December 08, 2015

Using GenEvent With Ecto Callbacks

Callbacks. It’s common to write tons of callback methods in Ruby ActiveRecord models, and they’re one reason ActiveRecord models tend to end up so complicated.

Ecto also has callback functions such as before_insert, before_update and so on. In my most recent Elixir project, I didn’t want to have my Ecto models end up as cluttered as my ActiveRecord models did.

Eliminating Callbacks

The most obvious way to eliminate callbacks in your models is simply not to use them at all. Every time you need to perform an action before_insert or after_update, you simply do that action.

So, in your controller, if you need to send an email after a user is created, you could do:

def create(conn, %{"user" => user_params}) do
  changeset = User.changeset(%User{}, user_params)

  case Repo.insert(changeset) do
    {:ok, user} ->
      send_email(user)
      # ...
    {:error, changeset} ->
      # ...
  end
end

However, this can become fragile if you start creating users in multiple places. You have to remember to add the send_email/1 call in each place. So, for this sort of thing, you really need an after_insert callback function to do it reliably after every user is inserted.

But then we’re back where we started, with the threat of an ever-expanding set of callback functions polluting our Ecto models.

Use GenEvent

The solution I came up with for this is very simple. We can think of the model’s lifecycle as a series of events: insert, update and delete. Therefore, we could use OTP’s built-in event management feature to handle this: GenEvent.

First, set up an Event broadcaster, like so:

defmodule MyApp.Event do
  @handlers [
    # Insert names of handler modules here
  ]

  def start_link do
    {:ok, manager} = GenEvent.start_link(name: __MODULE__)
    Enum.each(@handlers, &GenEvent.add_handler(manager, &1, []))
    {:ok, manager}
  end

  def broadcast(event) do
    GenEvent.notify(__MODULE__, event)
  end
end

We’re setting up a publish/subscribe model, and so the @handlers attribute is a list of subscriber modules. start_link/0 will be used to start the event broadcaster, and add register all of the subscribers. The broadcast/1 function will notify all subscribers of a new event.

We then add MyApp.Event to our supervisor to ensure it starts up:

children = [
  worker(MyApp.Event, [])
]

Broadcasting All Lifecyle Events

We now will create a module which will automatically broadcast lifecyle events. It looks like this:

defmodule MyApp.ModelLifecycle do
  alias MyApp.Event

  defmacro __using__(_) do
    quote do
      import unquote(__MODULE__)

      after_insert :broadcast_event, [:insert]
      after_update :broadcast_event, [:update]
      after_delete :broadcast_event, [:delete]
    end
  end

  # Broadcasts events in this format:
  # {:model, :update, changeset}
  def broadcast_event(changeset, type) do
    Event.broadcast({:model, type, changeset})
    changeset
  end
end

The __using__ macro imports the module and inserts all of the needed callbacks, pointing at broadcast_event/2, which then broadcasts that event on MyApp.Event, notifying all subscribers.

We can then use this ModelLifecyle module in our model:

defmodule MyApp.User do
  use Ecto.Model
  use MyApp.ModelLifecyle

  # ...
end

Now, our User model will automatically broadcast each lifecyle event after it occurs. Using the GenEvent API, we can subscribe to those notifications and do something with them.

For example, we could write an EmailSubscriber, like so:

defmodule MyApp.EmailSubscriber do
  use GenEvent

  alias MyApp.User

  def handle_event({:model, :insert, %{model: %User{}} = changeset}, state) do
    send_email(changeset.model)
    {:ok, state}
  end
end

With a little tender love and care, (not shown) we can pretty up this API to look like so:

defmodule MyApp.EmailSubscriber do
  use GenEvent

  alias MyApp.User

  def handle_insert(%User{} = user) do
    send_email(user)
  end

  # ...
end

Benefits

This approach provides a few benefits:

  • We can have multiple subscribers to the same events
  • New subscribers require no changes to the model
  • Event handling logic for multiple models can live in one place
  • Lifecycle events are handled in a separate process, and therefore don’t prevent your model from being saved if they fail. This can be a good or bad thing depending on your use case.

Downsides

Like everything, this approach also has its downsides.

  • Added complexity
  • Implicit behaviour: it isn’t immediately obvious how the logic for certain events is triggered. You need to know about the pub/sub behaviour to know where to look.

If either of these downsides is too much for you, you can also reference a foreign module in an Ecto callback, like so:

after_insert OtherModule, :function_name

This allows you to delegate callback work to other modules, keeping your model clean, but requires you to change each model if you want to add another “subscriber”.

Conclusion

I hope you found this as interesting as I did when I discovered it. Hopefully I’ll have more time for blog posts once my LearnElixir.tv screencast series is finished.

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