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Redisquery~7 mins

Resharding hash slots in Redis

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Introduction
Resharding hash slots helps balance data across Redis cluster nodes so no single node gets too busy or full.
When adding a new node to a Redis cluster and you want to move some data to it.
When some nodes have too many keys and others have fewer, causing uneven load.
When you want to improve performance by spreading data more evenly.
When removing a node and you need to move its data to other nodes.
Syntax
Redis
CLUSTER SETSLOT <slot> IMPORTING <node_id>
CLUSTER SETSLOT <slot> MIGRATING <node_id>
CLUSTER GETKEYSINSLOT <slot> <count>
MIGRATE <host> <port> <key> 0 <timeout>
You move hash slots one by one between nodes using IMPORTING and MIGRATING states.
Use CLUSTER GETKEYSINSLOT to find keys in a slot before migrating.
Examples
Mark slot 121 as migrating from the current node to the node with id 07c3...
Redis
CLUSTER SETSLOT 121 MIGRATING 07c37dfeb235512dbd9f8d7a0e8f0f8a0e7a1f3a
Mark slot 121 as importing on the node with id 07c3...
Redis
CLUSTER SETSLOT 121 IMPORTING 07c37dfeb235512dbd9f8d7a0e8f0f8a0e7a1f3a
Get up to 10 keys that belong to slot 121 to migrate them.
Redis
CLUSTER GETKEYSINSLOT 121 10
Move key1 to the node at 192.168.1.2 port 6379 with a 5-second timeout.
Redis
MIGRATE 192.168.1.2 6379 "key1" 0 5000
Sample Program
This example moves slot 121 from the current node to the node with id 07c3... It marks the slot as migrating and importing, gets keys in the slot, migrates a key, then assigns the slot to the new node.
Redis
CLUSTER SETSLOT 121 MIGRATING 07c37dfeb235512dbd9f8d7a0e8f0f8a0e7a1f3a
CLUSTER SETSLOT 121 IMPORTING 07c37dfeb235512dbd9f8d7a0e8f0f8a0e7a1f3a
CLUSTER GETKEYSINSLOT 121 2
MIGRATE 192.168.1.2 6379 "user:1001" 0 5000
CLUSTER SETSLOT 121 NODE 07c37dfeb235512dbd9f8d7a0e8f0f8a0e7a1f3a
OutputSuccess
Important Notes
Always migrate keys in small batches to avoid blocking the server.
Make sure to update the slot ownership after migrating keys to keep cluster state consistent.
Use the node IDs from CLUSTER NODES command to identify nodes.
Summary
Resharding hash slots balances data across Redis cluster nodes.
Use CLUSTER SETSLOT with MIGRATING and IMPORTING to mark slot movement.
Migrate keys with MIGRATE command and update slot ownership after.

Practice

(1/5)
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
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