Reducers

Reducers are shared Chronicle read-model behavior. Use the shared reducer docs for the model, lifecycle, filtering, and client-tabbed examples.

• Reducers
• Getting started with reducers
• Reducer event processing
• Elixir client setup

Passive reducers

use Chronicle.Reducers.Reducer accepts an :active option, defaulting to true. Set it to false for a passive reducer: its read model is only computed on demand (e.g. via Chronicle.ReadModels.get_instance_by_id/3) instead of being kept warm in the background as events are appended — useful for a reducer whose read model backs a command-side decision that’s read rarely, where continuous observation would be wasted work. Mirrors C#‘s ReducerAttribute.IsActive.

defmodule MyApp.Reducers.AccountReducer do
  use Chronicle.Reducers.Reducer, model: MyApp.ReadModels.AccountInfo, active: false

  alias MyApp.Events.OrderPlaced

  @handles OrderPlaced

  @impl true
  def reduce(%OrderPlaced{}, model, _context), do: model
end

Replay lifecycle hooks

Reducers support the same optional on_replay_begin/0, on_replay_end/0, on_partition_replay_begin/1, and on_partition_replay_end/1 callbacks as reactors — see Replay lifecycle hooks for details. They run outside normal reduce/3 dispatch and are not events to reduce.