PUBLISH to channels in Redis - Step-by-Step Execution

Concept Flow - PUBLISH to channels

Client sends PUBLISH command

↓

Redis server receives message

↓

Server identifies channel

↓

Server sends message to all subscribers

↓

Subscribers receive message

↓

End

The client sends a message to a channel; Redis server receives it and forwards the message to all clients subscribed to that channel.

Execution Sample

Redis

PUBLISH news "Hello subscribers!"

This command sends the message "Hello subscribers!" to all clients subscribed to the 'news' channel.

Execution Table

Step	Action	Channel	Message	Subscribers Notified	Result
1	Client sends PUBLISH command	news	"Hello subscribers!"	N/A	Command received by server
2	Server identifies channel	news	"Hello subscribers!"	N/A	Channel found
3	Server sends message to subscribers	news	"Hello subscribers!"	3 subscribers	Message delivered to all subscribers
4	Subscribers receive message	news	"Hello subscribers!"	3 subscribers	Subscribers process message
5	End	news	"Hello subscribers!"	3 subscribers	Publish complete

💡 All subscribers of 'news' channel have received the message; publish operation ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Channel	None	news	news	news	news
Message	None	"Hello subscribers!"	"Hello subscribers!"	"Hello subscribers!"	"Hello subscribers!"
Subscribers Count	0	0	3	3	3
Publish Status	Not started	Command received	Channel identified	Message sent	Complete

Key Moments - 3 Insights

Why does the server need to identify the channel before sending messages?

The server must know which channel to send the message to so it can notify the correct subscribers, as shown in step 2 of the execution_table.

What happens if no clients are subscribed to the channel?

If no clients are subscribed, the server still processes the PUBLISH command but no subscribers are notified, so the message is effectively discarded.

Does the PUBLISH command wait for subscribers to acknowledge the message?

No, the PUBLISH command sends the message and returns immediately; it does not wait for any acknowledgment from subscribers.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the server send the message to subscribers?

A Step 1

B Step 4

C Step 3

D Step 2

💡 Hint

Check the 'Action' column in the execution_table for when the message is sent.

According to variable_tracker, how many subscribers are notified after Step 3?

A 0

B 3

C 1

D Unknown

💡 Hint

Look at the 'Subscribers Count' row after Step 3 in variable_tracker.

If no clients subscribed to 'news', how would the 'Subscribers Notified' column change in the execution_table?

A "0 subscribers"

B "3 subscribers"

C "N/A"

D "Unknown"

💡 Hint

Consider what happens when no clients are subscribed as explained in key_moments.

Concept Snapshot

PUBLISH command syntax:
PUBLISH <channel> <message>

Sends a message to all clients subscribed to the channel.
Does not wait for acknowledgments.
If no subscribers, message is discarded.
Useful for real-time notifications.

Full Transcript

The PUBLISH command in Redis allows a client to send a message to a named channel. When the server receives this command, it identifies the channel and forwards the message to all clients subscribed to that channel. Subscribers receive the message asynchronously. If no clients are subscribed, the message is discarded. The command returns immediately after sending the message without waiting for any acknowledgment from subscribers.