Multi-key transactions for consistency in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a multi-key transaction in Redis?

A multi-key transaction in Redis is a way to execute multiple commands on different keys as a single atomic operation, ensuring all commands succeed or none do.

beginner

Which Redis commands start and execute a multi-key transaction?

The MULTI command starts a transaction, and EXEC executes all queued commands atomically.

intermediate

How does Redis ensure consistency when multiple keys are involved in a transaction?

Redis queues all commands after MULTI and runs them together with EXEC. If any watched key changes, the transaction aborts to ensure consistency.

intermediate

What role does the WATCH command play in multi-key transactions?

WATCH monitors one or more keys for changes. If any watched key changes before EXEC, the transaction aborts to prevent inconsistent updates.

advanced

Can Redis transactions be rolled back automatically if a command fails?

No, Redis transactions do not support automatic rollback. All commands are queued and executed atomically, but if a command fails during execution, previous commands are not undone.

Which command starts a multi-key transaction in Redis?

A EXEC

B MULTI

C WATCH

D DISCARD

MULTI starts the transaction by queuing commands.

What happens if a watched key changes before EXEC is called?

A Redis automatically retries the transaction

B The transaction continues normally

C The transaction aborts and EXEC returns null

D The watched key is reset

If a watched key changes, Redis aborts the transaction to avoid inconsistent updates.

Which command executes all queued commands in a Redis transaction?

A EXEC

B DISCARD

C WATCH

D MULTI

EXEC runs all commands queued after MULTI atomically.

Can Redis transactions include commands on multiple keys?

A Yes, Redis supports multi-key transactions

B No, transactions are limited to one key

C Only if keys are in the same hash slot

D Only with Lua scripts

Redis transactions can include commands on multiple keys, executed atomically.

What does the DISCARD command do in Redis transactions?

A Starts watching keys

B Executes the queued commands

C Locks keys for the transaction

D Cancels all queued commands and ends the transaction

DISCARD cancels the transaction and clears queued commands.

Explain how Redis uses MULTI, EXEC, and WATCH commands to maintain consistency in multi-key transactions.

Think about how Redis prevents conflicts when multiple clients update keys.

Describe the limitations of Redis transactions regarding rollback and error handling.

Consider what happens if a command inside EXEC fails.

Practice

1. What is the main purpose of using MULTI and EXEC commands in Redis transactions?

easy

A. To group multiple commands so they execute atomically for data consistency

B. To speed up the execution of individual Redis commands

C. To create backups of the Redis database automatically

D. To delete multiple keys at once

Solution

1. Step 1: Understand MULTI command

   The MULTI command starts a transaction block in Redis, grouping commands.

2. Step 2: Understand EXEC command

   The EXEC command executes all commands queued after MULTI atomically, ensuring consistency.

3. Final Answer:

   To group multiple commands so they execute atomically for data consistency -> Option A

4. Quick Check:

   Atomic execution = Data consistency [OK]

Hint: MULTI starts, EXEC runs all commands atomically [OK]

Common Mistakes:

• Thinking MULTI speeds up commands individually
• Confusing transactions with backups
• Assuming EXEC deletes keys

2. Which of the following is the correct syntax to start a Redis transaction?

easy

A. EXEC

B. START

C. BEGIN

D. MULTI

Solution

1. Step 1: Identify Redis transaction start command

   Redis uses MULTI to begin a transaction block.

2. Step 2: Check other options

   EXEC ends the transaction, BEGIN and START are not Redis commands.

3. Final Answer:

   MULTI -> Option D

4. Quick Check:

   Start transaction = MULTI [OK]

Hint: Start with MULTI, end with EXEC [OK]

Common Mistakes:

• Using EXEC to start instead of end
• Using SQL-like BEGIN or START commands
• Confusing transaction commands with key commands

3. Consider the following Redis commands executed in a transaction:

MULTI
SET key1 "value1"
SET key2 "value2"
EXEC

What will be the state of key1 and key2 after EXEC?

medium

A. key1 and key2 will both have their new values set atomically

B. Only key1 will be set, key2 will remain unchanged

C. Neither key1 nor key2 will be set because EXEC fails

D. key1 will be set, but key2 will be deleted

Solution

1. Step 1: Understand MULTI and EXEC behavior

   Commands between MULTI and EXEC are queued and executed atomically.

2. Step 2: Analyze the commands

   Both SET key1 "value1" and SET key2 "value2" will be applied together when EXEC runs.

3. Final Answer:

   key1 and key2 will both have their new values set atomically -> Option A

4. Quick Check:

   Atomic set of keys = Both keys updated [OK]

Hint: EXEC applies all queued commands together [OK]

Common Mistakes:

• Thinking commands run immediately after MULTI
• Assuming partial execution on EXEC failure
• Confusing EXEC with DISCARD

4. You wrote this Redis transaction:

MULTI
SET key1 "val1"
GET key1
EXEC

But the GET key1 command returns null inside the transaction. Why?

medium

A. SET command failed, so GET returns null

B. GET command syntax is incorrect

C. GET inside MULTI queues the command and returns null until EXEC runs

D. EXEC was not called, so commands did not run

Solution

1. Step 1: Understand command queuing in MULTI

   Inside a transaction, commands are queued and do not execute immediately.

2. Step 2: Effect on GET command

   GET returns null because the SET has not executed yet; actual results come after EXEC.

3. Final Answer:

   GET inside MULTI queues the command and returns null until EXEC runs -> Option C

4. Quick Check:

   Queued commands return null before EXEC [OK]

Hint: GET returns null inside MULTI before EXEC [OK]

Common Mistakes:

• Thinking GET runs immediately inside MULTI
• Assuming SET failed without checking
• Forgetting to call EXEC

5. You want to update two related keys account:balance and account:points atomically in Redis. Which approach ensures both keys update together or not at all?

hard

A. Use GET to check keys, then SET commands separately

B. Use MULTI, queue both SET commands, then EXEC to apply atomically

C. Use WATCH on both keys, then run SET commands without MULTI

D. Run two separate SET commands without MULTI

Solution

1. Step 1: Understand atomic updates need transactions

   To update multiple keys together, Redis transactions with MULTI and EXEC ensure atomicity.

2. Step 2: Analyze other options

   Separate SET commands risk partial updates; WATCH helps optimistic locking but needs MULTI for atomicity.

3. Final Answer:

   Use MULTI, queue both SET commands, then EXEC to apply atomically -> Option B

4. Quick Check:

   Atomic multi-key update = MULTI + EXEC [OK]

Hint: MULTI + EXEC ensures all-or-nothing updates [OK]

Common Mistakes:

• Running SET commands separately risking partial updates
• Using WATCH without MULTI for atomicity
• Relying on GET before SET without transaction