Cluster architecture in Redis - Step-by-Step Execution

Concept Flow - Cluster architecture

Client calculates key slot

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Client sends command to correct node

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Node processes command

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Return result to client

The client calculates the slot for the key, determines the correct node, sends the command directly there, and receives the result.

Execution Sample

Redis

SET user:1 "Alice"
GET user:1
DEL user:1

Commands to set, get, and delete a key in a Redis cluster, showing routing and execution.

Execution Table

Step	Action	Key	Slot Calculation	Node Chosen	Result
1	Client sends SET command	user:1	Calculate slot for 'user:1' (e.g. slot 1234)	Node A (owns slot 1234)	Sent to Node A
2	Node A executes SET	user:1	Slot 1234	Node A	OK
3	Client sends GET command	user:1	Calculate slot for 'user:1' (slot 1234)	Node A	Sent to Node A
4	Node A executes GET	user:1	Slot 1234	Node A	"Alice"
5	Client sends DEL command	user:1	Calculate slot for 'user:1' (slot 1234)	Node A	Sent to Node A
6	Node A executes DEL	user:1	Slot 1234	Node A	1 (deleted)
7	Client sends GET command	user:1	Calculate slot for 'user:1' (slot 1234)	Node A	Sent to Node A
8	Node A executes GET	user:1	Slot 1234	Node A	(nil)
9	End of commands				No more commands to process

💡 All commands processed; cluster routes commands based on key slots to correct nodes.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	After Step 8
user:1	does not exist	"Alice"	"Alice"	deleted	does not exist

Key Moments - 2 Insights

Why does the client need to calculate the slot before sending the command?

The client must calculate the slot to send the command directly to the right node (see execution_table steps 1, 3, 5).

What happens if the key does not exist on the node?

The node returns nil or 0 depending on the command, as shown in step 8 where GET returns (nil) after the key was deleted in step 6.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what is the result of the GET command?

A OK

B "Alice"

C 1 (deleted)

D (nil)

💡 Hint

Check the 'Result' column at step 4 in the execution_table.

At which step does the key 'user:1' get deleted from the cluster?

A Step 6

B Step 4

C Step 2

D Step 8

💡 Hint

Look at the 'Action' and 'Result' columns in the execution_table to find when DEL is executed.

If the client did not calculate the slot, what would happen?

A The command would be ignored

B The cluster would automatically find the correct node without issue

C The client would receive an error immediately

D The command would be routed to the wrong node and fail

💡 Hint

Refer to the concept_flow showing the importance of slot calculation before sending to the node.

Concept Snapshot

Redis Cluster distributes data by key slots.
Client calculates slot from key.
Slot determines which node handles the command.
Commands are sent directly to the correct node.
Nodes execute commands and return results.
This ensures data is distributed and accessible.

Full Transcript

In Redis Cluster architecture, when a client sends a command involving a key, it first calculates the key's slot. This slot number determines which node in the cluster owns the data for that key. The client then sends the command directly to that node. The node executes the command and returns the result to the client. For example, setting a key 'user:1' involves calculating its slot, routing the SET command to the correct node, storing the value, and confirming success. Later GET and DEL commands follow the same routing process. If the key is deleted, subsequent GET commands return nil. This process ensures efficient data distribution and retrieval across the cluster.