Reactors

Reactors are documented in the shared Chronicle docs with synchronized examples for C#, Kotlin, Java, Elixir, and TypeScript.

• Getting started with reactors
• Reactor event processing
• Reactors overview

Use the Elixir get started page for client startup and supervision setup.

Side-effect appending

handle/2 can return {:ok, event_or_events} in addition to :ok/{:error, reason}, and Chronicle.Elixir appends the returned event(s) as a side effect right after handle/2 returns. Mirrors C#'s Reactors/SideEffects and Kotlin's ReactorsService.appendSideEffects.

Returning a single event struct appends it to the same event source that triggered the handler:

defmodule MyApp.Reactors.OrderNotifier do
  use Chronicle.Reactors.Reactor

  alias MyApp.Events.{OrderPlaced, OrderConfirmationQueued}

  @handles OrderPlaced

  @impl true
  def handle(%OrderPlaced{} = event, _context) do
    {:ok, %OrderConfirmationQueued{customer_id: event.customer_id}}
  end
end

Return a list of events to append them all atomically to that same event source. To target a different event source, wrap the event in a Chronicle.EventSequences.EventForEventSourceId; a list of these (optionally mixed with bare event structs, which are normalized to the triggering event source) is appended atomically across all their event source ids in one call.

If appending the side effect fails, handle/2's overall result becomes {:error, reason} — reported the same way as a plain {:error, reason} return.

Replay lifecycle hooks

Implement any of on_replay_begin/0, on_replay_end/0, on_partition_replay_begin/1, or on_partition_replay_end/1 to be notified when Chronicle starts or finishes replaying this reactor — either as a whole, or for a single partition (event source). All four are optional. Mirrors C#'s ICanBeNotifiedWhenReplay / ICanBeNotifiedWhenPartitionReplayed.

defmodule MyApp.Reactors.ReplayAwareNotifier do
  use Chronicle.Reactors.Reactor

  require Logger

  alias MyApp.Events.OrderPlaced

  @handles OrderPlaced

  @impl true
  def handle(%OrderPlaced{}, _context), do: :ok

  @impl true
  def on_replay_begin, do: Logger.info("Replay starting")

  @impl true
  def on_replay_end, do: Logger.info("Replay finished")
end

These are notifications, not events to handle — they run outside normal handle/2 dispatch, are not subject to failed-partition tracking, and a raised exception is logged and swallowed rather than reported to Chronicle.