Hash slots distribution in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to get the hash slot for a key.

Redis

CLUSTER KEYSLOT [1]

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

A1000

Buser:1000

C"user:1000"

DKEYSLOT

Attempts:

3 left

💡 Hint

Common Mistakes

Putting quotes around the key name causes an error.

Using a number instead of the key name.

The CLUSTER KEYSLOT command requires the key name without quotes to return its hash slot.

2fill in blank

medium

Complete the command to get all keys in a specific hash slot.

Redis

CLUSTER GETKEYSINSLOT [1] 5

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

A0

B5000

C1000

Dslot

Attempts:

3 left

💡 Hint

Common Mistakes

Using a key name instead of a slot number.

Using a slot number outside the valid range.

The command CLUSTER GETKEYSINSLOT requires the slot number as the first argument. Slot 0 is a valid slot number.

3fill in blank

hard

Fix the error in the command to get the hash slot for a key.

Redis

CLUSTER KEYSLOT [1]

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

Auser:123

BKEYSLOT

C123

D"user:123"

Attempts:

3 left

💡 Hint

Common Mistakes

Including quotes around the key name.

Using a number instead of the key name.

The CLUSTER KEYSLOT command expects the key name without quotes. Using quotes causes a syntax error.

4fill in blank

hard

Fill both blanks to get keys from slot 1000 with a count of 3.

Redis

CLUSTER GETKEYSINSLOT [1] [2]

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

A1000

B3

C5

Dslot

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the slot number and count.

Using invalid slot numbers or counts.

The first argument is the slot number (1000), and the second is the count of keys to return (3).

5fill in blank

hard

Fill all three blanks to get the hash slot for key 'session:42' and then get 2 keys from that slot.

Redis

SET key session:42
SLOT = CLUSTER KEYSLOT [1]
CLUSTER GETKEYSINSLOT [2] [3]

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

Asession:42

BSLOT

C2

Dsession42

Attempts:

3 left

💡 Hint

Common Mistakes

Using an incorrect key name.

Using a literal slot number instead of the variable.

Using wrong count number.

First, provide the key name 'session:42' to get its slot. Then use the variable SLOT to get keys from that slot with count 2.

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