Using Commanded

Commanded provides the building blocks for you to create your own Elixir applications following the CQRS/ES pattern.

A separate guide is provided for each of the components you can build:

• Aggregates.
• Commands, registration and dispatch.
• Events and handlers.
• Process managers.

Commanded uses strong consistency for command dispatch (write model) and eventual consistency, by default, for the read model. Receiving an :ok reply from dispatch indicates the command was successfully handled and any created domain events fully persisted to your chosen event store. You may opt-in to strong consistency for individual event handlers and command dispatch as required.

Quick overview

Here's an example bank account opening feature built using Commanded to demonstrate its usage.

1. Define an OpenBankAccount command:

    defmodule OpenBankAccount do
      defstruct [:account_number, :initial_balance]
    end

2. Define a corresponding BankAccountOpened domain event:

    defmodule BankAccountOpened do
      @derive Jason.Encoder
      defstruct [:account_number, :initial_balance]
    end

3. Build a BankAccount aggregate to handle the command, protect its business invariants, and return a domain event when successfully handled:

    defmodule BankAccount do
      defstruct [:account_number, :balance]
   
      # Public command API
   
      def execute(%BankAccount{account_number: nil}, %OpenBankAccount{account_number: account_number, initial_balance: initial_balance})
        when initial_balance > 0
      do
        %BankAccountOpened{account_number: account_number, initial_balance: initial_balance}
      end
   
      # Ensure initial balance is never zero or negative
      def execute(%BankAccount{}, %OpenBankAccount{initial_balance: initial_balance})
        when initial_balance <= 0
      do
        {:error, :initial_balance_must_be_above_zero}
      end
   
      # Ensure account has not already been opened
      def execute(%BankAccount{}, %OpenBankAccount{}) do
        {:error, :account_already_opened}
      end
   
      # State mutators
   
      def apply(%BankAccount{} = account, %BankAccountOpened{account_number: account_number, initial_balance: initial_balance}) do
        %BankAccount{account |
          account_number: account_number,
          balance: initial_balance
        }
      end
    end

4. Define a router module to route the open account command to the bank account aggregate:

    defmodule BankRouter do
      use Commanded.Commands.Router
   
      dispatch OpenBankAccount, to: BankAccount, identity: :account_number
    end

5. Create an event handler module that updates a bank account balance:

    defmodule AccountBalanceHandler do
      use Commanded.Event.Handler, name: __MODULE__
   
      def init do
        with {:ok, _pid} <- Agent.start_link(fn -> 0 end, name: __MODULE__) do
          :ok
        end
      end
   
      def handle(%BankAccountOpened{initial_balance: initial_balance}, _metadata) do
        Agent.update(__MODULE__, fn _ -> initial_balance end)
      end
   
      def current_balance do
        Agent.get(__MODULE__, fn balance -> balance end)
      end
    end

Finally, we can dispatch a command to open a new bank account:

:ok = BankRouter.dispatch(%OpenBankAccount{account_number: "ACC123456", initial_balance: 1_000})