PubSub vs LiveView Whats the difference

Happy Friday,

Phoenix LiveView is a powerful library that allows for real-time user experience updates. Meanwhile PubSub claims to publish and broadcast messages to a topic.

My question is why do we need both? why not just use LiveView and disregard PubSub. Can you give me scenario where PubSub and LiveView can work together?

Thank You

LiveView uses (phoenix) PubSub. Without PubSub there would be neither phoenix channels nor LV, which builds on top of the channels implementation.

So they are one Unit?

I’d rather say they build on top of each other. PubSub is just plain pubsub between elixir nodes, channels adds clients connecting to elixir nodes through various transports to the mix (by default long poll or websockets) and LV builds on top of channels to allow for dynamic html driven from the server.

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To over simplify it, PubSub is the communication between elixir code, both running on the server side. LiveView and Channel are the communication between elixir code on the server side and javascript on the client side.

I am sorry I don’t quite understand, would you be able to give me an analogy or use Liveview and Pubsub in an example?

May I suggest you to read a book? Asking on a forum works best if you already have the right context and only need a nudge in the right direction.

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Let’s start with the basics, you could use LiveView without PubSub. Phoenix has a dependency in the underlying channel in order to provide some convenience, but that’s not core to how LiveView/Channels work. Digging in:

On the surface LiveView lets you write (mostly) Elixir to build rich UIs. The messaging that happens back/forth to the frontend/backend is not tied to PubSub. For example, clicking a button will send a message from client->server but PubSub is not involved.

But LiveView isn’t just a library to let you build rich UIs in Elixir. It has a real-time component that lets you do things like update the UI based on a change that happened somewhere else. This is “magical” in some ways, but is critically backed by PubSub.

Let’s say that user A is connected to server 1. An event happens over a webhook and arrives on server 2. User A is not connected to server 2, so it wouldn’t get the real-time update and your UI is stale (not real-time).

This is where PubSub comes in. When the event happens on server 2, you can broadcast that event over PubSub. Other servers receive that event and they see that there’s a listener that cares about the event (your LiveView process). The event is processed and the UI is updated—your app is real-time.

Just to be clear though, that’s optional. If you just want the rich UI building and worry about real-time later, then you likely won’t even think about PubSub other than the setup instructions.

edit1: Added bit about how Phoenix has a dependency on PubSub, so it’s required to use LiveView. Added bit about how you don’t need to worry about PubSub unless you want real-time updates

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sure what book did you have in mind?

Would you say that pubSub is like Middleware?

In JS we often use Middleware to allow software to communicate with data management and the application.

PubSub is like, well, PubSub. It’s the Publish-Subscribe pattern. In JS I think there’s the EventBus thing which is similar?

Processes subscribe to a topic they are interested in, like "user:541:notifications"
A publisher is someone that sends a message to that topic, and what the PubSub server(a message bus) does is to relay the message to every process subscribed to that topic.

You can read more about that here: Publish–subscribe pattern - Wikipedia

So, the publisher sends messages to the PubSub indicating the topic, and the PubSub takes care of figuring out who are the recipients and relays the message to them. The Publisher doesn’t need to know anything about the subcribers, it only sends messages to the PubSub server.

Channels and Liveview use this under the hood to relay messages to the right components. Channel topics in Phoenix are essentially PubSub topics.

Now Liveview is a complete different beast and it’s about building interactive user interfaces. It is built on top of Phoenix Channels, so by extension it also uses PubSub under the hood.

Because PubSub is a component implementing a pattern, you can use it for your own purposes, not only Liveview. That’s the example Steve was mentioning.

For a real world example you can check the LiveBeats app by Chris McCord. Here a liveview is subscribing to pubsub events:

And here are some functions that handle the messages coming through PubSub to update the UI or the liveview state:

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The author of real-time Phoenix, @sb8244 has answered your question. So maybe you can start with that book.

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