Prerequisites:

1. You don't need to know anything before reading this post.
2. A Redis Instance.

What is Publish-Subscribe?

Publish-Subscribe, or also commonly known as Pub/Sub, is a pattern to implement data streaming. It is super fast and asynchronous in nature.

How does it work?

You'd basically have a queue called Channel where messages are transmitted.
Then there are Publishers who publish/send the message and Subscribers who receive the message.

The Publisher would send a message into the channel and whoever's (Subscribers) subscribed to the Channel would receive that message.

The Publisher would not know about who is subscribing to the channel and the Subscriber would not know about who is publishing a particular message.

This removes all the work of programming the subscription mechanism.

A big problem with this pattern is, the Channel is a SPOF(Single Point Of Failure) if the Channel somehow crashes or breaks down.

To understand this pattern more, let us get our hands dirty by using the Pub/Sub feature of Redis.

Pub/Sub in Redis?

We all know that Redis is an In-Memory Key-Value Database. But Redis also has the feature of Pub/Sub.

If you have installed Redis, start the server and we are good to go.
Let us create a Pub/Sub for publishing a list of favorite TV Series.

1. To publish a message in the tv_series channel, we can use the PUBLISH command.

   127.0.0.1:6379> PUBLISH tv_series the_big_bang_theory
   (integer) 0
   

   This sends the the_big_bang_theory message into the tv_series channel.
   This command returns an integer denoting the number of subscribers that received the message. There are no subscribers as of now, so we should get the return value of 0.

2. To subscribe to the tv_series channel, we can use the SUBSCRIBE command.

   127.0.0.1:6379> SUBSCRIBE tv_series
   Reading messages... (press Ctrl-C to quit)
   1) "subscribe"
   2) "tv_series"
   3) (integer) 1
   

   We can subscribe to multiple channels at a time by just adding more channel names with space separation.
   We are now subscribed to the tv_series channel. The integer return is denoting the ith channel number we have subscribed to.

3. But we can't see the message the_big_bang_theory we have published in the channel. It is because we have subscribed to the channel after we have published the message. To be able to read a message. we have to have a subscriber in the channel while publishing the message.
   So let us publish the message again.

   127.0.0.1:6379> PUBLISH tv_series the_big_bang_theory
   (integer) 1
   

   This time we can see the return value is 1 as there is a subscriber currently in the channel, and also the message is printed in the output of the subscriber.

   127.0.0.1:6379> SUBSCRIBE tv_series
   Reading messages... (press Ctrl-C to quit)
   1) "subscribe"
   2) "tv_series"
   3) (integer) 1
   1) "message"
   2) "tv_series"
   3) "the_big_bang_theory"
   

   The output says we have received a message from the tv_series channel with the_big_bang_theory as the message.

4. To see the state of the Pub/Sub system, we can use the PUBSUB command.

   127.0.0.1:6379> PUBSUB CHANNELS
   1) "tv_series"
   

   PUBSUB CHANNELS gives us what channels that are currently there.

   127.0.0.1:6379> PUBSUB NUMSUB tv_series
   1) "tv_series"
   2) (integer) 1
   

   PUBSUB NUMSUB <channel> gives us the number of subscribers that a channel has.

5. To stop receiving messages, we can unsubscribe from the channel by using the UNSUBSCRIBE command or by pressing CTRL + C if you are using the CLI.

   127.0.0.1:6379> UNSUBSCRIBE tv_series
   

So, this is a basic demonstration of Pub/Sub in Redis. We'll look at another concept called Patterned Subscriptions in another blog post.

What can I try?

1. Try subscribing to multiple channels at the same time and observe the output.
2. Try doing the same thing from Python or JavaScript clients instead of CLI.

Reference Docs:

1. Redis Pub/Sub Docs

Thanks for reading the post, have a good day, and stay safe.