Multi-key operations in cluster in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a key limitation of multi-key operations in a Redis cluster?

Multi-key operations can only be performed on keys that are in the same hash slot. Keys in different slots cannot be used together in a single multi-key command.

beginner

How does Redis cluster determine which keys belong to the same hash slot?

Redis cluster uses a hash function on the key name or a part of it (called a hash tag) to assign keys to one of 16384 hash slots. Keys with the same hash slot number are stored on the same node.

intermediate

What is a hash tag in Redis cluster multi-key operations?

A hash tag is a substring enclosed in curly braces {} within a key name. It forces Redis to use only that substring for hashing, allowing multiple keys with the same hash tag to be in the same slot.

intermediate

Why can't you run a command like MGET on keys in different hash slots in Redis cluster?

Because Redis cluster distributes keys across nodes by hash slots, multi-key commands require all keys to be on the same node. Different slots mean keys are on different nodes, so the command would fail.

intermediate

How can you design keys to enable multi-key operations in Redis cluster?

Use hash tags by including the same substring in curly braces in all related keys. This ensures they hash to the same slot and can be used together in multi-key commands.

In Redis cluster, what must be true for keys used in a multi-key command like MGET?

A Keys must be on different nodes

B Keys can be in any hash slot

C All keys must be in the same hash slot

D Keys must have different hash tags

Multi-key commands require all keys to be in the same hash slot so they reside on the same node.

What is the purpose of a hash tag in Redis cluster keys?

A To encrypt the key

B To force keys to the same hash slot

C To make keys unique

D To store keys on different nodes

Hash tags force Redis to hash only the substring inside curly braces, placing keys with the same tag in the same slot.

How many hash slots does a Redis cluster have?

A 16384

B 8192

C 1024

D 32768

Redis cluster uses 16384 hash slots to distribute keys across nodes.

What happens if you try to run a multi-key command on keys in different hash slots in Redis cluster?

A The command succeeds normally

B The command runs but returns partial results

C The command merges keys automatically

D The command fails with a CROSSSLOT error

Redis cluster returns a CROSSSLOT error because multi-key commands require keys in the same slot.

Which of these keys will be in the same hash slot in Redis cluster?

A user:{1000}:name and user:{1000}:email

B user:{1000} and user:{1001}

C user:1000 and user:{1001}

D user:1000 and user:1001

Keys with the same hash tag inside curly braces hash to the same slot.

Explain why multi-key operations in Redis cluster require keys to be in the same hash slot and how hash tags help achieve this.

Think about how Redis cluster splits data and how hash tags control key placement.

Describe how you would design keys to support multi-key commands like MGET in a Redis cluster environment.

Focus on key naming patterns that control hash slot assignment.

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