EXEC to execute in Redis - Mini Project: Build & Apply

Using Redis EXEC to Execute Transactions

📖 Scenario: You are managing a simple Redis database for a small online store. You want to update the stock quantities of two products together to keep the data consistent.

🎯 Goal: Build a Redis transaction using MULTI and EXEC commands to update the stock of two products atomically.

📋 What You'll Learn

Create two keys with initial stock values

Start a transaction with MULTI

Queue two DECRBY commands to reduce stock

Execute the transaction with EXEC

💡 Why This Matters

🌍 Real World

Redis transactions help keep data consistent when multiple related changes must happen together, like updating stock levels in an online store.

💼 Career

Understanding Redis transactions is useful for backend developers and database administrators working with caching and real-time data stores.

1

DATA SETUP: Create initial stock keys

Create two Redis keys called product:1001:stock and product:1002:stock with values 50 and 30 respectively using the SET command.

Redis

# Your Redis commands here

Hint

Use the SET command to assign values to keys.

2

CONFIGURATION: Start the transaction

Start a Redis transaction by issuing the MULTI command.

Redis

SET product:1001:stock 50
SET product:1002:stock 30
# Start the transaction with MULTI

Hint

Use MULTI to begin queuing commands for a transaction.

3

CORE LOGIC: Queue stock decrement commands

Queue two DECRBY commands inside the transaction to reduce product:1001:stock by 5 and product:1002:stock by 3.

Redis

SET product:1001:stock 50
SET product:1002:stock 30
MULTI
# Queue DECRBY commands here

Hint

Use DECRBY key amount to decrease stock values inside the transaction.

4

COMPLETION: Execute the transaction

Execute all queued commands atomically by issuing the EXEC command.

Redis

SET product:1001:stock 50
SET product:1002:stock 30
MULTI
DECRBY product:1001:stock 5
DECRBY product:1002:stock 3
# Execute the transaction with EXEC

Hint

Use EXEC to run all queued commands in the transaction.

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