Where is a Phoenix app's 'main' entry-point function?

The :phoenix application has it’s application start callback in Phoenix.

https://github.com/phoenixframework/phoenix/blob/main/lib/phoenix.ex

But you can not really call anything from there, that code has already been written.

And even though you can reload modules at will, the application has probably already been started.

Maybe explain a bit better what you are trying to achieve.

I am experimenting with Ecto and I have a module called Custom.DB that creates an in-memory database.

I have included the module as one of the children in application.ex.

However, before I can use it, I have to first create the tables, and populate the database. I’ve created a function for doing so, but I don’t know how to get it to run when the program starts with iex -S mix phx.server.

defmodule Custom.DB do
  use Ecto.Repo,
    otp_app: :testing,
    adapter: Ecto.Adapters.SQLite3

  def create_tables do
    :testing
    |> :code.priv_dir()
    |> Path.join("repo/testing.sql")
    |> File.read!()
    |> query()
  end

  def show_tables do
    query("select * from sqlite_schema;")
  end
end

in application.ex:

    children = [
    ...

      {Custom.DB, database: :memory, pool_size: 1}
      ...
    ]

Where do I run Custom.DB.create_tables()?

There’s no single entrypoint on the beam, which you can customize. That’s not how the abstractions are setup. The beam when starting will start a set of application. Each application can optionally provide a callback module, which (among others) will have the beam execute the start/2 function of that application. That’s where you put code to execute on startup.

You’re already have that callback module and function, because that’s where the children list is defined, which you posted.

Mhmm… thank you for trying, but I still don’t get it. Maybe my question wasn’t specific enough.

My question is: Where do I run Custom.DB.create_tables() so my database is populated with whatever I have in ‘repo/testing.sql’ when I run iex -S mix phx.server?

That totally depends on your exact setup, though I would probably do that kind of seeding in the process that owns the database, such that it’s reseeded if the process ever has to be restarted.

Do you want to do that whenever your application starts, only in development, only when using the phx.server mix task, …. There’s a lot of versions to this question, which do not share their answer.

I see. So there’s a lot more to this.

I want to have this run whenever my application starts. So, Custom.DB.create_tables() should be executed everytime I start the server, in dev or prod.

Once the database is populated, I can render views using the data within wih Ecto (Repo, Query, Schema).

Then the start/2 callback of your application module (usually MyApp.Application) is the place to run such things.

I really think the owning process should be responsible, not the application, to ensure that a crash in the DB process at least recovers into “empty but usable” database, rather than a database that doesn’t even have a schema.

Maybe. Sometimes that additional level of rigor is worth it. But I’d argue that’s step two. It’s no longer just the question of where the beams “main” is.