Command handler behaviour for CQRS pattern implementation in Raxol.

Command handlers are responsible for processing commands and executing the business logic associated with write operations. Each handler should focus on a single command type and maintain consistency.

Handler Design Principles

1. Single Responsibility: One handler per command type
2. Business Logic Focus: Contain domain logic, not infrastructure concerns
3. Idempotent: Handlers should be safe to retry
4. Event Generation: Generate domain events for state changes
5. Error Handling: Provide meaningful error messages

Usage

defmodule MyApp.Handlers.CreateUserHandler do
  use Raxol.Architecture.CQRS.CommandHandler

  alias MyApp.Commands.CreateUserCommand
  alias MyApp.Events.UserCreatedEvent
  alias MyApp.Repositories.UserRepository

  @impl true
  def handle(%CreateUserCommand{} = command, context) do
    with {:ok, user} <- UserRepository.create(command),
         {:ok, event} <- create_user_created_event(user, command),
         :ok <- publish_event(event, context) do
      {:ok, %{user_id: user.id, status: :created}}
    else
      {:error, :user_already_exists} ->
        {:error, :user_already_exists}
      {:error, reason} ->
        {:error, {:user_creation_failed, reason}}
    end
  end

  defp create_user_created_event(user, command) do
    event = %UserCreatedEvent{
      user_id: user.id,
      name: user.name,
      email: user.email,
      created_by: command.created_by,
      correlation_id: command.correlation_id
    }
    {:ok, event}
  end
end