Usage.md

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:

• Application
• 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 into 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{} = event) do
       %BankAccountOpened{account_number: account_number, initial_balance: initial_balance} = event
   
       %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. Define an application to host the aggregate and supporting processes:

   defmodule BankApp do
     use Commanded.Application,
       otp_app: :bank,
       event_store: [adapter: Commanded.EventStore.Adapters.InMemory]
   
     router BankRouter
   end

   This application is configured to use in-memory event store included with Commanded for testing.

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

   defmodule AccountBalanceHandler do
     use Commanded.Event.Handler,
       application: BankApp,
       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

7. Start the application and event handler processes:

   {:ok, _pid} = BankApp.start_link()
   {:ok, _pid} = AccountBalanceHandler.start_link()

   In a real application you would use a supervisor to start these processes.

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

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