Multi-key operations in cluster in Redis - Mini Project: Build & Apply

Multi-key Operations in Redis Cluster

📖 Scenario: You are managing a Redis cluster that stores user session data. You want to perform operations on multiple keys efficiently, but Redis cluster requires special handling for multi-key commands.

🎯 Goal: Build a Redis key setup and perform multi-key operations using hash tags to ensure keys are in the same slot for cluster compatibility.

📋 What You'll Learn

Create keys with hash tags to ensure they belong to the same hash slot

Set values for multiple keys

Use a multi-key command like MGET or MSET on these keys

Demonstrate how to retrieve multiple keys in one command

💡 Why This Matters

🌍 Real World

In real-world Redis clusters, multi-key commands require keys to be in the same hash slot. Using hash tags ensures keys are grouped correctly, enabling efficient multi-key operations like MGET and MSET.

💼 Career

Understanding multi-key operations in Redis cluster is essential for backend developers and database administrators working with distributed caching and session management systems.

1

Create keys with hash tags for cluster slot alignment

Create three Redis keys named user:{1001}:name, user:{1001}:email, and user:{1001}:age with empty string values using the SET command.

Redis

# Your code here

Hint

Use the SET command with keys containing {1001} as the hash tag to ensure they are in the same cluster slot.

2

Set values for the keys

Set the value "Alice" for user:{1001}:name, "[e-mail]" for user:{1001}:email, and "30" for user:{1001}:age using the SET command.

Redis

SET user:{1001}:name ""
SET user:{1001}:email ""
SET user:{1001}:age ""
# Your code here

Hint

Use SET to assign the exact values to each key.

3

Use MGET to retrieve multiple keys at once

Use the MGET command to retrieve the values of user:{1001}:name, user:{1001}:email, and user:{1001}:age in one command.

Redis

SET user:{1001}:name "Alice"
SET user:{1001}:email "[e-mail]"
SET user:{1001}:age "30"
# Your code here

Hint

Use MGET followed by the keys with the same hash tag to get all values at once.

4

Use MSET to set multiple keys at once

Use the MSET command to set user:{1001}:name to "Bob", user:{1001}:email to "[e-mail]", and user:{1001}:age to "25" in one command.

Redis

MGET user:{1001}:name user:{1001}:email user:{1001}:age
# Your code here

Hint

Use MSET with key-value pairs for all keys sharing the same hash tag.

Practice

1. In a Redis cluster, why must keys be in the same hash slot for multi-key operations?

easy

A. Because Redis cluster does not support multi-key commands at all

B. Because keys in different slots cause syntax errors

C. Because keys in different slots are automatically merged

D. Because Redis cluster routes commands to a single node per hash slot

Solution

1. Step 1: Understand Redis cluster hash slots

   Redis cluster divides keys into 16384 hash slots, each handled by one node.

2. Step 2: Multi-key commands require single node routing

   Multi-key commands must operate on keys in the same slot to be routed to one node.

3. Final Answer:

   Because Redis cluster routes commands to a single node per hash slot -> Option D

4. Quick Check:

   Same hash slot = single node routing [OK]

Hint: Keys must share hash slot for multi-key commands [OK]

Common Mistakes:

• Thinking keys in different slots can be used together
• Assuming multi-key commands work cluster-wide automatically
• Confusing syntax errors with routing errors

2. Which of the following is the correct syntax to ensure two keys are in the same hash slot using key tags?

easy

A. SET user:1:name 'Alice'
SET user:1:age 30

B. SET {user:1}:name 'Alice'
SET {user:1}:age 30

C. SET {user1}:1:name 'Alice'
SET {user}:1:age 30

D. SET {user:1:name} 'Alice'
SET {user:1:age} 30

Solution

1. Step 1: Identify key tag usage

   Key tags are enclosed in curly braces {} and define the hash slot.

2. Step 2: Check which keys share the same tag

   Keys with the same substring inside {} share the same slot, e.g., {user:1}.

3. Final Answer:

   SET {user:1}:name 'Alice' and SET {user:1}:age 30 -> Option B

4. Quick Check:

   Same key tag inside {} means same slot [OK]

Hint: Use identical curly braces content for same slot keys [OK]

Common Mistakes:

• Placing braces incorrectly around parts of the key
• Using different tags for keys meant to be grouped
• Omitting braces and expecting same slot

3. Given these Redis commands in a cluster:

SET {order}:1001:status 'pending'
SET {order}:1001:amount 250
MGET {order}:1001:status {order}:1001:amount

What will be the output of the MGET command?

medium

A. ["pending", "250"]

B. ["pending", null]

C. [null, "250"]

D. Error: CROSSSLOT Keys in request don't hash to the same slot

Solution

1. Step 1: Check keys and their hash slots

   Both keys have the same tag {order}, so they share the same slot.

2. Step 2: Verify keys exist and MGET output

   Both keys were set, so MGET returns their values in order: 'pending' and '250'.

3. Final Answer:

   ["pending", "250"] -> Option A

4. Quick Check:

   Same slot keys return values correctly [OK]

Hint: MGET works if keys share same hash slot [OK]

Common Mistakes:

• Expecting CROSSSLOT error despite same key tags
• Confusing string and integer values
• Assuming null for existing keys

4. You run this Redis cluster command:

MGET user:1:name user:2:name

But get an error: CROSSSLOT Keys in request don't hash to the same slot. What is the best fix?

medium

A. Use key tags to put both keys in the same hash slot, e.g., {user}:1:name and {user}:2:name

B. Run the command on each key separately

C. Rename keys to have the same prefix without braces

D. Disable cluster mode to allow multi-key commands

Solution

1. Step 1: Understand CROSSSLOT error cause

   Keys user:1:name and user:2:name hash to different slots, causing the error.

2. Step 2: Use key tags to group keys

   Adding the same tag in braces, e.g., {user}, forces keys into the same slot.

3. Final Answer:

   Use key tags to put both keys in the same hash slot, e.g., {user}:1:name and {user}:2:name -> Option A

4. Quick Check:

   Key tags fix CROSSSLOT errors [OK]

Hint: Add identical key tags in braces to fix CROSSSLOT [OK]

Common Mistakes:

• Ignoring the error and retrying same command
• Assuming prefix alone groups keys
• Trying to disable cluster mode without reason

5. You want to atomically increment two counters in a Redis cluster using a multi-key command. The counters are counter:{2024}:a and counter:{2024}:b. Which approach correctly increments both counters in one command without errors?

hard

A. Use INCR counter:2024:a and INCR counter:2024:b separately

B. Use INCR counter:{2024}:a counter:{2024}:b in one command

C. Use MULTI with two INCR commands on the keys, then EXEC

D. Use INCR counter:{2024}:a and INCR counter:{2025}:b in a pipeline

Solution

1. Step 1: Confirm keys share the same hash slot

   Both keys have the same tag {2024}, so they are in the same slot.

2. Step 2: Use MULTI/EXEC for atomic multi-key operations

   Redis supports atomic transactions with MULTI/EXEC on keys in the same slot.

3. Final Answer:

   Use MULTI with two INCR commands on the keys, then EXEC -> Option C

4. Quick Check:

   Atomic multi-key needs MULTI/EXEC and same slot keys [OK]

Hint: Use MULTI/EXEC with same-slot keys for atomic multi-key ops [OK]

Common Mistakes:

• Trying to INCR multiple keys in one command
• Using keys without same hash tag
• Assuming pipeline is atomic