Resharding hash slots in Redis - Mini Project: Build & Apply

Resharding Hash Slots in Redis Cluster

📖 Scenario: You are managing a Redis cluster that distributes data across multiple nodes using hash slots. To balance the load, you need to move some hash slots from one node to another.This project will guide you through the steps to reshard hash slots manually using Redis commands.

🎯 Goal: Learn how to reshard hash slots by moving a specific range of slots from one Redis node to another, ensuring data consistency and cluster stability.

📋 What You'll Learn

Create a list of hash slots assigned to the source node

Define the target node ID to receive the hash slots

Use Redis commands to migrate keys belonging to the hash slots

Update the cluster configuration to assign the hash slots to the target node

💡 Why This Matters

🌍 Real World

Redis clusters use hash slots to distribute data. Resharding hash slots helps balance load and improve performance.

💼 Career

Database administrators and backend engineers often need to reshard Redis clusters to maintain system health and scalability.

1

Set up the list of hash slots to move

Create a variable called slots_to_move that contains the list of hash slots from 1000 to 1010 inclusive.

Redis

# Your code here
# Create the list of hash slots from 1000 to 1010

Hint

Use the range function to create a list of numbers from 1000 to 1010.

2

Define the target node ID

Create a variable called target_node_id and set it to the string 'node-02' which represents the Redis node that will receive the hash slots.

Redis

slots_to_move = list(range(1000, 1011))
# Your code here
# Define the target node ID as 'node-02'

Hint

Assign the string 'node-02' to the variable target_node_id.

3

Generate commands to migrate keys for each hash slot

Write a for loop using slot as the iterator over slots_to_move. Inside the loop, create a Redis command string called migrate_command that uses the format "CLUSTER SETSLOT {slot} MIGRATING {target_node_id}".

Redis

slots_to_move = list(range(1000, 1011))
target_node_id = 'node-02'
# Your code here
# Loop over slots_to_move and create migrate_command for each slot

Hint

Use an f-string to format the command with the current slot and target_node_id.

4

Assign the hash slots to the target node

After migrating keys, write a for loop over slots_to_move using slot as the iterator. Inside the loop, create a Redis command string called assign_command with the format "CLUSTER SETSLOT {slot} NODE {target_node_id}" to assign each slot to the target node.

Redis

slots_to_move = list(range(1000, 1011))
target_node_id = 'node-02'
for slot in slots_to_move:
    migrate_command = f"CLUSTER SETSLOT {slot} MIGRATING {target_node_id}"
# Your code here
# Loop again to assign slots to the target node

Hint

Use a second loop to assign each slot to the target node with the correct command format.

Practice

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

easy

A. To balance data evenly across cluster nodes

B. To delete unused keys from the database

C. To backup data to an external storage

D. To increase the size of each hash slot

Solution

1. Step 1: Understand hash slots in Redis cluster

   Redis cluster divides data into 16384 hash slots distributed among nodes.

2. Step 2: Purpose of resharding

   Resharding moves hash slots to balance load and storage evenly across nodes.

3. Final Answer:

   To balance data evenly across cluster nodes -> Option A

4. Quick Check:

   Resharding = balance data [OK]

Hint: Resharding means redistributing data evenly [OK]

Common Mistakes:

• Thinking resharding deletes keys
• Confusing resharding with backup
• Assuming hash slot size changes

2. Which Redis command is used to mark a hash slot as migrating during resharding?

easy

A. CLUSTER ADDSLOTS <slot>

B. CLUSTER SETSLOT <slot> MIGRATING <node_id>

C. CLUSTER DELSLOTS <slot>

D. CLUSTER INFO

Solution

1. Step 1: Identify commands related to slot state changes

   CLUSTER SETSLOT command changes slot ownership or state.

2. Step 2: Marking migrating slot

   Using MIGRATING flag with CLUSTER SETSLOT marks slot as migrating from a node.

3. Final Answer:

   CLUSTER SETSLOT <slot> MIGRATING <node_id> -> Option B

4. Quick Check:

   MIGRATING slot = CLUSTER SETSLOT MIGRATING [OK]

Hint: Use CLUSTER SETSLOT with MIGRATING to mark slot move [OK]

Common Mistakes:

• Using ADDSLOTS or DELSLOTS instead of SETSLOT
• Confusing INFO command with slot state change
• Omitting node_id in command

3. Given the following commands during resharding, what is the correct order to migrate a hash slot 5000 from node A to node B?

1. CLUSTER SETSLOT 5000 MIGRATING nodeB_id
2. MIGRATE nodeB_ip 6379 "" 0 5000
3. CLUSTER SETSLOT 5000 IMPORTING nodeA_id
4. CLUSTER SETSLOT 5000 NODE nodeB_id

medium

A. 1, 2, 3, 4

B. 3, 1, 2, 4

C. 2, 1, 3, 4

D. 1, 3, 2, 4

Solution

1. Step 1: Mark slot as migrating on source node

   Use CLUSTER SETSLOT 5000 MIGRATING nodeB_id to mark slot migrating from node A.

2. Step 2: Mark slot as importing on destination node

   Use CLUSTER SETSLOT 5000 IMPORTING nodeA_id on node B to accept keys.

3. Step 3: Migrate keys with MIGRATE command

   Use MIGRATE to move keys of slot 5000 from node A to node B.

4. Step 4: Assign slot ownership to destination node

   Use CLUSTER SETSLOT 5000 NODE nodeB_id to finalize ownership.

5. Final Answer:

   1, 3, 2, 4 -> Option D

6. Quick Check:

   Mark migrating -> mark importing -> migrate keys -> assign slot [OK]

Hint: MIGRATING then IMPORTING before MIGRATE command [OK]

Common Mistakes:

• Migrating keys before marking IMPORTING
• Assigning slot ownership before migration
• Skipping IMPORTING step

4. You run the following commands to reshard slot 7000 but keys are not migrating correctly:

CLUSTER SETSLOT 7000 MIGRATING nodeB_id
MIGRATE nodeB_ip 6379 "" 0 7000
CLUSTER SETSLOT 7000 NODE nodeB_id

What is missing to fix the migration?

medium

A. Mark slot as IMPORTING on node B before MIGRATE

B. Run CLUSTER DELSLOTS 7000 on node A first

C. Use CLUSTER ADDSLOTS 7000 on node B after migration

D. Restart both nodes before migration

Solution

1. Step 1: Identify missing IMPORTING state

   Destination node must mark slot as IMPORTING before receiving keys.

2. Step 2: Add missing command

   Use CLUSTER SETSLOT 7000 IMPORTING nodeA_id on node B before MIGRATE.

3. Final Answer:

   Mark slot as IMPORTING on node B before MIGRATE -> Option A

4. Quick Check:

   IMPORTING state needed on destination [OK]

Hint: Always mark IMPORTING on target node before MIGRATE [OK]

Common Mistakes:

• Skipping IMPORTING step
• Deleting slots prematurely
• Restarting nodes unnecessarily

5. During a large-scale resharding, you want to move multiple hash slots from node A to node B without downtime. Which sequence of actions ensures data consistency and minimal disruption?

hard

A. Assign slots to node B first, then migrate keys, finally mark MIGRATING on node A

B. Migrate keys first, then mark slots IMPORTING on node B, finally mark MIGRATING on node A

C. Mark slots MIGRATING on node A, mark IMPORTING on node B, migrate keys, then assign slots to node B

D. Delete slots on node A, migrate keys, then assign slots to node B

Solution

1. Step 1: Mark slots as MIGRATING on source node

   This prevents writes to those slots on node A during migration.

2. Step 2: Mark slots as IMPORTING on destination node

   Allows node B to accept keys for those slots.

3. Step 3: Migrate keys using MIGRATE command

   Safely transfer keys from node A to node B.

4. Step 4: Assign slots ownership to node B

   Finalizes the slot ownership after migration completes.

5. Final Answer:

   Mark slots MIGRATING on node A, mark IMPORTING on node B, migrate keys, then assign slots to node B -> Option C

6. Quick Check:

   MIGRATING -> IMPORTING -> MIGRATE -> ASSIGN [OK]

Hint: Follow MIGRATING, IMPORTING, MIGRATE, then assign slots [OK]

Common Mistakes:

• Assigning slots before migration
• Migrating keys without marking IMPORTING
• Deleting slots before migration