Hash slots distribution in Redis - Step-by-Step Execution

Concept Flow - Hash slots distribution

Start with 16384 hash slots

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Calculate key's hash value

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Modulo operation: hash_value % 16384

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Assign key to resulting hash slot

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Distribute slots evenly across cluster nodes

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Client queries node responsible for slot

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Node handles key operations

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END

Redis cluster divides all keys into 16384 slots by hashing keys and assigning each to a slot, which are then distributed across nodes.

Execution Sample

Redis

HASH_SLOT = CRC16(key) % 16384
Assign key to HASH_SLOT
Distribute slots to cluster nodes
Client queries node for HASH_SLOT
Node processes key commands

This process shows how Redis calculates a key's hash slot and routes commands to the correct node.

Execution Table

Step	Action	Key	CRC16 Hash	Hash Slot (CRC16 % 16384)	Node Assigned
1	Calculate CRC16 hash	user:1000	12345	12345 % 16384 = 12345	Node A
2	Calculate CRC16 hash	order:500	23456	23456 % 16384 = 7072	Node B
3	Calculate CRC16 hash	session:abc	34567	34567 % 16384 = 1799	Node C
4	Calculate CRC16 hash	cart:789	45678	45678 % 16384 = 12910	Node A
5	Calculate CRC16 hash	product:321	56789	56789 % 16384 = 10637	Node B
6	End	-	-	-	-

💡 All keys assigned to slots between 0 and 16383, distributed to nodes accordingly.

Variable Tracker

Variable	Start	After user:1000	After order:500	After session:abc	After cart:789	After product:321	Final
Key	-	user:1000	order:500	session:abc	cart:789	product:321	-
CRC16 Hash	-	12345	23456	34567	45678	56789	-
Hash Slot	-	12345	7072	1799	12910	10637	-
Node Assigned	-	Node A	Node B	Node C	Node A	Node B	-

Key Moments - 3 Insights

Why do we use modulo 16384 in the hash slot calculation?

Because Redis cluster has exactly 16384 hash slots numbered 0 to 16383, modulo ensures the hash value fits into this range (see execution_table rows 1-5).

How does Redis decide which node handles a key?

Each node is assigned a range of hash slots; the key's slot determines the node (see Node Assigned column in execution_table).

What happens if two keys have the same hash slot?

They are stored on the same node responsible for that slot, allowing efficient grouping (implied by slot assignment in execution_table).

Visual Quiz - 3 Questions

Look at the execution_table, what is the hash slot for the key 'order:500'?

A 23456

B 12345

C 7072

D 8415

💡 Hint

Check the 'Hash Slot' column for 'order:500' in execution_table row 2.

At which step does the key get assigned to Node C?

A Step 3

B Step 1

C Step 4

D Step 5

💡 Hint

Look at the 'Node Assigned' column in execution_table to find Node C.

If the modulo was changed from 16384 to 10000, what would happen to the hash slot for 'cart:789'?

A It would remain 12910

B It would be 45678 % 10000 = 5678

C It would be 45678 % 16384 = 12910

D It would be 45678

💡 Hint

Recall the hash slot is calculated as CRC16(key) % number_of_slots (see execution_sample).

Concept Snapshot

Redis Cluster uses 16384 hash slots.
Each key's CRC16 hash is computed.
Hash slot = CRC16(key) % 16384.
Slots are distributed evenly across nodes.
Clients query nodes by slot to access keys.
This enables scalable, distributed key storage.

Full Transcript

Redis Cluster divides all keys into 16384 hash slots. Each key is hashed using CRC16, then the hash value is taken modulo 16384 to find its slot number. This slot number determines which cluster node stores the key. The cluster distributes these slots evenly among nodes. When a client wants to access a key, it calculates the key's slot and sends the command to the node responsible for that slot. This process ensures keys are spread across nodes for load balancing and scalability.