Distributed Transactions Concepts
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Questions and Answers

In the context of distributed transactions, what is the primary role of the coordinator?

  • To manage the client's requests and assign them to different servers.
  • To optimize data transfer between servers during the transaction.
  • To execute the transaction requests sequentially across all servers.
  • To ensure all servers either commit or abort the transaction consistently. (correct)

How do flat and nested distributed transactions differ in their execution?

  • Flat transactions involve multiple coordinators, while nested transactions have only one.
  • Flat transactions are initiated by the server, while nested transactions are initiated by the client.
  • Flat transactions execute each request completely before moving to the next, while nested transactions allow concurrent subtransactions. (correct)
  • Flat transactions allow parallel execution of requests, while nested transactions execute them sequentially.

What is a key advantage of using nested transactions over flat transactions in a distributed system?

  • Improved performance through parallel execution of subtransactions. (correct)
  • Increased fault tolerance by replicating transaction logs across all servers.
  • Reduced network traffic due to localized transaction management.
  • Simplified error handling due to fewer transaction states.

When a client initiates a distributed transaction, what is the first step that occurs?

<p>The client sends a request to a coordinator server to begin the transaction. (C)</p> Signup and view all the answers

What must a server do when it commits during a distributed transaction, and why?

<p>Communicate with other servers to coordinate actions and ensure atomicity. (B)</p> Signup and view all the answers

Which of the following is NOT a typical characteristic of a distributed transaction?

<p>Operates exclusively on a single server to ensure data locality. (B)</p> Signup and view all the answers

A distributed system uses a two-phase commit protocol. What potential issue does the coordinator address by implementing this protocol?

<p>Ensuring all servers commit or abort the transaction atomically. (A)</p> Signup and view all the answers

In which scenario would a nested transaction be most beneficial compared to a flat transaction in a distributed system?

<p>When the transaction involves multiple independent operations that can run concurrently. (D)</p> Signup and view all the answers

In a distributed transaction, what is the primary responsibility of the coordinator?

<p>Deciding whether to commit or abort the transaction at the end. (C)</p> Signup and view all the answers

What role does a 'participant' play in a distributed transaction?

<p>It keeps track of objects involved in the transaction and cooperates in the commit protocol. (C)</p> Signup and view all the answers

In the context of a distributed banking transaction involving multiple branches (servers), how does a participant join the transaction?

<p>By invoking the <code>join</code> method in the coordinator. (B)</p> Signup and view all the answers

In a distributed transaction, if a participant encounters an issue, what action can it take regarding the transaction?

<p>It can call <code>abortTransaction</code> in the coordinator. (D)</p> Signup and view all the answers

During a distributed banking transaction, what information is passed with each request made to different servers (e.g., BranchX, BranchY, BranchZ)?

<p>The transaction ID (TID). (D)</p> Signup and view all the answers

Consider a banking transaction involving accounts A, B, C, and D across different servers. Which of the following is NOT a typical responsibility of the participant at each server?

<p>Independently confirming the success of transactions at other servers. (B)</p> Signup and view all the answers

In the context of distributed transactions, what is the significance of the coordinator residing in one of the servers (e.g., BranchX)?

<p>It indicates that the server hosts the central transaction authority and manages the overall commit process. (D)</p> Signup and view all the answers

How does the use of a transaction ID (TID) contribute to ensuring the consistency and reliability of a distributed transaction?

<p>It facilitates tracking and correlating operations across different servers. (A)</p> Signup and view all the answers

In the two-phase commit protocol, what action does a participant take immediately after voting 'Yes' in Phase 1?

<p>Saves objects in permanent storage in preparation to commit. (B)</p> Signup and view all the answers

Under what condition does the coordinator decide to abort a transaction in the two-phase commit protocol?

<p>If there are failures or if not all participants vote 'Yes'. (C)</p> Signup and view all the answers

What message does the coordinator send to participants that voted 'Yes' if it decides to commit the transaction?

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

What action should a participant take if it voted 'Yes' and subsequently receives a 'doAbort' request from the coordinator?

<p>Abort the transaction. (B)</p> Signup and view all the answers

In the TimeOut Protocol, what is the immediate consequence if a cohort cannot communicate with the coordinator at step 4?

<p>The cohort must block until communication is re-established. (A)</p> Signup and view all the answers

Under the TimeOut Protocol, what options does the coordinator have if it doesn't collect all commit votes from participants?

<p>The coordinator will vote to 'abort' the transaction. (D)</p> Signup and view all the answers

According to the content, in which phase of the TimeOut Protocol is it permissible to abort?

<p>Steps 2 and 3. (C)</p> Signup and view all the answers

In what scenarios can blocking occur, causing delays in the completion of a distributed transaction?

<p>When the coordinator crashes or a cohort cannot communicate with the coordinator. (D)</p> Signup and view all the answers

What is the primary purpose of the join method in the Coordinator interface?

<p>To inform the coordinator about a new participant joining the transaction. (D)</p> Signup and view all the answers

In a two-phase commit protocol, what is the significance of a participant voting to commit in phase 1?

<p>The participant is prepared to commit and has saved updates in permanent storage, and cannot change its mind. (B)</p> Signup and view all the answers

What is a key limitation of the one-phase atomic commit protocol?

<p>Participants cannot unilaterally abort the transaction if they encounter issues like deadlocks or crashes. (A)</p> Signup and view all the answers

Which of the following scenarios would necessitate a participant to call the getDecision(trans) method in the two-phase commit protocol?

<p>The participant has crashed and restarted, needing to determine the transaction's outcome. (B)</p> Signup and view all the answers

In the two-phase commit protocol, what action does the coordinator take after receiving 'Yes' votes from all participants during the voting phase?

<p>The coordinator records the votes and proceeds to the next phase, but is not explicitly described in this text. (D)</p> Signup and view all the answers

If a participant in a two-phase commit protocol crashes after voting 'Yes' but before receiving the doCommit or doAbort message, what action should it take upon recovery?

<p>Call the <code>getDecision(trans)</code> method on the Coordinator to determine the transaction's outcome. (B)</p> Signup and view all the answers

What is the purpose of the haveCommitted(trans, participant) call in the two-phase commit protocol?

<p>To enable the participant to confirm to the coordinator that it has successfully committed the transaction. (D)</p> Signup and view all the answers

In the context of distributed transactions and the two-phase commit protocol, what does it mean for a participant to be 'prepared'?

<p>The participant has saved all updates in permanent storage and is ready to commit if instructed. (B)</p> Signup and view all the answers

How can a client finalize a series of nested transactions?

<p>By calling <code>closeTransaction</code> or <code>abortTransaction</code> in the top-level transaction. (D)</p> Signup and view all the answers

What does the Coordinator return status provisional indicate about a transaction?

<p>The transaction's fate depends on the decision of its parent transaction. (B)</p> Signup and view all the answers

What is the relationship between a subtransaction's TID and its parent's TID?

<p>The subtransaction's TID is derived from, and is an extension of, its parent's TID. (C)</p> Signup and view all the answers

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

<p>The subtransactions must also abort. (B)</p> Signup and view all the answers

In a nested transaction scenario, when does a subtransaction typically start and finish relative to its parent?

<p>The subtransaction starts after its parent and finishes before it. (B)</p> Signup and view all the answers

Transaction T1 has a subtransaction T11. T1 provisionally commits, but later T11 aborts. What is the final outcome for T1?

<p>T1 remains committed, as a parent can commit even if a subtransaction aborts. (D)</p> Signup and view all the answers

What is the primary purpose of the openSubTransaction operation in a transactional coordinator?

<p>To initiate a new subtransaction within an existing transaction. (A)</p> Signup and view all the answers

Consider a scenario where transaction T has two subtransactions, T1 and T2. T1 provisionally commits while T2 aborts. Later, the top-level transaction also provisionally commits. What is the likely final outcome for T1?

<p>T1's fate is still dependent on the final decision of the top-level transaction, despite its provisional commit. (D)</p> Signup and view all the answers

Flashcards

Distributed Transaction

A transaction accessing objects managed by multiple servers.

Atomicity in Distributed Transactions

All servers either commit the transaction, or all abort the transaction.

Coordinator Role

Ensures the same outcome (commit or abort) at all servers in a distributed transaction.

Nested Transaction

A transaction that can open subtransactions, which can further open subtransactions.

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Concurrency in Nested Transactions

Subtransactions at the same level can run concurrently, increasing efficiency.

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Flat Transaction

A transaction that completes each request before moving to the next one.

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Server Communication (Commit)

Servers communicate to coordinate their actions when committing a distributed transaction.

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Transaction Initiation

A client sends a request to a coordinator on any server to begin a transaction.

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Transaction ID (TID)

Uniquely identifies a distributed transaction across multiple servers.

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Transaction Coordinator

The server responsible for initiating and managing the distributed transaction, ensuring atomicity across all participants.

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Transaction Participant

A server that manages objects (e.g., database records) accessed by a distributed transaction and participates in the commit protocol.

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Transaction Join

The process where a server informs the transaction coordinator that it's involved in a particular distributed transaction.

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Commit or Abort

A method called to either permanently apply the changes made during the transaction or to undo them, ensuring data consistency.

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Flat Distributed Transaction

A type of distributed transaction where all operations occur at the same level, without nested sub-transactions.

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openTransaction

Requesting the start of a distributed transaction, typically initiated by a client application.

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closeTransaction

Requesting the completion of a distributed transaction, triggering the commit or abort process.

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Join Operation

Used when a new participant joins a transaction, informing the coordinator of the new member.

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Coordinator Interface

An interface with openTransaction, closeTransaction, abortTransaction methods, used by servers to track which transaction is accessing objects.

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join(Trans, participant)

A method in the Coordinator interface used to inform the coordinator that a new participant has joined the transaction.

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Coordinator/Participant Knowledge

All coordinators know all participants and each participant knows the coordinator, will enable them to collect the information that will be needed at commit time.

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One-Phase Atomic Commit

The coordinator instructs participants to either commit or abort.

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Two-Phase Atomic Commit

Allows participants to abort if needed, with phase 1 for voting and phase 2 for executing the decision.

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canCommit?(trans)

Queries whether a participant can commit; the participant responds with its vote (Yes/No).

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getDecision(trans)

Call from participant to coordinator to ask for transaction decision after voting Yes, used after delays or crashes.

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Atomic Commit Protocol

A protocol ensuring all or none of the servers commit a distributed transaction.

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Two-Phase Commit (2PC)

A distributed atomic commitment protocol in which all processes cooperate to achieve agreement on whether to commit or abort a transaction.

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2PC: Phase 1

Phase 1 involves a participant of a 2PC replying with its vote (Yes or No) to the coordinator. Before voting Yes, it prepares to commit.

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2PC: Phase 2

The coordinator collects the votes. If all votes are Yes, the coordinator decides to commit; otherwise, it aborts.

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TimeOut Protocol - Abort

A commit decision is not made; it's OK to abort the transaction.

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TimeOut Protocol - Blocking

Cohort is unable to communicate with coordinator so the cohort must block, until communication is re-established.

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Blocking

A situation in a distributed transaction where a process must wait indefinitely for some event (e.g., a message).

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Blocking Occurrence

Coordinator crashes or cohort cannot communicate with coordinator.

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doAbort

Ends a nested transaction, which can either commit or abort all changes made.

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Subtransaction TID

An extension of its parent's TID. Allows each subtransaction to trace back to the top-level transaction.

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openSubTransaction(trans)

Opens a new subtransaction under a parent transaction, returning a unique identifier.

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getStatus(trans)

Reports the status of a transaction, which could be committed, aborted, or provisional.

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Subtransaction Start

A subtransaction starts executing its instructions after its parent transaction has begun.

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Subtransaction Finish

A subtransaction must finish its instruction execution before the parent transaction completes.

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Parent Commit, Sub Abort

A parent transaction can provisionally commit even if a subtransaction aborts.

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Parent Abort, Sub Abort

If a transaction aborts, then all of its subtransactions must also abort.

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Study Notes

  • Distributed transactions can be flat or nested, and they access objects managed by multiple servers.
  • When a distributed transaction ends, either all servers commit or all abort the transaction, maintaining atomicity.
  • One server acts as the coordinator, ensuring the same outcome across all servers.

Flat vs. Nested Transactions

  • In nested transactions, a top-level transaction can open subtransactions, which can further open subtransactions to any depth.
  • Flat client transactions complete each request sequentially, accessing servers' objects in order.
  • Nested transactions allow subtransactions at the same level to run concurrently, increasing efficiency by enabling parallel execution on several servers.

Coordinator Role

  • Servers must communicate with each other during the commit process to coordinate actions.
  • A client initiates a transaction by sending a request to a coordinator on any server, which returns a unique Transaction ID (TID).
  • The coordinator is responsible for committing or aborting the transaction.
  • Each server managing accessed objects becomes a participant, joining the transaction.
  • Participants keep track of involved objects and cooperate with the coordinator for the commit protocol.
  • Participants can request the coordinator to abort the transaction.

Banking Transaction Example

  • A client's banking transaction involving accounts on multiple servers.
  • The client sends an openTransaction request to the coordinator
  • Each server's participant joins the transaction and TID is passed with each server, using the join method in the coordinator.

Join Operation

  • The coordinator interface includes openTransaction, closeTransaction, and abortTransaction methods.
  • openTransaction returns a Transaction ID(TID) passed with each operation, identifying the accessed servers.
  • The join method is for new participants (join(Trans, reference to participant))
  • The coordinator records the new participant in its participant list.
  • Knowing all participants helps collect necessary information at commit time.

Atomic Commit Protocols

One-Phase Commit Protocol

  • The coordinator instructs participants to commit or abort.
  • Drawback: No server can independently abort if a deadlock is detected or if the server has crashed and restarted.

Two-Phase Commit Protocol

  • Participants can choose to abort.
  • Phase 1 involves participants voting to commit, becoming "prepared" and saving updates in permanent storage.
  • Phase 2 involves participants carrying out the joint decision.

Two-Phase Commit Protocol Operations

  • Coordinator sends a canCommit? request to the participant, that then responds with a Yes or No vote for the transaction.
  • Coordinator sends a doCommit or doAbort request to the participant, telling them whether to commit or abort.
  • Participants then confirm by sending haveCommitted to the coordinator, that they have committed the transaction.
  • Participants then send a getDecision request to the coordinator, to ascertain if that is the case.

Two-Phase Commit Protocol Step-by-Step

Phase 1 (Voting)

  • The coordinator sends a canCommit? request to all participants.
  • Participants respond with Yes or No, saving objects if voting Yes.

Phase 2 (Completion)

  • The coordinator collects votes.
  • If all votes are Yes, the coordinator sends a doCommit request. Otherwise, it sends a doAbort request to participants who voted Yes.
  • Participants then apply doCommit or doAbort request and confirm to the coordinator

TimeOut Protocol

  • If a commit decision is not reached in the TimeOut protocol, then it can be aborted.
  • If there is a Time Out, then the cohort must block until there is either a re-establishment of communication or ask the other cohorts what to do

Blocking Conditions

  • Blocking can occur if the coordinatoor crashes or the cohort cannot communicate with the coordinatoor, in between the phases.

Nested Transactions Operations

openSubTransaction(trans) -> subTrans

  • Opens a subtransaction with the specified parent, returning a unique subtransaction identifier.

getStatus(trans) -> committed, aborted, provisional

  • Asks about the status of a subtransaction.

Coordinator Interface

  • Supports opening further subtransactions.
  • Allows subtransactions to inquire about their status.

Client Operations

  • Starts with openTransaction to open a top-level transaction, returning a Transaction ID(TID).
  • This Transaction ID(TID) is utilized to open subtransactions
  • A subtransaction automatically joins its parent and a Transaction ID(TID) gets returned

Transaction Tree and Commit Decisions

  • The fate of a subtransaction depends on its parent and the top-level transaction.
  • A parent can commit even if a subtransaction aborts.
  • If a parent aborts, all its subtransactions must abort.

Coordinator Information in Nested Transactions

  • Each coordinator maintains a list of its subtransactions.
  • At provisional commit, a subtransaction reports its status and its descendants' status to its parent.
  • Aborted subtransactions notify their parent.

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Explore distributed transactions, including the role of the coordinator, differences between flat and nested transactions, and the two-phase commit protocol. Understand how servers commit and the benefits of nested transactions. Learn about the coordinator's responsibilities and the participant's role.

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