Displaying data from database in real time in Phoenix

I have a question about broadcasting data from Postgres. I understand the way that you could do this if you’re building for example a chat application using Javascript, etc, etc… but what I had in mind is some kind of counter that is updated when someone interacts with a specific bot, for example, from top of my mind, there’s a big number (Just 1 number) in the web page and someone sends a message to the bot saying that he wants to add 10 to that number then the number updates in real time when the transaction to the database is done, my question is. How can I update this from channels or something else? Do I still need the JavaScript to interact with he DOM in real time? Perhaps use something like RethinkDB? I’m pretty lost with the implementation in this one.

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If I understand correctly then you can use a GenServer with an optional PubSub abstraction to achieve that and use database for persistence without building the whole thing around the database.

Say you have a GenServer that loads its initial state (number) from the database and has a function that updates (or adds to) this number, this function can be used anywhere in the application.

To be able to notify other processes about changes, this GenServer can store a list of pids and send messages to them or use one of existing abstractions, registry would do good.

Web layer: the webpage should have a channel connected and subscribed to changes (in join function), it will then receive updates and can act accordingly (e.g. push changed number to the client where the DOM can be updated).

Finally the database: depending on the number of updates you could subscribe to the same GenServer and update the table on each change or (if that happens a lot) add another lightweight GenServer, it can fetch the state from the publisher onece every minute or every X seconds and update the database.

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Good news! The hard work to send updates to other places in your application is already done for you by Phoenix.PubSub. There is no need to re-build it yourself (storing a list of pids, handle pids disappearing and reappearing, etc).

Depending on the scope of your application, you might either allow the location that performs the update send a broadcast to the registered Phoennix channels directly, or add a layer of indirection by manually subscribing your channels to a manually made PubSub location, in which case the broadcasting location does not need to know anything about the existence of a web layer (and you might be able to reuse it later for other types of data pushing).

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You can also use Boltun to take advantage of Postgres LISTEN/NOTIFY if you do want to build it around the DB.

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I now am at a computer, so it is easier to mock out a code example.

This is what I have done lately for a proof-of-concept ‘online game’, where game state is regularly updated, and then pushed towards all connected players. It does not depend on a database, since updating the data usually does not depend on the database (unless another application connects directly with the database, in which case Boltun might be what you want).

In the module that maintains the player state, I run the following function when this is updated:

  defp broadcast_update(state) do
    Phoenix.PubSub.broadcast(OnlineGame.PubSub, "player_state_update:#{state.user_id}", {:player_state_update, state})

(Note that OnlineGame.PubSub is the name configured for Phoenix Pubsub, see your Phoenix config file, it will by default be MyAppName.PubSub.)

This will send an update to any other process that would like to listen to the player_state_update:2 topic.
To subscribe a Phoenix Channel to this, which forwards the information over a Phoenix Channel to a user’s Browser, I have the following

code in the channel:

defmodule OnlineGame.Web.GameChannel do
  use Phoenix.Channel

  def join("player_state_update", _params, socket) do
    user_id = get_user_id_in_some_way()
    Phoenix.PubSub.subscribe(OnlineGame.PubSub, "player_state_update:#{user_id}")
    # Some other stuff not relevant for this example, and finally:
    {:ok, socket}

  def handle_info({:player_state_update, player_state}, socket) do
    push socket, "player_state_update", player_state
    {:noreply, socket}

So you just override the handle_info from another GenServer. Note that, since the process just receives the message without being given information about what topic it was the message arrived on, it makes sense to send a tuple like this.
The implementation of handle_info here just forwards the information to the socket.

The cool thing about this approach, is that my Game context does not need to know that I use Phoenix. (But just that I use Phoenix.PubSub). I could register to this update in any other location of the application.


Ah perfect, thanks for your example, it really helped me a lot, by the way, there is an error in your handle_info/2, push's message needs to be a map. With this example and the other answers I’ll be able to work my way in this ^-^, again, thanks!

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Ah yes, in my code the player_state happens to be a Struct, which happens to be a map.
Nicer would be to transform it in a special representation to ensure only publicly available data will be shown, of course.

Glad to have helped :smiley: .

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