Structuring Contexts and Channels in a Real Time Weather App

Phoenix Forum Questions / Help
,
1

After reading the documentation on Contexts, I’m wondering how to best structure contexts for a realtime weather app that broadcasts temperature readings via channels from either Agent or GenServer (streaming CSV or JSON files), or using Postgres LISTEN/NOTIFY.

Any feedback is highly appreciated!

city.ex

schema "city" do
field :city_name, :string
has_many :temp, App.Temperature
has_many :precipitation, App.Temperature
end

temperature.ex

schema "temperature" do
field :temp, :float
field :precipitation, :float
belongs_to :city, App.City
end

-lib
– my_app
— city
---- city.ex
---- temperature.ex
– my_app_web
— channels
---- temperature_socket.ex

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2

One important thing to notice: it’s better to use contexts to separate concerns on your system and not just grouping schemas by itself. So I would do something like:

  • lib
    • my_app/weather
      • city.ex
      • temperature.ex
    • my_app_web/channels
      • temperature_channels.ex

Maybe, once your system grows with more and more schemas, you could separate the city on another context called “locations” or something like that, but I would not do that on the beginning because it would just make your system more complex needlessly.

3

Thank you for your input.

In your example, “weather” is the context and city and temperature are the schemas?

Do you think it’s a good idea to then have the following setup? My concern is with channels. Specifically, I want to broadcast temperature updates in real time but separating each city to its own lobby/room so I can broadcast updates for each city individually. I don’t know if it’s better to create a JSON api that will trigger (e.g when there’s a new temperature update) a broadcast event for the channel, or to rely on Postgres listen/notify feature (hence the brief excerpt of the simple associations below)

2 tables:
city (parent) and temperature (child)

City has many temperatures.
Temperature belongs to City.

city_id is the foreign key in Temperature.

1 Like
4

Correct.

Well, I would do this:

  • lib
    • my_app/weather
      • city.ex # schema
      • temperature.ex # schema
      • weather.ex # context
    • my_app_web/channels
      • user_socket.ex # socket
      • city_channel.ex # channel

Then in my_app_web/channels/user_socket.ex:

defmodule MyAppWeb.UserSocket do
  use Phoenix.Socket

  ## Channels
  channel "city:*", MyAppWeb.CityChannel

  # ...
end

And in my_app_web/channels/city_channel.ex:

defmodule MyAppWeb.DashboardChannel do
  use MyAppWeb, :channel

  alias MyApp.Weather

  def join("city:" <> id, _payload, socket) do
    {:ok, Socket.assign(socket, :city_id, id)}
  end

  def handle_in("update_temperature", params, socket) do
    case Weather.update_temperature(socket.assigns[:city_id], params) do
      {:ok, temperature} -> broadcast! socket, "update_temperature", %{body: temperature}
      {:error, cs} -> # handle your error
    end
    {:noreply, socket}
  end
end
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