MULTI command to start in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the MULTI command do in Redis?

The MULTI command starts a transaction block in Redis. It queues multiple commands so they can be executed together atomically with EXEC.

beginner

How do you execute all commands queued after MULTI in Redis?

You use the EXEC command to execute all the commands queued after MULTI as a single atomic transaction.

intermediate

What happens if you call DISCARD after MULTI in Redis?

DISCARD cancels the transaction and clears all queued commands after MULTI without executing them.

intermediate

Can commands inside a MULTI block in Redis fail individually?

No, commands are queued and only executed together with EXEC. If one command fails during execution, the whole transaction still runs but errors are reported per command.

beginner

Why use MULTI in Redis instead of running commands one by one?

Using MULTI ensures commands run atomically, so no other client can run commands in between. This keeps data consistent and avoids race conditions.

What is the first command to start a transaction in Redis?

A EXEC

B WATCH

C DISCARD

D MULTI

MULTI starts the transaction block by queuing commands.

Which command executes all queued commands after MULTI?

A MULTI

B DISCARD

C EXEC

D FLUSH

EXEC runs all commands queued after MULTI atomically.

What does DISCARD do in a Redis transaction?

A Starts a transaction

B Cancels the transaction and clears queued commands

C Locks keys for transaction

D Executes queued commands

DISCARD cancels the transaction and clears all queued commands.

If a command inside MULTI fails, what happens?

A All commands run, errors reported per command

B Only the failed command is skipped

C The whole transaction is aborted

D Redis crashes

All commands run atomically; errors are reported individually.

Why use MULTI in Redis?

A To run commands atomically

B To delete keys

C To speed up commands

D To monitor commands

MULTI ensures commands run atomically without interference.

Explain how the MULTI command works in Redis and how it relates to EXEC and DISCARD.

Think about starting, running, or canceling a group of commands.

Why is atomicity important in Redis transactions started with MULTI?

Consider what happens if commands run one by one without control.

Practice

1.

What does the MULTI command do in Redis?

easy

A. Immediately executes all commands

B. Starts a transaction block to queue commands

C. Deletes all keys in the database

D. Shows the current database size

Solution

1. Step 1: Understand the purpose of MULTI

   The MULTI command begins a transaction block where commands are queued instead of executed immediately.

2. Step 2: Compare with other options

   The other options describe different Redis commands or actions unrelated to MULTI.

3. Final Answer:

   Starts a transaction block to queue commands -> Option B

4. Quick Check:

   MULTI = start transaction block [OK]

Hint: MULTI queues commands for atomic execution [OK]

Common Mistakes:

• Thinking MULTI executes commands immediately
• Confusing MULTI with FLUSHDB or DBSIZE
• Assuming MULTI deletes keys

2.

Which of the following is the correct syntax to start a transaction block in Redis?

?

easy

A. START TRANSACTION

B. EXEC

C. MULTI

D. BEGIN

Solution

1. Step 1: Identify Redis transaction start command

   In Redis, MULTI is used to start a transaction block.

2. Step 2: Check other options

   EXEC runs queued commands, BEGIN and START TRANSACTION are SQL commands, not Redis.

3. Final Answer:

   MULTI -> Option C

4. Quick Check:

   Start transaction = MULTI [OK]

Hint: MULTI starts, EXEC runs queued commands [OK]

Common Mistakes:

• Using SQL syntax like BEGIN or START TRANSACTION
• Confusing EXEC as start command
• Typing lowercase commands (Redis commands are case-insensitive but usually uppercase)

3.

Consider the following Redis commands executed in order:

MULTI
SET key1 value1
SET key2 value2
EXEC
GET key1
GET key2

What will be the output of the two GET commands?

medium

A. ["value1", "value2"]

B. [null, null]

C. ["value1", null]

D. [null, "value2"]

Solution

1. Step 1: Understand MULTI and EXEC behavior

   The commands between MULTI and EXEC are queued and executed atomically when EXEC runs. So both SET commands run together.

2. Step 2: Check GET commands after EXEC

   After EXEC, both keys are set, so GET key1 returns "value1" and GET key2 returns "value2".

3. Final Answer:

   ["value1", "value2"] -> Option A

4. Quick Check:

   Queued SETs run on EXEC, GET returns set values [OK]

Hint: EXEC runs queued commands, so GET sees updated keys [OK]

Common Mistakes:

• Assuming SET commands run immediately before EXEC
• Expecting null because commands are queued
• Confusing output format of GET commands

4.

What is wrong with this Redis transaction block?

MULTI
SET key1 value1
GET key1
EXEC

medium

A. MULTI must be followed by WATCH

B. EXEC must come before GET

C. SET cannot be used inside MULTI

D. GET inside MULTI queues the command, does not return value immediately

Solution

1. Step 1: Analyze GET inside MULTI block

   Inside MULTI, commands are queued and do not return results immediately, so GET queues but does not return the value now.

2. Step 2: Understand EXEC behavior

   EXEC runs all queued commands and returns their results together, so GET's result is only available after EXEC.

3. Final Answer:

   GET inside MULTI queues the command, does not return value immediately -> Option D

4. Quick Check:

   Commands queue inside MULTI, results after EXEC [OK]

Hint: GET inside MULTI queues, no immediate output [OK]

Common Mistakes:

• Expecting GET to return value immediately inside MULTI
• Thinking EXEC must come before GET
• Believing SET is disallowed inside MULTI

5.

You want to update two keys user:1 and user:2 atomically in Redis. Which sequence correctly uses MULTI and EXEC to ensure both updates happen together?

?

hard

A. MULTI
SET user:1 "Alice"
SET user:2 "Bob"
EXEC

B. SET user:1 "Alice"
MULTI
SET user:2 "Bob"
EXEC

C. MULTI
SET user:1 "Alice"
EXEC
SET user:2 "Bob"

D. EXEC
MULTI
SET user:1 "Alice"
SET user:2 "Bob"

Solution

1. Step 1: Identify correct transaction block

   MULTI SET user:1 "Alice" SET user:2 "Bob" EXEC starts with MULTI, queues both SET commands, then runs them together with EXEC.

2. Step 2: Check other options for errors

   SET user:1 "Alice" MULTI SET user:2 "Bob" EXEC queues only one SET inside MULTI; MULTI SET user:1 "Alice" EXEC SET user:2 "Bob" runs EXEC too early; EXEC MULTI SET user:1 "Alice" SET user:2 "Bob" starts with EXEC which is invalid.

3. Final Answer:

   MULTI SET user:1 "Alice" SET user:2 "Bob" EXEC -> Option A

4. Quick Check:

   Queue all commands between MULTI and EXEC [OK]

Hint: Queue all commands between MULTI and EXEC for atomic update [OK]

Common Mistakes:

• Calling EXEC before queuing all commands
• Starting EXEC before MULTI
• Mixing commands outside transaction block