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Event handling

Domain events

Domain events indicate that something of importance has occurred, within the context of an aggregate. They are named in the past tense: account registered; funds transferred; fraudulent activity detected.

Create a module per domain event and define the fields with defstruct. An event should contain a field to uniquely identify the aggregate instance (e.g. account_number).

defmodule BankAccountOpened do
  defstruct [:account_number, :initial_balance]

Note, due to event serialization you should expect that only: strings, numbers and boolean values defined in an event are preserved; any other value will be converted to a string. You can control this behaviour as described in serialization section.

Event handlers

Event handlers allow you to execute code that reacts to domain events: to build read model projections; dispatch commands to other aggregates; and to interact with third-party systems such as sending emails.

Use the Commanded.Event.Handler macro within your event handler module to implement the defined behaviour. This consists of a single handle/2 function that receives each published domain event and its metadata, including the event's unique event number. It should return :ok on success or {:error, :reason} on failure. You can return {:error, :already_seen_event} to skip events that have already been handled, due to the at-least-once event delivery of the supported event stores.

Use pattern matching to match on each type of event you are interested in. A catch-all handle/2 function is included, so all other events will be ignored by default.

defmodule ExampleHandler do
  use Commanded.Event.Handler, name: "ExampleHandler"

  def handle(%AnEvent{..}, _metadata) do
    # ... process the event

The name given to the event handler must be unique and remain unchanged between releases. It is used when subscribing to the event store to track which events the handler has seen during restarts.

{:ok, _handler} = ExampleHandler.start_link()

Subscription options

You can choose to start the event handler's event store subscription from the :origin, :current position, or an exact event number 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 number (e.g. 1234)
defmodule ExampleHandler do
  use Commanded.Event.Handler, name: "ExampleHandler", start_from: :origin

  # ...

You can optionally override :start_from by passing it as param:

{:ok, _handler} = ExampleHandler.start_link(start_from: :current)

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 start your event handlers using a supervisor to ensure they are restarted on error.

Subscribing to an individual stream

By default event handlers will subscribe to all events appended to any stream. Provide a subscribe_to option to subscribe to a single stream.

defmodule ExampleHandler do
  use Commanded.Event.Handler,
    name: __MODULE__,
    subscribe_to: "stream1234"

This will ensure the handler only receives events appended to that stream.

init/0 callback

You can define an init/0 function in your handler to be called once it has started and successfully subscribed to the event store.

This callback function must return :ok, any other return value will terminate the event handler with an error.

defmodule ExampleHandler do
  use Commanded.Event.Handler, name: "ExampleHandler"

  def init do
    # optional initialisation

  def handle(%AnEvent{..}, _metadata) do
    # ... process the event

error/3 callback

You can define an error/3 callback function to handle any exceptions or errors returned from your event handler's handle/2 functions. The error/3 function is passed the actual error (e.g. {:error, :failure}), the failed event, and a failure context.

Use pattern matching on the error and/or failed event to explicitly handle certain errors or events. You can choose to retry, skip, or stop the event handler after an error.

The default behaviour if you don't provide an error/3 callback is to stop the event handler using the exact error reason returned from the handle/2 function. You should supervise event handlers to ensure they are correctly restarted on error.

Example error handling

defmodule ExampleHandler do
  use Commanded.Event.Handler, name: __MODULE__

  require Logger

  alias Commanded.Event.FailureContext

  def handle(%AnEvent{}, _metadata) do
    # Simulate event handling failure
    {:error, :failed}

  def error({:error, :failed}, %AnEvent{} = event, %FailureContext{context: context}) do
    context = record_failure(context)

    case Map.get(context, :failures) do
      too_many when too_many >= 3 ->
        # Skip bad event after third failure
        Logger.warn(fn -> "Skipping bad event, too many failures: " <> inspect(event) end)


      _ ->
        # Retry event, failure count is included in context map
        {:retry, context}

  defp record_failure(context) do
    Map.update(context, :failures, 1, fn failures -> failures + 1 end)


The handle/2 function in your handler receives the domain event and a map of metadata associated with that event. You can provide the metadata key/value pairs when dispatching a command:

:ok = ExampleRouter.dispatch(command, metadata: %{"issuer_id" => issuer_id, "user_id" => ""})

In addition to the metadata key/values you provide, the following system values will be included in the metadata passed to an event handler:

  • event_id - a globally unique UUID to identify the event.
  • event_number - a globally unique, monotonically incrementing and gapless integer used to order the event amongst all events.
  • stream_id - the stream identity for the event.
  • stream_version - the version of the stream for the event.
  • causation_id - an optional UUID identifier used to identify which command caused the event.
  • correlation_id - an optional UUID identifier used to correlate related commands/events.
  • created_at - the date/time, in UTC, indicating when the event was created.

These key/value metadata pairs will use atom keys to differentiate them from the user provided metadata:

defmodule ExampleHandler do
  use Commanded.Event.Handler, name: "ExampleHandler"

  def handle(event, metadata) do
    # %{
    #   :causation_id => "db1ebd30-7d3c-40f7-87cd-12cd9966df32",
    #   :correlation_id => "1599630b-9c38-433c-9548-0dd793108ba0",
    #   :created_at => ~N[2017-10-30 11:19:56.178901],
    #   :event_id => "5e4a0f38-385b-4d57-823b-a1bcf705b7bb",
    #   :event_number => 12345,
    #   :stream_id => "e42a588d-2cda-4314-a471-5d008cce01fc",
    #   :stream_version => 1,
    #   "issuer_id" => "0768d69a-d2b7-48f4-d0e9-083a97f7ebe0",
    #   "user_id" => ""
    # }


Consistency guarantee

You can specify an event handler's consistency guarantee using the consistency option:

defmodule ExampleHandler do
  use Commanded.Event.Handler,
    name: "ExampleHandler",
    consistency: :eventual

The available options are :eventual (default) and :strong:

  • Strong consistency offers up-to-date data but at the cost of high latency.
  • Eventual consistency offers low latency but read model queries may reply with stale data since they may not have processed the persisted events.

You request the consistency guarantee, either :strong or :eventual, when dispatching a command. Strong consistency will block the command dispatch and wait for all strongly consistent event handlers to successfully process all events created by the command. Whereas eventual consistency will immediately return after command dispatch, without waiting for any event handlers, even those configured for strong consistency.