Distributed transactions and 2PC in DBMS Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a distributed transaction in SQL.

DBMS Theory

BEGIN [1] TRANSACTION;

Drag options to blanks, or click blank then click option'

ALOCAL

BBATCH

CSIMPLE

DDISTRIBUTED

Attempts:

3 left

💡 Hint

Common Mistakes

Using LOCAL instead of DISTRIBUTED starts a transaction only on one database.

Using SIMPLE or BATCH are not valid transaction types for distributed transactions.

The DISTRIBUTED keyword is used to start a distributed transaction that spans multiple databases or nodes.

2fill in blank

medium

Complete the code to prepare a transaction in the two-phase commit protocol.

DBMS Theory

PREPARE [1];

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ATRANSACTION

BCOMMIT

CROLLBACK

DSAVEPOINT

Attempts:

3 left

💡 Hint

Common Mistakes

Using PREPARE COMMIT is incorrect; COMMIT is a separate command.

ROLLBACK and SAVEPOINT are unrelated to the prepare phase.

In the two-phase commit protocol, the PREPARE TRANSACTION command is used to prepare the transaction for commit.

3fill in blank

hard

Fix the error in the two-phase commit SQL command to finalize the transaction.

DBMS Theory

COMMIT [1];

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ATRANSACTION

BWORK

CPREPARE

DSAVEPOINT

Attempts:

3 left

💡 Hint

Common Mistakes

Using COMMIT TRANSACTION is not standard in some DBMS for 2PC.

COMMIT PREPARE is invalid syntax.

SAVEPOINT is unrelated to commit.

The correct command to finalize a prepared transaction is COMMIT WORK. This commits the transaction that was previously prepared.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps transaction IDs to their status if the status is 'prepared'.

DBMS Theory

{ [1]: [2] for [1] in transactions if transactions[[1]] == 'prepared' }

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Atx_id

Bstatus

Ctransaction

Dstate

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for key and loop variable causes errors.

Using 'status' without defining it in the loop is incorrect.

The variable tx_id is used as the key and state as the value in the dictionary comprehension. The comprehension filters transactions with status 'prepared'.

5fill in blank

hard

Fill all three blanks to write a SQL query that selects transaction IDs, their status, and the coordinator node from a distributed transaction log where status is 'committed'.

DBMS Theory

SELECT [1], [2], [3] FROM transaction_log WHERE status = 'committed';

Drag options to blanks, or click blank then click option'

Atransaction_id

Bstatus

Ccoordinator_node

Dparticipant_node

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting participant_node instead of coordinator_node changes the meaning.

Omitting status column loses transaction state information.

The query selects transaction_id, status, and coordinator_node columns to show committed transactions and their coordinator.

Practice

1. What is the main purpose of the Two-Phase Commit (2PC) protocol in distributed transactions?

easy

A. To ensure all participating systems agree to commit or abort a transaction

B. To speed up transaction processing by skipping checks

C. To allow partial commits in case of failures

D. To encrypt data during transaction processing

Solution

1. Step 1: Understand the role of 2PC in distributed systems

   2PC coordinates multiple systems to either all commit or all abort a transaction, ensuring consistency.

2. Step 2: Analyze the options

   Only To ensure all participating systems agree to commit or abort a transaction correctly describes 2PC's goal of agreement before finalizing changes.

3. Final Answer:

   To ensure all participating systems agree to commit or abort a transaction -> Option A

4. Quick Check:

   2PC ensures agreement = To ensure all participating systems agree to commit or abort a transaction [OK]

Hint: 2PC means all or nothing commit agreement [OK]

Common Mistakes:

• Thinking 2PC speeds up transactions
• Believing partial commits are allowed
• Confusing 2PC with encryption

2. Which of the following is the correct sequence of phases in the Two-Phase Commit protocol?

easy

A. Commit phase followed by Prepare phase

B. Commit phase only

C. Abort phase followed by Prepare phase

D. Prepare phase followed by Commit phase

Solution

1. Step 1: Recall the 2PC phases

   The protocol first asks participants to prepare (vote), then commits if all agree.

2. Step 2: Match phases to options

   Prepare phase followed by Commit phase correctly lists Prepare phase first, then Commit phase.

3. Final Answer:

   Prepare phase followed by Commit phase -> Option D

4. Quick Check:

   2PC phases = Prepare then Commit [OK]

Hint: Prepare before commit in 2PC sequence [OK]

Common Mistakes:

• Reversing the order of phases
• Ignoring the Prepare phase
• Thinking Commit happens alone

3. Consider a distributed transaction using 2PC with three participants: P1, P2, and P3. If P1 and P2 vote to commit but P3 votes to abort during the Prepare phase, what will be the final outcome?

medium

A. Only P1 and P2 commit, P3 aborts

B. All participants abort the transaction

C. All participants commit the transaction

D. Transaction is left in uncertain state

Solution

1. Step 1: Understand voting in 2PC Prepare phase

   All participants must vote to commit for the transaction to proceed; any abort vote causes abort.

2. Step 2: Apply voting results

   Since P3 votes to abort, the coordinator instructs all to abort to keep data consistent.

3. Final Answer:

   All participants abort the transaction -> Option B

4. Quick Check:

   Any abort vote = all abort [OK]

Hint: One abort vote cancels entire transaction [OK]

Common Mistakes:

• Assuming partial commits are allowed
• Thinking transaction stays uncertain
• Ignoring abort votes

4. A distributed transaction using 2PC is stuck indefinitely in the Commit phase. What is the most likely cause of this problem?

medium

A. A participant failed to send its vote during the Prepare phase

B. All participants voted to abort during Prepare phase

C. The coordinator crashed after sending Commit messages but before receiving acknowledgments

D. The transaction was never started

Solution

1. Step 1: Identify causes of blocking in Commit phase

   If the coordinator crashes after sending Commit but before acknowledgments, participants wait indefinitely.

2. Step 2: Analyze options

   The coordinator crashed after sending Commit messages but before receiving acknowledgments matches this scenario; other options relate to earlier phases or no transaction.

3. Final Answer:

   The coordinator crashed after sending Commit messages but before receiving acknowledgments -> Option C

4. Quick Check:

   Coordinator crash during Commit causes blocking [OK]

Hint: Coordinator crash after commit message causes blocking [OK]

Common Mistakes:

• Confusing Prepare phase failures with Commit phase blocking
• Assuming abort votes cause commit blocking
• Ignoring coordinator role

5. In a distributed system using 2PC, how can the protocol be improved to avoid the blocking problem caused by coordinator failure during the Commit phase?

hard

A. Use a Three-Phase Commit protocol that adds a pre-commit phase

B. Skip the Prepare phase to speed up commits

C. Allow participants to commit independently without coordinator

D. Increase the timeout for participant responses

Solution

1. Step 1: Understand blocking in 2PC

   2PC can block if coordinator fails after sending commit but before acknowledgments.

2. Step 2: Identify protocol improvements

   Three-Phase Commit adds a pre-commit phase to reduce blocking by ensuring participants can safely decide without coordinator.

3. Final Answer:

   Use a Three-Phase Commit protocol that adds a pre-commit phase -> Option A

4. Quick Check:

   3PC adds pre-commit to avoid blocking [OK]

Hint: 3PC adds pre-commit phase to prevent blocking [OK]

Common Mistakes:

• Skipping Prepare phase breaks consistency
• Allowing independent commits causes inconsistency
• Just increasing timeout doesn't fix blocking