Hash slots distribution in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Hash slot calculation for a given key

Given the Redis key {user:1000}.data, what is the hash slot number assigned to this key in a Redis cluster?

A The hash slot is randomly assigned and not related to the key content.

B The hash slot is calculated on the entire key <code>{user:1000}.data</code> including the braces and suffix.

C The hash slot is always zero for keys with curly braces.

D The hash slot is calculated only on the substring inside curly braces, so the slot corresponds to the hash of <code>user:1000</code>.

💡 Hint

Redis cluster uses the substring inside curly braces to compute the hash slot.

Redis cluster uses the substring inside curly braces {} as the key for hash slot calculation. This allows grouping related keys in the same slot.

🧠 Conceptual

intermediate

Purpose of hash slots in Redis cluster

What is the main purpose of hash slots in a Redis cluster?

A To encrypt data stored in Redis for security.

B To compress keys to save memory space.

C To distribute keys evenly across multiple nodes for load balancing.

D To store keys in a single node only.

💡 Hint

Think about how Redis cluster manages data across nodes.

Hash slots allow Redis cluster to split the keyspace into 16384 slots, distributing them across nodes to balance load and enable scaling.

📝 Syntax

advanced

Identify the correct command to view hash slot allocation

Which Redis CLI command correctly shows the hash slot allocation for cluster nodes?

A CLUSTER SLOTS

B CLUSTER INFO

C CLUSTER NODES

D CLUSTER KEYSLOT <key>

💡 Hint

The command should show slot ranges assigned to nodes.

The CLUSTER SLOTS command returns the slot ranges and the nodes responsible for them.

🔧 Debug

advanced

Troubleshooting key slot mismatch error

You get a MOVED error when accessing a key in Redis cluster. What is the most likely cause?

A The client sent the key to the wrong node that does not own the key's hash slot.

B The key does not exist in the cluster.

C The key contains invalid characters.

D The Redis server is down.

💡 Hint

MOVED errors indicate redirection to the correct node.

A MOVED error means the client tried to access a key on a node that does not own the hash slot; the client should redirect to the correct node.

❓ optimization

expert

Optimizing multi-key operations with hash slots

You want to perform a multi-key operation (e.g., MGET) on keys user:1, user:2, and user:3 in a Redis cluster. How should you design the keys to ensure the operation succeeds without errors?

A Use completely different key names without curly braces.

B Use the same hash tag in curly braces for all keys, like <code>{user}:1</code>, <code>{user}:2</code>, <code>{user}:3</code>.

C Use keys with different hash tags, like <code>{user1}:1</code>, <code>{user2}:2</code>, <code>{user3}:3</code>.

D Use keys with no hash tags but ensure they are stored on the same node manually.

💡 Hint

Multi-key commands require keys to be in the same hash slot.

Using the same hash tag inside curly braces forces all keys to hash to the same slot, allowing multi-key commands to work.

Practice

1. What is the total number of hash slots used by Redis Cluster to distribute data?

easy

A. 16384

B. 1024

C. 65536

D. 2048

Solution

1. Step 1: Recall Redis Cluster design

   Redis Cluster divides data into hash slots to distribute keys evenly.

2. Step 2: Identify the number of hash slots

   The standard number of hash slots in Redis Cluster is 16384.

3. Final Answer:

   16384 -> Option A

4. Quick Check:

   Redis Cluster uses 16384 hash slots [OK]

Hint: Remember Redis Cluster always uses 16384 slots [OK]

Common Mistakes:

• Confusing hash slots with number of keys
• Thinking slots are 1024 or 2048
• Assuming slots equal to max integer value

2. Which Redis command shows the current distribution of hash slots across cluster nodes?

easy

A. CLUSTER NODES

B. CLUSTER KEYS

C. CLUSTER SLOTS

D. CLUSTER INFO

Solution

1. Step 1: Understand Redis cluster commands

   Redis provides commands to inspect cluster state and slot distribution.

2. Step 2: Identify the command for slot distribution

   The CLUSTER SLOTS command returns how hash slots are assigned to nodes.

3. Final Answer:

   CLUSTER SLOTS -> Option C

4. Quick Check:

   CLUSTER SLOTS shows slot distribution [OK]

Hint: Use CLUSTER SLOTS to see slot assignments [OK]

Common Mistakes:

• Using CLUSTER NODES which shows node info but not slots
• Confusing CLUSTER INFO with slot details
• Assuming CLUSTER KEYS exists

3. Given a Redis Cluster with 3 nodes and the following slot ranges: Node1: 0-5460, Node2: 5461-10922, Node3: 10923-16383. Which node holds the key with hash slot 7000?

medium

A. None of the above

B. Node1

C. Node3

D. Node2

Solution

1. Step 1: Understand slot ranges per node

   Node1 covers slots 0 to 5460, Node2 covers 5461 to 10922, Node3 covers 10923 to 16383.

2. Step 2: Locate slot 7000

   Slot 7000 falls between 5461 and 10922, so it belongs to Node2.

3. Final Answer:

   Node2 -> Option D

4. Quick Check:

   7000 is in Node2's slot range [OK]

Hint: Check which node's slot range includes the slot number [OK]

Common Mistakes:

• Choosing Node1 because 7000 is less than 10922
• Choosing Node3 by mistake
• Not checking slot ranges carefully

4. You run CLUSTER SLOTS but get an error: "ERR This instance has cluster support disabled". What is the likely cause?

medium

A. The Redis instance is not running in cluster mode

B. The command syntax is incorrect

C. The cluster has no slots assigned yet

D. The Redis version is too new

Solution

1. Step 1: Analyze the error message

   The error says cluster support is disabled, meaning the instance is not a cluster node.

2. Step 2: Understand cluster mode requirement

   Only Redis instances running in cluster mode support CLUSTER SLOTS command.

3. Final Answer:

   The Redis instance is not running in cluster mode -> Option A

4. Quick Check:

   Cluster commands need cluster mode enabled [OK]

Hint: Check if Redis is started with cluster enabled [OK]

Common Mistakes:

• Assuming syntax error causes this
• Thinking slots are unassigned causes error
• Blaming Redis version without checking mode

5. In a Redis Cluster, if you want to evenly distribute 16384 hash slots across 4 nodes, which slot range should Node3 handle?

hard

A. 12288 to 16383

B. 8192 to 12287

C. 4096 to 8191

D. 0 to 4095

Solution

1. Step 1: Calculate slots per node

   16384 slots divided by 4 nodes = 4096 slots per node.

2. Step 2: Determine Node3's slot range

   Node1: 0-4095, Node2: 4096-8191, Node3: 8192-12287, Node4: 12288-16383.

3. Final Answer:

   8192 to 12287 -> Option B

4. Quick Check:

   Each node gets 4096 slots; Node3 starts at 8192 [OK]

Hint: Divide 16384 by nodes, assign consecutive ranges [OK]

Common Mistakes:

• Off-by-one errors in slot ranges
• Assigning wrong ranges to nodes
• Not dividing slots evenly