State Management

Act models domain logic as state machines — each entity is a state definition with actions that emit events, and events that patch state.

State Builder

States are built using a fluent API:

import { state } from "@rotorsoft/act";
import { z } from "zod";

const Counter = state({ Counter: z.object({ count: z.number() }) })
  .init(() => ({ count: 0 }))
  .emits({ Incremented: z.object({ amount: z.number() }) })
  .patch({
    Incremented: ({ data }, state) => ({ count: state.count + data.amount }),
  })
  .on({ increment: z.object({ by: z.number() }) })
    .emit((action) => ["Incremented", { amount: action.by }])
  .build();

Builder Chain

state({}) → .init() → .emits({}) → optional .patch({}) → .on({}) → optional .given([]) → .emit() → .build()

• .emits() declares events with passthrough reducers by default (({ data }) => data)
• .patch() overrides only events that need custom reducers
• .emit("EventName") passes the action payload through directly as event data
• .emit((action, snapshot, target) => [name, data]) for computed event data. The handler receives:
  • action — the validated action payload
  • snapshot — the current state snapshot ({ state, version, event, ... })
  • target — the dispatch target ({ stream, actor }), useful when the actor or stream id needs to flow into the event

Partial States

Multiple states sharing the same name merge automatically when composed in slices or the act orchestrator:

const TicketCreation = state({ Ticket: z.object({ title: z.string() }) })
  .init(() => ({ title: "" }))
  .emits({ TicketOpened: z.object({ title: z.string() }) })
  .on({ OpenTicket: z.object({ title: z.string() }) })
    .emit("TicketOpened")
  .build();

const TicketOperations = state({ Ticket: z.object({ status: z.string() }) })
  .init(() => ({ status: "open" }))
  .emits({ TicketClosed: z.object({ reason: z.string() }) })
  .on({ CloseTicket: z.object({ reason: z.string() }) })
    .emit("TicketClosed")
  .build();

// These merge into a single "Ticket" state with both actions and events

Patch merge priority: When partials are merged and both declare the same event:

• One custom, one passthrough → keep the custom one (order doesn't matter)
• Same function reference → re-registration from another slice, allowed
• Two different custom patches → throw error at build time

This means a partial can redeclare an event in .emits() (to react to it via .on()) without overwriting the custom reducer from the partial that owns the event.

Invariants

Business rules enforced before actions execute:

import { type Invariant } from "@rotorsoft/act";

const mustBeOpen: Invariant<{ status: string }> = {
  description: "Ticket must be open",
  valid: (state) => state.status === "open",
};

.on({ CloseTicket: z.object({ reason: z.string() }) })
  .given([mustBeOpen])
  .emit("TicketClosed")

When an invariant fails, the framework throws an InvariantError with the description.

Snapshots

For long-lived streams, configure snapshotting to avoid replaying the entire event history on cold starts:

.snap((snap) => snap.patches >= 50)  // snapshot every 50 events

Snapshots are persisted as __snapshot__ events in the store and used as a starting point when the cache is cold (process restart, LRU eviction).

Cache

Cache is always-on with InMemoryCache (LRU, maxSize 1000) as the default. It stores the latest state checkpoint per stream:

• On load() — cache is checked first; only events after the cached position are replayed from the store
• On action() — cache is updated after each successful commit
• On ConcurrencyError — stale cache entries are invalidated automatically

Cache and snapshots are the same checkpoint pattern at different layers. Cache eliminates store round-trips on warm hits; snapshots limit replay on cache miss.

Projections

Projections are read-model updaters that react to events:

import { projection } from "@rotorsoft/act";

const TicketProjection = projection("tickets")
  .on({ TicketOpened })
    .do(async ({ stream, data }) => {
      await db.insert(tickets).values({ id: stream, ...data });
    })
  .build();

Projection handlers receive (event, stream) — no dispatcher, no state mutations.

Slices

Slices group partial states with scoped reactions into vertical feature modules:

import { slice } from "@rotorsoft/act";

const TicketSlice = slice()
  .withState(TicketCreation)
  .withState(TicketOperations)
  .withProjection(TicketProjection)
  .on("TicketOpened")
    .do(async (event, stream, app) => {
      // reactingTo is auto-injected — no need to pass `event` explicitly
      await app.do("AssignTicket", { stream: event.stream, actor }, payload);
    })
    .to((event) => ({ target: event.stream }))
  .build();

Slice handlers receive (event, stream, app) where app implements IAct (do, load, query, query_array).

Auto-injected reactingTo

When a slice handler calls app.do() without an explicit fourth argument, the framework automatically threads the triggering event in as reactingTo, propagating the correlation chain (correlation and causation.event) through the new commit. Pass an explicit fourth argument only if you want to override that default — e.g., to attribute a side-effect commit to a different upstream event.

Act Orchestrator

The orchestrator composes everything:

const app = act()
  .withState(Counter)
  .withSlice(TicketSlice)
  .withProjection(AuditProjection)
  .build();

const snaps = await app.do("increment", { stream: "counter1", actor }, { by: 5 });
const snapshot = await app.load(Counter, "counter1");

Snapshot shape

Both app.do() (returns one snapshot per emitted event) and app.load() (returns one snapshot for the latest replayed state) yield objects of this shape:

type Snapshot<TState> = {
  state: TState;       // current state after this event
  version: number;     // 0-indexed stream version
  event?: Committed;   // the event that produced this state (undefined on init)
  patch?: Partial<TState>; // the diff applied by this event's reducer
  patches: number;     // events since last __snapshot__
  snaps: number;       // total __snapshot__ events seen on this stream
  cache_hit: boolean;  // true when load() served from cache without store I/O
  replayed: number;    // events processed past the cache point (0 on a warm hit)
};

patches and snaps drive the .snap() predicate; cache_hit and replayed show in the trace breadcrumbs and tell you whether the load round-tripped to the store.

Utility Types

Extract inferred types from built State objects:

import type { InferEvents, InferActions } from "@rotorsoft/act";

type Events = InferEvents<typeof Counter>;
type Actions = InferActions<typeof Counter>;