Your First Aggregate.md

Your First Aggregate

To build first aggregate, you need to implement Helios.Aggregate behaviour that providies extendable facility for aggregate command pipeline.

Helios Installation

Available in Hex, the package can be installed by adding helios_aggregate to your list of dependencies in mix.exs:

def deps do
  [
    {:helios, "~> 0.1"}
  ]
end

Defining Domain Events

defmodule CustomerCreated do
  defstruct [:customer_id, :first_name, :last_name]
end

defmodule CustomerContactCreated do
  defstruct [:customer_id, :email]
end

Customer Aggregate Example

defmodule CustomerAggregate do
  use Helios.Aggregate

  # Aggregate state
  defstruct [:id, :first_name, :last_name, :email]

  def create_customer(ctx, %{id: id, first_name: fname, last_name: lname, email: email}) do
    if id == ctx.aggregate.id do
      raise RuntimeError, "Already created"
    end

    ctx
    |> emit(%CustomerCreated{
        customer_id: id,
        first_name: fname,
        last_name: lname
      })
    |> emit(%CustomerContactCreated{
        customer_id: id,
        email: email
      })
    |> ok(%{status: :created, payload: %{id: id}})
  end

  def apply_event(%CustomerCreated{}=event, customer) do
    %{customer
      | id: event.id,
        first_name: event.first_name,
        last_name: event.last_name
    }
  end

  def apply_event(%CustomerContactCreated{email: email}, customer) do
    %{customer| email: email}
  end

  # sometimes we generate event but it is not needed in recovery and it is safe to
  # skip it
  def apply_event(_, agg), do: agg
end

Message Handler Sample Code

  
  ctx = %Helios.Context{
    aggregate: %CustomerAggregate{},
    aggregate_module: CustomerAggregate,
    correlation_id: "1234567890",
    command: :create_customer,
    peer: self(),
    params: %{first_name: "Jhon", last_name: "Doe", email: "jhon.doe@gmail.com"}
  }

  ctx = Cusomer.call(ctx, :create_customer)
    

Pipeline Logger configuration

We built logger which should log each command sent to aggregate. Since commands can cary some confidential information, or you have to be PCI DSS compliant, we expsed configuration like below where you could configure which filed values in command should be retracted.

Below is example where list of field names are given. Please note, the logger plug will not try to conver string to atom or other way round, if you have both case please state them in list as both, string and atom.

use Mix.Config

# filter only specified
config :helios, 
  :filter_parameters, [:password, "password", :credit_card_number]

# filter all but keep original values
config :helios, 
  :filter_parameters, {:keep, [:email, "email", :full_name]}

# retract only specified field values
config :helios, 
  :filter_parameters, [:password, "password", :credit_card_number]