Use Commanded to build your own Elixir applications following the CQRS/ES pattern.

Provides support for:

• Command registration and dispatch.
• Hosting and delegation to aggregate roots.
• Event handling.
• Long running process managers.

Uses eventstore for event data persistence to a PostgreSQL database.

MIT License

• Getting started
• Aggregate roots
• Commands
  • Command handlers
  • Command dispatch and routing
• Middleware
• Event handlers
• Process managers
• Supervision
• Serialization
• Used in production?
• Contributing

The package can be installed from hex as follows.

1. Add commanded to your list of dependencies in mix.exs:

```elixir
def deps do
  [{:commanded, "~> 0.9"}]
end
```

2. Ensure commanded is started before your application:

```elixir
def application do
  [applications: [:commanded]]
end
```

3. Configure the eventstore in each environment's mix config file (e.g. config/dev.exs), specifying usage of the included JSON serializer:

```elixir
config :eventstore, EventStore.Storage,
  serializer: Commanded.Serialization.JsonSerializer,
  username: "postgres",
  password: "postgres",
  database: "eventstore_dev",
  hostname: "localhost",
  pool_size: 10
```

4. Create the eventstore database and tables using the mix task.

```
mix event_store.create
```

Including commanded in the applications section of mix.exs will ensure it is started.

You may manually start the top level Supervisor process.

{:ok, _} = Commanded.Supervisor.start_link

Build your aggregate roots using standard Elixir modules and functions, with structs to hold state. There is no external dependency requirement.

An aggregate root is comprised of its state, public command functions, and state mutators.

A command function receives the aggregate root's state and the command to execute. It must return the resultant domain events. This may be none, one, or multiple events.

For business rule violations and errors you may return an {:error, reason} tagged tuple or raise an exception.

The state of an aggregate root can only be mutated by applying a raised domain event to its state. This is achieved by an apply/2 function that receives the state and the domain event. It returns the modified state.

Pattern matching is used to invoke the respective apply/2 function for an event. These functions must never fail as they are used when rebuilding the aggregate state from its history of raised domain events. You cannot reject the event once it has occurred.

defmodule BankAccount do
  defstruct [account_number: nil, balance: nil]

  # public command API

  def open_account(%BankAccount{} = account, account_number, initial_balance)
    when initial_balance > 0
  do
    %BankAccountOpened{account_number: account_number, initial_balance: initial_balance}
  end

  def open_account(%BankAccount{} = account, account_number, initial_balance)
    when initial_balance <= 0
  do
    {:error, :initial_balance_must_be_above_zero}
  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

Create a module per command and define the fields with defstruct. A command must contain a field to uniquely identify the aggregate instance (e.g. account_number).

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

Implement the Commanded.Commands.Handler behaviour consisting of a single handle/2 function.

It receives the aggregate root state and the command to be handled. It must return the raised domain events from the aggregate root. It may return an {:error, reason} tuple on failure.

defmodule OpenAccountHandler do
  @behaviour Commanded.Commands.Handler

  def handle(%BankAccount{} = aggregate, %OpenAccount{account_number: account_number, initial_balance: initial_balance}) do
    aggregate
    |> BankAccount.open_account(account_number, initial_balance)
  end
end

You must create a router to register each command with its associated handler.

defmodule BankRouter do
  use Commanded.Commands.Router

  dispatch OpenAccount, to: OpenAccountHandler, aggregate: BankAccount, identity: :account_number
  dispatch DepositMoney, to: DepositMoneyHandler, aggregate: BankAccount, identity: :account_number
end

It is also possible to route a command directly to an aggregate root. Without requiring an intermediate command handler.

defmodule BankRouter do
  use Commanded.Commands.Router

  dispatch OpenAccount, to: BankAccount, identity: :account_number
end

The aggregate root must implement an execute/2 function that receives the aggregate's state and the command to execute.

You can then dispatch a command using the router.

:ok = BankRouter.dispatch(%OpenAccount{account_number: "ACC123", initial_balance: 1_000})

A command handler has a default timeout of 5 seconds. The same default as a GenServer process call. It must handle the command in this period, otherwise the call fails and the caller exits.

You can configure a different timeout value during command registration or dispatch.

defmodule BankRouter do
  use Commanded.Commands.Router

  # configure a timeout of 1 second for the open account command handler
  dispatch OpenAccount, to: OpenAccountHandler, aggregate: BankAccount, identity: :account_number, timeout: 1_000
end

# dispatch the open account command with a timeout of 2 seconds
:ok = BankRouter.dispatch(%OpenAccount{account_number: "ACC123", initial_balance: 1_000}, 2_000)

Command routers support multi command registration so you can group related command handlers into the same module.

defmodule BankRouter do
  use Commanded.Commands.Router

  dispatch [OpenAccount,CloseAccount], to: BankAccountHandler, aggregate: BankAccount, identity: :account_number
end

Allows a command router to define middleware modules that are executed before and after success or failure of each command dispatch.

This provides an extension point to add in command validation, authorization, logging, and other behaviour that you want to be called for every command the router dispatches.

defmodule BankingRouter do
  use Commanded.Commands.Router

  middleware CommandLogger
  middleware MyCommandValidator
  middleware AuthorizeCommand

  dispatch OpenAccount, to: OpenAccountHandler, aggregate: BankAccount, identity: :account_number
  dispatch DepositMoney, to: DepositMoneyHandler, aggregate: BankAccount, identity: :account_number
end

The middleware modules are executed in the order they’ve been defined. They will receive a Commanded.Middleware.Pipeline struct containing the command being dispatched.

Implement the Commanded.Middleware behaviour in your module and define the before_dispatch, after_dispatch, and after_failure callback functions.

defmodule NoOpMiddleware do
  @behaviour Commanded.Middleware

  alias Commanded.Middleware.Pipeline
  import Pipeline

  def before_dispatch(%Pipeline{command: command} = pipeline) do
    pipeline
  end

  def after_dispatch(%Pipeline{} = pipeline) do
    pipeline
  end

  def after_failure(%Pipeline{} = pipeline) do
    pipeline
  end
end

Commanded provides a Commanded.Middleware.Logger middleware for logging the name of each dispatched command and its execution duration.

Create an event handler module which implements the Commanded.Event.Handler behaviour.

This consists of a single handle/2 function that receives each published domain event and its metadata, including the event's unique id. It should return :ok on success or {:error, :reason} on failure.

Use pattern matching to match on each type of event you are interested in. Add a catch-all handle/2 function for all other events to be ignored.

defmodule AccountBalanceHandler do
  @behaviour Commanded.Event.Handler

  def start_link do
    Agent.start_link(fn -> 0 end, name: __MODULE__)
  end

  def handle(%BankAccountOpened{initial_balance: initial_balance}, _metadata) do
    Agent.update(__MODULE__, fn _ -> initial_balance end)
  end

  def handle(%MoneyDeposited{balance: balance}, _metadata) do
    Agent.update(__MODULE__, fn _ -> balance end)
  end

  # ignore all other events
  def handle(_event, _metadata), do: :ok

  def current_balance do
    Agent.get(__MODULE__, fn balance -> balance end)
  end
end

Register the event handler with a given name. This is used when subscribing to the event store to track which events the handler has seen during restarts.

{:ok, _balance} = AccountBalanceHandler.start_link
{:ok, _handler} = Commanded.Event.Handler.start_link("account_balance", AccountBalanceHandler)

You can choose to start the event handler's event store subscription from the :origin, :current position or an exact event id using the start_from option. The default is to use the origin so your handler will receive all events.

# start from :origin, :current, or an explicit event id
{:ok, _handler} = Commanded.Event.Handler.start_link("account_balance", AccountBalanceHandler, start_from: :origin)

Use the :current position when you don't want newly created event handlers to go through all previous events. An example would be adding an event handler to send transactional emails to an already deployed system containing many historical events.

You should use a supervisor to start your event handlers to ensure they are restarted on error.

A process manager is responsible for coordinating one or more aggregate roots.

It handles events and may dispatch commands in response. Process managers have state that can be used to track which aggregate roots are being orchestrated.

A process manager must implement the Commanded.ProcessManagers.ProcessManager behaviour. It defines three callback functions: interested?/1, handle/2, and apply/2.

The interested?/1 function is used to indicate which events the process manager receives. The response is used to route the event to an existing instance or start a new process instance.

• Return {:start, process_uuid} to create a new instance of the process manager.
• Return {:continue, process_uuid} to continue execution of an existing process manager.
• Return {:stop, process_uuid} to stop an existing process manager and shutdown its process.
• Return false to ignore the event.

A handle/2 function must exist for each :start and :continue tagged event previously specified. It receives the process manager's state and the event to be handled. It must return the commands to be dispatched. This may be none, a single command, or many commands.

The apply/2 function is used to mutate the process manager's state. It receives its current state and the interested event. It must return the modified state.

defmodule TransferMoneyProcessManager do
  @behaviour Commanded.ProcessManagers.ProcessManager

  defstruct [
    transfer_uuid: nil,
    debit_account: nil,
    credit_account: nil,
    amount: nil,
    status: nil
  ]

  def interested?(%MoneyTransferRequested{transfer_uuid: transfer_uuid}), do: {:start, transfer_uuid}
  def interested?(%MoneyWithdrawn{transfer_uuid: transfer_uuid}), do: {:continue, transfer_uuid}
  def interested?(%MoneyDeposited{transfer_uuid: transfer_uuid}), do: {:continue, transfer_uuid}
  def interested?(_event), do: false

  def handle(%TransferMoneyProcessManager{}, %MoneyTransferRequested{transfer_uuid: transfer_uuid, debit_account: debit_account, amount: amount}) do
    %WithdrawMoney{account_number: debit_account, transfer_uuid: transfer_uuid, amount: amount}
  end

  def handle(%TransferMoneyProcessManager{transfer_uuid: transfer_uuid, credit_account: credit_account, amount: amount}, %MoneyWithdrawn{}) do
    %DepositMoney{account_number: credit_account, transfer_uuid: transfer_uuid, amount: amount}
  end

  def handle(%TransferMoneyProcessManager{}, %MoneyDeposited{}), do: []

  ## state mutators

  def apply(%TransferMoneyProcessManager{} = transfer, %MoneyTransferRequested{transfer_uuid: transfer_uuid, debit_account: debit_account, credit_account: credit_account, amount: amount}) do
    %TransferMoneyProcessManager{transfer |
      transfer_uuid: transfer_uuid,
      debit_account: debit_account,
      credit_account: credit_account,
      amount: amount,
      status: :withdraw_money_from_debit_account
    }
  end

  def apply(%TransferMoneyProcessManager{} = transfer, %MoneyWithdrawn{}) do
    %TransferMoneyProcessManager{transfer |
      status: :deposit_money_in_credit_account
    }
  end

  def apply(%TransferMoneyProcessManager{} = transfer, %MoneyDeposited{}) do
    %TransferMoneyProcessManager{transfer |
      status: :transfer_complete
    }
  end
end

Register the process manager router with a uniquely identified name. This is used when subscribing to events from the event store to track the last seen event and ensure they are only received once.

{:ok, _} = Commanded.ProcessManagers.Router.start_link("transfer_money_process_manager", TransferMoneyProcessManager, start_from: :current)

You can choose to start the process router's event store subscription from the :origin, :current position or an exact event id using the start_from option. The default is to use the origin so it will receive all events. You typically use :current when adding a new process manager to an already deployed system containing historical events.

Process manager instance state is persisted to storage after each handled event. This allows the process manager to resume should the host process terminate.

Use a supervisor to host your process managers and event handlers.

defmodule Bank.Supervisor do
  use Supervisor

  def start_link do
    Supervisor.start_link(__MODULE__, :ok)
  end

  def init(:ok) do
    children = [
      supervisor(Commanded.Supervisor, []),

      # process manager
      worker(Commanded.ProcessManagers.ProcessRouter, ["TransferMoneyProcessManager", TransferMoneyProcessManager, BankRouter, [start_from: :current]], id: :transfer_money_process_manager),

      # event handler
      worker(Commanded.Event.Handler, ["AccountBalanceHandler", AccountBalanceHandler, [start_from: :origin]], id: :account_balance_handler)
    ]

    supervise(children, strategy: :one_for_one)
  end
end

Your application should include the supervisor as a worker.

defmodule Bank do
  use Application

  def start(_type, _args) do
    import Supervisor.Spec, warn: false

    children = [
      worker(Bank.Supervisor, [])
    ]

    opts = [strategy: :one_for_one, name: __MODULE__]
    Supervisor.start_link(children, opts)
  end
end

JSON serialization is used by default for event and snapshot data.

The included Commanded.Serialization.JsonSerializer module provides an extension point to allow additional decoding of the deserialized value. This can be used for parsing data into valid structures, such as date/time parsing from a string.

The example event below has an implementation of the Commanded.Serialization.JsonDecoder protocol to parse the date into a NaiveDateTime struct.

defmodule ExampleEvent do
  defstruct [:name, :date]
end

defimpl Commanded.Serialization.JsonDecoder, for: ExampleEvent do
  @doc """
  Parse the date included in the event
  """
  def decode(%ExampleEvent{date: date} = event) do
    %ExampleEvent{event |
      date: NaiveDateTime.from_iso8601!(date)
    }
  end
end

You can implement the EventStore.Serializer behaviour to use an alternative serialization format if preferred.

Yes, Commanded is being used in production.

• Case Study: Building a CQRS/ES web application in Elixir using Phoenix

Pull requests to contribute new or improved features, and extend documentation are most welcome.

Please follow the existing coding conventions, or refer to the Elixir style guide.

You should include unit tests to cover any changes. Run mix test to execute the test suite.

• Andrey Akulov
• Andrzej Sliwa
• Henry Hazan