PSUBSCRIBE for pattern matching in Redis - Step-by-Step Execution

Concept Flow - PSUBSCRIBE for pattern matching

Client sends PSUBSCRIBE with pattern

↓

Server registers pattern subscription

↓

Messages published to channels

↓

Server matches published channel names to patterns

↓

If match, server sends message to client

↓

Client receives messages matching pattern

The client subscribes to channel name patterns. The server sends messages for any published channel matching those patterns.

Execution Sample

Redis

PSUBSCRIBE news.*

# Server sends messages published to channels like 'news.sports', 'news.weather' to client

Client subscribes to all channels starting with 'news.' and receives messages published to those channels.

Execution Table

Step	Action	Pattern Subscribed	Published Channel	Pattern Match?	Message Sent to Client
1	Client sends PSUBSCRIBE	news.*		N/A	No
2	Message published	news.*	news.sports	Yes	Yes
3	Message published	news.*	news.weather	Yes	Yes
4	Message published	news.*	sports.news	No	No
5	Message published	news.*	news	No	No
6	Message published	news.*	news.sports.football	Yes	Yes
7	Client unsubscribes	news.*		N/A	No
8	Message published	news.*	news.sports	No (unsubscribed)	No

💡 Client unsubscribed from pattern; no further messages sent.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 7	Final
Subscribed Patterns	[]	["news.*"]	["news.*"]	[]	[]
Messages Received	[]	[]	["news.sports"]	["news.sports", "news.weather", "news.sports.football"]	["news.sports", "news.weather", "news.sports.football"]

Key Moments - 3 Insights

Why does 'news.sports.football' match the pattern 'news.*'?

Because the pattern 'news.*' matches any channel starting with 'news.' followed by any characters, including dots. This is shown in execution_table row 6 where the match is Yes.

Why doesn't 'sports.news' match the pattern 'news.*'?

Because 'sports.news' does not start with 'news.', so it does not match the pattern. This is shown in execution_table row 4 where the match is No.

What happens when the client unsubscribes from the pattern?

After unsubscribing (step 7), the client no longer receives messages even if channels match the pattern, as shown in step 8 where no message is sent.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, which channel was published and did it match the pattern?

A "news.sports", Yes

B "sports.news", No

C "news.weather", Yes

D "news", No

💡 Hint

Check row 3 under Published Channel and Pattern Match? columns.

At which step does the client stop receiving messages?

A Step 8

B Step 7

C Step 5

D Step 6

💡 Hint

Look at the exit_note and step 8 in execution_table where no message is sent after unsubscribe.

If the pattern was changed to 'news.sports*', which published channel would NOT send a message?

A "news.sports.football"

B "news.weather"

C "news.sports"

D "news.sports.basketball"

💡 Hint

Consider which channels start exactly with 'news.sports' as per pattern matching rules.

Concept Snapshot

PSUBSCRIBE lets clients subscribe to channel name patterns.
Patterns use '*' as wildcard for any characters.
Server sends messages for any published channel matching the pattern.
Useful to listen to groups of related channels.
Unsubscribe stops receiving matching messages.
Pattern matching is prefix-based with wildcards.

Full Transcript

PSUBSCRIBE in Redis allows a client to subscribe to channels using patterns with wildcards. When the client sends PSUBSCRIBE with a pattern like 'news.*', the server registers this subscription. Whenever a message is published to any channel matching this pattern, such as 'news.sports' or 'news.weather', the server sends the message to the client. Channels that do not match the pattern, like 'sports.news', do not trigger messages. If the client unsubscribes from the pattern, no further messages are sent even if channels match. This visual trace shows each step: subscribing, messages published, pattern matching, messages sent, and unsubscribing. It helps beginners see how pattern matching works in real time and understand why some messages are received and others are not.