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

Concept Flow - SUBSCRIBE to channels

Client sends SUBSCRIBE command

↓

Server registers client to channel(s)

↓

Server waits for messages on channel(s)

↓

Message published to channel

↓

Server sends message to subscribed client

↓

Client receives message

↓

Repeat for new messages or client unsubscribes

The client sends a SUBSCRIBE command to the server, which registers the client to listen to specified channels. When messages are published to those channels, the server forwards them to the client.

Execution Sample

Redis

SUBSCRIBE news sports

(Publisher) PUBLISH news "Hello News"

(Publisher) PUBLISH sports "Game Tonight"

Client subscribes to 'news' and 'sports' channels; messages published to these channels are received by the client.

Execution Table

Step	Action	Server State	Client Output
1	Client sends SUBSCRIBE news sports	Client registered to channels: news, sports	Subscription confirmation for news and sports
2	Publisher sends PUBLISH news "Hello News"	Message queued for clients subscribed to news	Received message on channel 'news': "Hello News"
3	Publisher sends PUBLISH sports "Game Tonight"	Message queued for clients subscribed to sports	Received message on channel 'sports': "Game Tonight"
4	No more messages	Waiting for new messages or unsubscribe	No output, waiting

💡 Execution waits indefinitely until client unsubscribes or connection closes

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
Subscribed Channels	[]	[news, sports]	[news, sports]	[news, sports]	[news, sports]
Received Messages	[]	[]	["Hello News"]	["Hello News", "Game Tonight"]	["Hello News", "Game Tonight"]

Key Moments - 3 Insights

Why does the client receive a confirmation message after subscribing?

After the SUBSCRIBE command (Step 1), the server sends a confirmation to the client listing the channels subscribed, confirming registration as shown in the Client Output column.

Does the client receive messages from channels it did not subscribe to?

No, only messages published to channels the client subscribed to are forwarded. For example, messages on 'news' and 'sports' are received, but not others.

Why does the server keep waiting after delivering messages?

The SUBSCRIBE mode keeps the connection open to continuously receive messages until the client unsubscribes or disconnects, as shown in Step 4.

Visual Quiz - 3 Questions

Look at the execution table, what channels is the client subscribed to after Step 1?

A [news, sports]

B [news]

C [sports]

D []

💡 Hint

Check the Server State and Client Output columns in Step 1

At which step does the client receive the message "Game Tonight"?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the Client Output column for the message content

If the client unsubscribes after Step 3, what happens at Step 4?

A Server sends more messages

B Client receives no more messages

C Client automatically resubscribes

D Server closes connection immediately

💡 Hint

Refer to the exit_note and Step 4 description

Concept Snapshot

SUBSCRIBE command registers client to channels
Server forwards published messages on those channels
Client receives messages asynchronously
Connection stays open until unsubscribe or disconnect
Used for real-time message delivery

Full Transcript

The SUBSCRIBE command in Redis lets a client listen to one or more channels. When the client sends SUBSCRIBE with channel names, the server registers the client to those channels and confirms the subscription. Whenever a message is published to any of those channels, the server sends the message to the client. The client keeps receiving messages until it unsubscribes or disconnects. This allows real-time message delivery through channels.