Hash slots distribution in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a hash slot in Redis Cluster?

A hash slot is one of 16,384 buckets used by Redis Cluster to distribute keys evenly across multiple nodes. Each key is assigned to a hash slot based on its hash value.

beginner

How many hash slots does a Redis Cluster have by default?

A Redis Cluster has 16,384 hash slots by default. This fixed number helps evenly distribute keys across the cluster nodes.

intermediate

How does Redis determine which hash slot a key belongs to?

Redis calculates a CRC16 checksum of the key and then takes the remainder when divided by 16,384. This remainder is the hash slot number for the key.

intermediate

Why is hash slot distribution important in Redis Cluster?

Hash slot distribution ensures data is spread evenly across nodes, balancing load and enabling efficient scaling and failover in the cluster.

advanced

What happens if a node in Redis Cluster fails regarding hash slots?

If a node fails, Redis Cluster reassigns the hash slots owned by that node to other nodes to keep the cluster operational and data accessible.

How many hash slots are there in a Redis Cluster?

A 1,024

B 16,384

C 65,536

D 512

Redis Cluster uses exactly 16,384 hash slots to distribute keys.

What method does Redis use to assign a key to a hash slot?

A Random assignment

B MD5 hash mod 1,024

C SHA-1 hash mod 65,536

D CRC16 checksum mod 16,384

Redis calculates the CRC16 checksum of the key and then uses modulo 16,384 to find the hash slot.

Why does Redis Cluster use hash slots?

A To evenly distribute keys across nodes

B To encrypt data

C To compress keys

D To store keys in a single node

Hash slots help spread keys evenly across cluster nodes for load balancing.

What happens to hash slots if a Redis node fails?

A They are deleted permanently

B They remain unavailable

C They are reassigned to other nodes

D They are backed up automatically

Redis Cluster reassigns hash slots from the failed node to keep data accessible.

Which of these is NOT true about Redis hash slots?

A Hash slots encrypt the keys

B Each key belongs to exactly one hash slot

C There are 16,384 hash slots

D Hash slots help with cluster scaling

Hash slots do not encrypt keys; they only help distribute keys across nodes.

Explain how Redis Cluster uses hash slots to distribute keys across nodes.

Think about how Redis decides where to put each key in the cluster.

Describe what happens to hash slots and data when a Redis Cluster node fails.

Consider how Redis keeps the cluster working despite failures.

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