Multi-key operations in cluster in Redis

Introduction

Multi-key operations in a Redis cluster let you work with several keys at once. This helps when you want to get or set many values quickly.

You want to get values of multiple keys in one go to save time.

You need to set or update several keys together.

You want to delete many keys at once.

You want to check if multiple keys exist.

You want to perform operations on keys that are stored in the same part of the cluster.

Syntax

Redis

COMMAND key1 key2 ... keyN

All keys must be in the same hash slot for multi-key commands to work in a Redis cluster.

Use key tags (like {tag}) to force keys into the same slot.

Examples

Gets the values of key1, key2, and key3 if they are in the same hash slot.

Redis

MGET key1 key2 key3

Deletes key1 and key2 if they share the same hash slot.

Redis

DEL key1 key2

Sets key1 to value1 and key2 to value2 together, keys must be in the same slot.

Redis

MSET key1 value1 key2 value2

Sample Program

This sets two keys with the same tag {user} so they are in the same slot, then gets their values together.

Redis

MSET {user}:1:name "Alice" {user}:1:age "30"
MGET {user}:1:name {user}:1:age

Important Notes

Redis cluster splits keys into slots. Multi-key commands only work if keys share the same slot.

Use curly braces {} around part of the key to create a key tag that forces keys into the same slot.

If keys are in different slots, multi-key commands will fail with an error.

Summary

Multi-key operations let you handle many keys at once in Redis cluster.

Keys must be in the same hash slot to use multi-key commands.

Use key tags with curly braces to group keys into the same slot.

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