EXEC to execute in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a Redis transaction.

Redis

redis.call('[1]')

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

AEXEC

BMULTI

CWATCH

DDISCARD

Attempts:

3 left

💡 Hint

Common Mistakes

Using EXEC to start the transaction instead of MULTI.

Using WATCH which is for monitoring keys, not starting transactions.

The MULTI command starts a transaction block in Redis.

2fill in blank

medium

Complete the code to execute all commands in the current Redis transaction.

Redis

redis.call('[1]')

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

AMULTI

BDISCARD

CEXEC

DWATCH

Attempts:

3 left

💡 Hint

Common Mistakes

Using MULTI instead of EXEC to execute commands.

Using DISCARD which cancels the transaction.

The EXEC command executes all commands queued in the current transaction.

3fill in blank

hard

Fix the error in the code to properly execute a Redis transaction.

Redis

redis.call('MULTI')
redis.call('SET', 'key', 'value')
redis.call('[1]')

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

AEXEC

BDISCARD

CWATCH

DMULTI

Attempts:

3 left

💡 Hint

Common Mistakes

Calling MULTI again instead of EXEC.

Using DISCARD which cancels the transaction.

After queuing commands with MULTI, use EXEC to execute them.

4fill in blank

hard

Fill both blanks to start a transaction and then cancel it.

Redis

redis.call('[1]')
redis.call('[2]')

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

AMULTI

BEXEC

CDISCARD

DWATCH

Attempts:

3 left

💡 Hint

Common Mistakes

Using EXEC instead of DISCARD to cancel.

Using WATCH which does not cancel transactions.

Use MULTI to start and DISCARD to cancel the transaction.

5fill in blank

hard

Fill all three blanks to watch a key, start a transaction, and execute it.

Redis

redis.call('[1]', 'mykey')
redis.call('[2]')
redis.call('[3]')

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

AMULTI

BEXEC

CWATCH

DDISCARD

Attempts:

3 left

💡 Hint

Common Mistakes

Using DISCARD instead of EXEC to run the transaction.

Starting transaction before watching keys.

Use WATCH to monitor a key, MULTI to start the transaction, and EXEC to execute it.

Practice

1. What does the EXEC command do in Redis transactions?

easy

A. It runs all queued commands in a transaction at once.

B. It starts queuing commands for a transaction.

C. It cancels all queued commands in a transaction.

D. It checks the status of the Redis server.

Solution

1. Step 1: Understand Redis transaction commands

   The MULTI command starts queuing commands, and EXEC runs them all together.

2. Step 2: Identify the role of EXEC

   EXEC executes all commands queued after MULTI in one atomic operation.

3. Final Answer:

   It runs all queued commands in a transaction at once. -> Option A

4. Quick Check:

   EXEC runs queued commands = B [OK]

Hint: Remember: MULTI queues, EXEC runs all queued commands [OK]

Common Mistakes:

• Confusing EXEC with MULTI
• Thinking EXEC cancels commands
• Assuming EXEC checks server status

2. Which of the following is the correct sequence to run multiple commands atomically in Redis?

easy

A. SET key value, EXEC, MULTI

B. EXEC, MULTI, SET key value

C. MULTI, SET key value, EXEC

D. SET key value, MULTI, EXEC

Solution

1. Step 1: Recall the transaction command order

   First, MULTI starts the transaction, then commands like SET queue, and finally EXEC runs them.

2. Step 2: Match the correct order

   MULTI, SET key value, EXEC follows this order: MULTI, then SET, then EXEC.

3. Final Answer:

   MULTI, SET key value, EXEC -> Option C

4. Quick Check:

   Start with MULTI, end with EXEC = A [OK]

Hint: MULTI starts, EXEC ends the transaction [OK]

Common Mistakes:

• Putting EXEC before MULTI
• Running SET before MULTI
• Mixing command order

3. Given the following Redis commands executed in order:

MULTI
SET user:1:name "Alice"
INCR user:1:visits
EXEC

What will be the result of the EXEC command?

medium

A. An error because INCR on a string key

B. ["OK", 1]

C. ["QUEUED", "QUEUED"]

D. Empty array []

Solution

1. Step 1: Analyze queued commands

   SET user:1:name "Alice" sets a string key, returns "OK"; INCR user:1:visits increments a numeric key, returns the new value.

2. Step 2: Understand EXEC output

   EXEC returns an array with results of each command: ["OK", 1].

3. Final Answer:

   ["OK", 1] -> Option B

4. Quick Check:

   EXEC returns results array = C [OK]

Hint: EXEC returns results of all queued commands in order [OK]

Common Mistakes:

• Expecting QUEUED responses from EXEC
• Thinking INCR causes error on string key
• Assuming EXEC returns empty array

4. You run the following Redis commands:

MULTI
SET key1 "value1"
GET key1
EXEC

What will be the result of the EXEC command?

medium

A. An error because GET command is not allowed inside MULTI block

B. You forgot to call MULTI before queuing commands

C. The GET command returns QUEUED, causing EXEC to fail

D. No error; EXEC always succeeds if MULTI was called

Solution

1. Step 1: Understand command behavior in transactions

   Some commands like GET are allowed but return QUEUED during MULTI, and EXEC returns their results after execution.

2. Step 2: Identify that no error occurs

   GET is fully supported inside MULTI/EXEC blocks, so EXEC succeeds and returns ["OK", "value1"].

3. Final Answer:

   No error; EXEC always succeeds if MULTI was called -> Option D

4. Quick Check:

   GET inside MULTI causes no error = D [OK]

Hint: Check if commands are allowed inside MULTI before EXEC [OK]

Common Mistakes:

• Assuming all commands work inside MULTI
• Forgetting to call MULTI before EXEC
• Expecting EXEC to never fail

5. You want to increment a counter atomically with optimistic locking (aborting if the key was modified meanwhile), using Redis transactions. Which sequence correctly uses WATCH, MULTI, and EXEC to do this safely?

hard

A. WATCH counter
MULTI
INCR counter
EXEC

B. MULTI
WATCH counter
INCR counter
EXEC

C. MULTI
INCR counter
EXEC
WATCH counter

D. INCR counter
WATCH counter
MULTI
EXEC

Solution

1. Step 1: Understand WATCH usage

   WATCH monitors keys for changes before starting a transaction with MULTI.

2. Step 2: Correct command order for safe increment

   First WATCH counter, then MULTI, queue INCR counter, and finally EXEC to run if no changes occurred.

3. Final Answer:

   WATCH counter MULTI INCR counter EXEC -> Option A

4. Quick Check:

   WATCH before MULTI for safe transactions = D [OK]

Hint: Always WATCH keys before MULTI to ensure safe EXEC [OK]

Common Mistakes:

• Calling WATCH after MULTI
• Running INCR before WATCH
• Not using WATCH for conditional updates