Multi-key operations in cluster in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the values of multiple keys in a Redis cluster.

Redis

MGET [1]

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

Akey1,key2,key3

B[key1 key2 key3]

C"key1" "key2" "key3"

D"key1 key2 key3"

Attempts:

3 left

💡 Hint

Common Mistakes

Passing all keys as one string separated by spaces or commas.

Using brackets or commas which Redis does not accept.

The MGET command requires each key as a separate argument, so you must provide each key as a separate string.

2fill in blank

medium

Complete the code to set multiple keys with their values atomically using a Redis cluster.

Redis

MSET [1]

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

A"key1" "value1" "key2" "value2"

B"key1:value1 key2:value2"

Ckey1=value1 key2=value2

D[key1 value1 key2 value2]

Attempts:

3 left

💡 Hint

Common Mistakes

Combining keys and values into one string.

Using brackets or commas which Redis does not accept.

The MSET command requires pairs of keys and values as separate arguments, each as a string.

3fill in blank

hard

Fix the error in the command to delete multiple keys in a Redis cluster.

Redis

DEL [1]

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

A"key1" "key2" "key3"

B"key1 key2 key3"

Ckey1,key2,key3

D[key1 key2 key3]

Attempts:

3 left

💡 Hint

Common Mistakes

Passing all keys as one string.

Using commas or brackets which Redis does not accept.

The DEL command requires each key as a separate argument, so you must provide each key separately in quotes.

4fill in blank

hard

Fill both blanks to correctly perform a multi-key atomic increment operation in Redis cluster using Lua script.

Redis

EVAL "return redis.call('INCRBY', [1], [2])" 0

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

A'key1'

B'key2'

C5

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong key names or missing quotes.

Passing increment as a string instead of number or vice versa.

The Lua script calls INCRBY on the key 'key1' with increment 5. Both must be strings or numbers as required.

5fill in blank

hard

Fill all three blanks to create a Redis Lua script that atomically sets a key only if it does not exist and returns the new value.

Redis

EVAL "if redis.call('SETNX', [1], [2]) == 1 then return redis.call('GET', [3]) else return nil end" 0

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

A'mykey'

B'100'

D'otherkey'

Attempts:

3 left

💡 Hint

Common Mistakes

Using different keys in SETNX and GET calls.

Passing values without quotes.

The script sets 'mykey' to '100' if it does not exist, then returns the value of 'mykey'. The same key must be used consistently.

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