allow participants to prepare for the transaction and vote on whether to commit. (B)</p> Signup and view all the answers

Which of the following is considered a key advantage of the Two-Phase Commit Protocol?

<p>It ensures that all participating nodes commit to the transaction, preventing inconsistencies. (D)</p> Signup and view all the answers

What is the main purpose of the 'canCommit?' request in the first phase of the Two-Phase Commit Protocol?

<p>To gather votes from the participants regarding the transaction. (D)</p> Signup and view all the answers

In the Two-Phase Commit Protocol, what happens if a participant votes 'No' in the first phase?

<p>The participant aborts the transaction immediately. (B)</p> Signup and view all the answers

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Flashcards

TimeOut Protocol

A protocol used in managing commit decisions in distributed transactions.

Abort Decision

A decision made when a coordinator votes for abort instead of commit.

Blocking

A state where a cohort cannot progress due to communication issues with the coordinator.

Coordinator Responsibilities

The coordinator manages communication and commit decisions for transactions.

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Nested Transactions

Transactions that can have subtransactions allowing hierarchical commits.

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Provisional Commit

An intermediate state where a transaction is tentatively committed but not finalized.

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Transaction Identifier (TID)

A unique identifier assigned to a top-level or subtransaction.

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Parent-Subtransaction Relationship

A parent transaction can commit independently of subtransactions, but their fate is linked upon abort.

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canCommit?

A request from the coordinator to ask if a participant can commit a transaction.

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doCommit

An instruction from the coordinator to a participant to commit to the transaction.

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doAbort

An instruction from the coordinator to a participant to abort the transaction.

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haveCommitted

A confirmation from the participant to the coordinator that it has committed the transaction.

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getDecision

A request from a participant to the coordinator for the decision on a transaction after voting Yes.

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Voting phase

The first phase of the Two-Phase Commit Protocol where participants cast votes on committing.

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Completion phase

The second phase in Two-Phase Commit, where the coordinator acts based on collected votes.

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Transaction outcome

The result of the voting phase in a Two-Phase Commit, either commit or abort.

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Study Notes

Two-Phase Commit Protocol

  • The protocol ensures atomicity in distributed transactions.
  • Two phases: voting and completion.
  • Phase 1 (Voting): Coordinator requests each participant if it can commit the transaction. Each participant confirms or rejects. If any participant rejects, the coordinator aborts immediately.
  • Phase 2 (Completion): Based on the votes, the coordinator decides to commit or abort. If all participants voted yes, it commits; otherwise, aborts. The coordinator instructs all participants to follow the decision.

Participant Interface

  • canCommit?(trans): The coordinator queries a participant whether it can commit the transaction. The participant responds with "Yes" or "No".
  • doCommit(trans): The coordinator requests the participant to commit its part of the transaction.
  • doAbort(trans): The coordinator requests the participant to abort its part of the transaction.

Coordinator Interface

  • haveCommitted(trans, participant): A participant notifies the coordinator that it has committed the transaction.
  • getDecision(trans): Indicates to ask the coordinator for a decision on a transaction. This is useful for recovery if the coordinator doesn't receive a response due to a crash or delay.

Time-Out Protocol

  • If the coordinator doesn't receive a response from a participant during the voting phase, it assumes a failure and aborts the transaction.

Nested Transactions

  • Subtransactions can be nested within a top-level transaction.
  • The transaction identifier (TID) of a subtransaction extends its parent.
  • A client finishes nested transactions by calling closeTransaction or abortTransaction.
  • The coordinator manages subtransactions, receiving commit or abort requests from them.

Coordinator Information

  • Coordinators keep track of their subtransactions.
  • During a provisional commit, subtransactions report their state and descendants to the parent.
  • If a subtransaction aborts, it informs its parent.

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