Multi-key operations in cluster in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this Redis cluster multi-key command?

Given a Redis cluster with keys {user1}:name and {user1}:age stored in the same hash slot, what is the output of the command MGET {user1}:name {user1}:age if {user1}:name = "Alice" and {user1}:age = "30"?

Redis

MGET {user1}:name {user1}:age

A [null, null]

B ["Alice"]

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

D ["Alice", "30"]

💡 Hint

Keys with the same hash tag (inside {}) are stored in the same slot and can be accessed together.

In Redis Cluster, keys with the same hash tag (the part inside curly braces) are stored in the same slot, allowing multi-key commands like MGET to work without error. Since both keys exist with values "Alice" and "30", MGET returns both values in order.

🧠 Conceptual

intermediate

Why does Redis Cluster reject multi-key commands on keys without the same hash tag?

In Redis Cluster, why does a command like MGET key1 key2 fail with a CROSSSLOT error if key1 and key2 do not share the same hash tag?

A Because keys must be in the same hash slot to ensure atomicity and consistency of multi-key operations.

B Because keys with different names cannot be accessed together.

C Because Redis Cluster does not support multi-key commands at all.

D Because the keys are stored in the same slot but the command syntax is invalid.

💡 Hint

Think about how Redis Cluster distributes keys and why atomic operations need keys in the same slot.

Redis Cluster partitions data by hash slots. Multi-key commands require all keys to be in the same slot to guarantee atomicity and avoid inconsistent states. If keys hash to different slots, the command fails with CROSSSLOT error.

📝 Syntax

advanced

Which command syntax correctly performs a multi-key increment in Redis Cluster?

You want to increment two counters stored as keys {counter}:a and {counter}:b atomically in Redis Cluster. Which command syntax is valid?

A INCR {counter}:a {counter}:b

B

MULTI
INCR {counter}:a
INCR {counter}:b
EXEC

C MSET {counter}:a {counter}:b

D INCR {counter}:a; INCR {counter}:b

💡 Hint

Consider how Redis transactions work and how multi-key commands are handled in clusters.

MULTI/EXEC starts a transaction block. Inside it, multiple commands like INCR can be queued and executed atomically. Option B correctly uses MULTI/EXEC with two INCR commands on keys sharing the same hash tag.

❓ optimization

advanced

How to optimize multi-key reads in Redis Cluster when keys have different hash tags?

You need to read values of keys user:1:name and user:2:name in Redis Cluster, but they hash to different slots. What is the best approach to optimize multi-key reads?

A Use MGET command directly on both keys.

B Rename keys to have the same hash tag and use MGET.

C Use pipelining to send multiple GET commands in one network round-trip.

D Use Lua scripting to read both keys atomically.

💡 Hint

Think about how to reduce network latency when keys are in different slots.

Since keys are in different slots, MGET fails. Renaming keys may not be feasible. Lua scripts cannot access multiple slots atomically. Pipelining sends multiple commands in one go, reducing latency and improving performance.

🔧 Debug

expert

Why does this Redis Cluster multi-key command fail with CROSSSLOT error?

You run the command MGET user:100:name user:101:name in a Redis Cluster and get the error: CROSSSLOT Keys in request don't hash to the same slot. Both keys exist. What is the root cause?

Redis

MGET user:100:name user:101:name

A The keys hash to different slots because they lack a common hash tag.

B The keys do not exist in the cluster.

C The MGET command is not supported in Redis Cluster.

D The keys are locked by another client.

💡 Hint

Check how Redis Cluster calculates hash slots for keys without explicit hash tags.

Without explicit hash tags (the part inside {}), Redis Cluster hashes the entire key name, so keys with different suffixes hash to different slots. Multi-key commands require keys in the same slot, so this causes CROSSSLOT error.