Pub/sub limitations (no persistence) in Redis - Step-by-Step Execution

Concept Flow - Pub/sub limitations (no persistence)

Publisher sends message

↓

Redis receives message

↓

Redis broadcasts to all subscribers currently connected

↓

Subscribers receive message

↓

If subscriber is offline, message is lost

↓

No message stored for later retrieval

Messages published are sent only to subscribers connected at that moment; no messages are saved for offline clients.

Execution Sample

Redis

PUBLISH news "Hello subscribers!"
-- Subscribers connected receive the message
-- Subscribers offline do not receive it

Shows that published messages reach only currently connected subscribers, no persistence.

Execution Table

Step	Action	Publisher State	Redis State	Subscribers Connected	Message Delivered
1	Publisher sends message 'Hello subscribers!'	Message ready	No message stored	Sub1, Sub2 connected	No
2	Redis broadcasts message to connected subscribers	Message sent	No message stored	Sub1, Sub2 connected	Sub1: Yes, Sub2: Yes
3	Subscriber Sub3 connects after message sent	No new message	No message stored	Sub1, Sub2, Sub3 connected	Sub3: No (missed message)
4	Publisher sends new message 'Update!'	Message ready	No message stored	Sub1, Sub2, Sub3 connected	Sub1: Yes, Sub2: Yes, Sub3: Yes
5	Subscriber Sub2 disconnects	No new message	No message stored	Sub1, Sub3 connected	No new message delivered
6	Publisher sends message 'Final news'	Message ready	No message stored	Sub1, Sub3 connected	Sub1: Yes, Sub3: Yes
7	Subscriber Sub2 reconnects after message sent	No new message	No message stored	Sub1, Sub2, Sub3 connected	Sub2: No (missed 'Final news')

💡 Execution ends because Redis pub/sub does not store messages; offline subscribers miss messages sent while disconnected.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7
Publisher Message	None	'Hello subscribers!'	None	'Update!'	None	'Final news'	None
Redis Stored Messages	None	None	None	None	None	None	None
Subscribers Connected	Sub1, Sub2	Sub1, Sub2	Sub1, Sub2, Sub3	Sub1, Sub2, Sub3	Sub1, Sub3	Sub1, Sub3	Sub1, Sub2, Sub3
Message Delivered to Sub2	No	Yes	No (missed message)	Yes	No	No	No (missed 'Final news')

Key Moments - 3 Insights

Why does subscriber Sub3 not receive the first message?

Because Sub3 connected after the message was published (see execution_table step 3), Redis pub/sub does not store messages for later delivery.

Does Redis store messages for offline subscribers to receive later?

No, Redis pub/sub sends messages only to currently connected subscribers and does not persist messages (execution_table shows Redis Stored Messages always None).

What happens when a subscriber disconnects and misses messages?

They do not receive any messages sent while disconnected, as shown by Sub2 missing the 'Final news' message after reconnecting (step 7).

Visual Quiz - 3 Questions

Look at the execution_table at step 3. Why does Sub3 not receive the message?

A Because Sub3 was not connected when the message was published

B Because Redis lost the message

C Because Sub3 unsubscribed

D Because the message was corrupted

💡 Hint

Check Subscribers Connected and Message Delivered columns at step 3 in execution_table

At which step does Sub2 miss a message due to being disconnected?

A Step 2

B Step 5

C Step 7

D Step 4

💡 Hint

Look at Subscribers Connected and Message Delivered to Sub2 in execution_table rows for steps 5-7

If Redis had message persistence, what would change in the execution_table?

A Redis Stored Messages would show stored messages and offline subscribers would receive missed messages

B Subscribers would disconnect more often

C Publisher would stop sending messages

D Subscribers would receive duplicate messages always

💡 Hint

Compare Redis Stored Messages and Message Delivered columns in execution_table

Concept Snapshot

Redis Pub/Sub sends messages only to subscribers connected at publish time.
No messages are saved for offline subscribers.
Subscribers connecting later miss past messages.
No message persistence means no delivery guarantees.
Useful for real-time but not for guaranteed delivery.

Full Transcript

In Redis Pub/Sub, when a publisher sends a message, Redis immediately broadcasts it to all subscribers currently connected. If a subscriber is offline or connects after the message is sent, it will not receive that message because Redis does not store messages for later delivery. This means messages are transient and only delivered in real-time to active subscribers. The execution table shows steps where subscribers connect or disconnect and whether they receive messages. Key moments highlight that offline subscribers miss messages and Redis does not persist messages. The quiz tests understanding of these points. This behavior is important to know when using Redis Pub/Sub for real-time messaging without persistence.