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.

Practice

1.

What does the PUBLISH command do in Redis?

easy

A. Stores a message permanently in the database

B. Sends a message to all clients subscribed to a specific channel

C. Deletes all messages from a channel

D. Creates a new channel for clients to subscribe

Solution

1. Step 1: Understand the purpose of PUBLISH

   The PUBLISH command sends messages to clients currently subscribed to a channel.

2. Step 2: Differentiate from storage commands

   PUBLISH does not store messages permanently; it only broadcasts to active subscribers.

3. Final Answer:

   Sends a message to all clients subscribed to a specific channel -> Option B

4. Quick Check:

   PUBLISH broadcasts messages [OK]

Hint: PUBLISH sends messages only to active subscribers [OK]

Common Mistakes:

• Thinking PUBLISH stores messages permanently
• Confusing PUBLISH with channel creation
• Assuming PUBLISH deletes messages

2.

Which of the following is the correct syntax to publish a message 'Hello World' to a channel named news in Redis?

?

easy

A. PUBLISH news 'Hello World'

B. PUBLISH 'Hello World' news

C. PUBLISH news Hello World

D. PUBLISH news, 'Hello World'

Solution

1. Step 1: Recall Redis PUBLISH syntax

   The syntax is: PUBLISH channel message. Quote the message if it contains spaces.

2. Step 2: Identify correct option

   PUBLISH news 'Hello World' matches the syntax exactly.

3. Final Answer:

   PUBLISH news 'Hello World' -> Option A

4. Quick Check:

   Correct Redis PUBLISH syntax = PUBLISH news 'Hello World' [OK]

Hint: Quote messages with spaces: PUBLISH channel 'msg with spaces' [OK]

Common Mistakes:

• Not quoting messages with spaces
• Swapping channel and message order
• Using commas or extra symbols

3.

Given the following Redis commands executed in order:

SUBSCRIBE sports
PUBLISH sports "Game tonight"
PUBLISH news "Weather update"

What messages will the subscriber receive?

medium

A. Only "Game tonight"

B. Both "Game tonight" and "Weather update"

C. Only "Weather update"

D. No messages

Solution

1. Step 1: Identify subscribed channel

   The subscriber is subscribed to the 'sports' channel only.

2. Step 2: Check published messages

   Messages published to 'sports' are received; messages to 'news' are ignored by this subscriber.

3. Final Answer:

   Only "Game tonight" -> Option A

4. Quick Check:

   Subscribers get messages only from their channels [OK]

Hint: Subscribers get messages only from their subscribed channels [OK]

Common Mistakes:

• Assuming subscriber gets all published messages
• Confusing channel names
• Ignoring subscription context

4.

What is wrong with this Redis command to publish a message?

PUBLISH

Choose the best explanation.

medium

A. PUBLISH cannot be used without SUBSCRIBE

B. Channel name is invalid

C. Message is too long

D. Missing channel and message arguments

Solution

1. Step 1: Check required arguments for PUBLISH

   PUBLISH requires two arguments: channel and message.

2. Step 2: Identify missing parts in the command

   The command only has PUBLISH with no arguments, so it is incomplete.

3. Final Answer:

   Missing channel and message arguments -> Option D

4. Quick Check:

   PUBLISH needs channel and message [OK]

Hint: PUBLISH needs channel and message arguments [OK]

Common Mistakes:

• Thinking PUBLISH works without arguments
• Assuming message length causes error here
• Believing SUBSCRIBE is required before PUBLISH

5.

You want to send a notification to multiple channels news and alerts at once. Which Redis approach is best?

hard

A. Create a new channel combining both names and publish once

B. Publish to one channel and expect all subscribers to receive it

C. Use multiple PUBLISH commands, one per channel

D. Use a Redis command to publish to multiple channels simultaneously

Solution

1. Step 1: Understand Redis PUBLISH limitations

   Redis PUBLISH sends messages to one channel at a time; no built-in multi-channel publish exists.

2. Step 2: Choose correct method to notify multiple channels

   Send separate PUBLISH commands for each channel to reach all subscribers.

3. Final Answer:

   Use multiple PUBLISH commands, one per channel -> Option C

4. Quick Check:

   Multiple channels need multiple PUBLISH calls [OK]

Hint: Publish separately to each channel for multiple notifications [OK]

Common Mistakes:

• Expecting one PUBLISH to reach multiple channels
• Combining channel names incorrectly
• Assuming Redis supports multi-channel publish natively