DDD/CQRS/ES/NoSQL and functional programming

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I am using CQRS in one project. So this is qorking, and I see no reason why it would not.

CQRS/DDD and friends are not bound to object oriented programming at all. Also, Elixir/Erlang are not that much different from object oriented languages when you think about it.

What I am doing at the moment is that I have two supervised workers, started when my application starts:

  • CQRS.Commands
  • CQRS.Events

there’s little bit of DSL in CQRS.Commands to define the list of commands my system accepts. These look like:

public MyApp.LogIn   
public MyApp.Register
private MyApp.CreatePost
private MyApp.DeletePost`

I have a single controller exposed on /api/commands that accepts params that have command name and payload. Public commands are ones that do not require User fetched (have plug for that). Private commands are the ones that get forwarded payload, but additionally current user.

Command looks like this inside:

defmodule MyApp.Commands.Register do
  alias MyApp.Repo
  alias MyApp.User
  alias MyApp.Events

  import  MyApp.Validator

  alias MyAppCommands.Register.Form

  def execute(params, _ \\ nil) do
    form = Form.from_params(params)

    if valid?(form) do
      Events.trigger("UserRegistered", event_data(form))

      {:ok, %{message: "email conrirmation needed"}}
    else
      {:error, errors(form)}
    end
  end

  defp event_data(form) do
    %{
      id: UUID.uuid4(),
      first_name: form.first_name,
      last_name: form.last_name,
      email: form.email,
      lang: form.lang,
      password_hash: Comeonin.Bcrypt.hashpwsalt(form.password),
      email_confirmation_token: Ecto.UUID.generate
    }
  end

  defp token_for_user(user) do
    Phoenix.Token.sign MyApp.Endpoint, "user_id", user.id
  end
end

and in my system commands can either succeed :ok , then generate some events, or fail (return tuple of :error and validation messages.

I do simple validations with https://github.com/CargoSense/vex.

Generated events are being dispatched by my CQRS.Events, that is also a bit of DSL that lists all the events that can happen. They return nothing and they should succeed. I do not have the strategy worked out yet on what to do when they fail. I am experimenting here, not everything is yet perfect.

But basically, commands gets executed, generates some events, those events are handled by events handlers. This is all happening in two workers that are GenServers, so one thing at a time.

In addiion to that I package all my queries into MyApp.Queries… etc. Events modify data, queries read data, I have clear separation here.

Queries are executed normally in Phoenix controllers.

I use Phoenix’ views to serialize results of my queries and send over to client either via JSON or just embed the thing into conn and pass to view.

I haven’t figured everything out yet. I will wrap it up in a library, and write a blog post soon. Maybe it’ll be useful for someone and maybe I get some good feedback. I am not CQRS/ddd expert, so will need that.

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