Reducers

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

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

Pinning the id

use Chronicle.Reducers.Reducer defaults the reducer’s id to its full module name. Renaming or moving the module then changes that id, and Chronicle treats it as a different reducer — its read model observation position resets. Pin the id explicitly so the module can be renamed or moved freely:

use Chronicle.Reducers.Reducer, id: "account-reducer", model: MyApp.ReadModels.AccountInfo

A reducer relying on the module-name default gets a one-time warning (per module, for the life of the BEAM node) from discovery, naming the module and showing this fix. See Discovery and artifact identity drift.

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.

Pinned generation delivery

By default (:compatibility), Chronicle upgrades every event to the handled module’s own generation before delivery, so reduce/3 always sees that generation’s shape regardless of which generation was actually appended. That is the right default when the reducer is written against the current generation.

Set generation_delivery: :pinned when the reducer is written against an older generation and must see events exactly as that generation was appended, instead of upgraded to the current one:

use Chronicle.Reducers.Reducer, model: MyApp.ReadModels.AccountInfo, generation_delivery: :pinned

context.appended_generation then tells you which generation was actually appended (0 means unknown — an older kernel, or an event appended outside a registered event source).

A client-wide default applies to every reducer that doesn’t set its own :generation_delivery:

{Chronicle.Client,
  ...
  event_generation_delivery: :pinned}

A reducer’s own option always wins over the client default. With :pinned, a reducer handling two generations of the same event type id has no single generation to pin to and raises Chronicle.Observers.MultipleGenerationsOfEventTypeInObserver on start.