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Envelope format

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When an actor has an eventAdapter() (or stateAdapter()), every persisted payload is wrapped in a versioned envelope before write:

{
  "_v": 2,                                    // version
  "_t": "deposited",                          // type tag
  "_e": { "kind": "deposited", "amount": 100, "currency": "USD" }
}

On read, the framework unwraps this into a stored frame { manifest, version, payload } and routes it through the adapter’s fromJournal(stored). The adapter sees the manifest, version, and payload; it returns the current-shape event.

The fields

Section titled “The fields”
FieldTypePurpose
_vnumberThe version this payload was written under.
_tstringThe type tag — typically the event/state name. Optional but useful for tooling.
_eobjectThe actual payload.

Underscores so they don’t collide with user fields. The framework considers any object with these three keys an envelope.

When envelopes are applied

Section titled “When envelopes are applied”
class Account extends PersistentActor<...> {
  override eventAdapter() {
    return new SomeAdapter();
  }
}

Setting an adapter makes the framework wrap events on write + unwrap on read. Without an adapter, events are written raw — no envelope.

This is strict on read: once you set an adapter, reading a non-envelope event throws MigrationError. You can’t mix enveloped + raw events for the same pid without WrapLegacy (see below).

For first-time deployments (fresh journal), you can either start with adapters from the beginning (every event has _v: 1) or defer adapter introduction until you actually need migration. Most teams add adapters only when the first breaking change arrives.

What the adapter sees

Section titled “What the adapter sees”
interface EventAdapter<DomainEvent, JournalShape = DomainEvent> {
  manifest(event: DomainEvent): string;
  toJournal(event: DomainEvent): OutboundFrame<JournalShape>;
  fromJournal(stored: StoredFrame): DomainEvent;
}


// what the adapter sees on the read path
type StoredFrame = {
  manifest: string;
  version: number;
  payload: unknown;
};


// what the adapter emits on the write path (wrapped into an envelope)
type OutboundFrame<JournalShape = unknown> = {
  manifest: string;
  version: number;
  payload: JournalShape;
};

On read, the framework unwraps the envelope into a StoredFrame and calls the adapter’s fromJournal(stored). stored.payload is the _e payload — without the envelope; stored.version is the _v value; stored.manifest is the _t tag. The adapter’s read-path job:

  • Inspect stored.version.
  • If it’s the current version, cast stored.payload to DomainEvent (the current shape) and return.
  • If it’s older, transform stored.payload to the current shape, then return.

manifest and toJournal cover the write path — they tag an outgoing event and package it as the triple the framework wraps into an envelope.

The framework’s built-in adapters (defaultsAdapter, migratingAdapter) encapsulate this pattern. Custom adapters implement the same.

Versioning your events

Section titled “Versioning your events”

When you want to bump the version:

class Account extends PersistentActor<...> {
  override eventAdapter() {
    const chain = MigrationChain.for<EventV2>('deposited', 2)
      .add({ fromVersion: 1, toVersion: 2, upcast: v1ToV2 });
    return migratingAdapter(chain);
  }
}

The adapter declares the current version. New events written by onCommand now get _v: 2 envelopes. Old _v: 1 events get upcast through the chain.

This works regardless of what version a given event was written under — the migration chain handles each.

Mixing with legacy non-envelope events

Section titled “Mixing with legacy non-envelope events”

If you’re adding an adapter to an existing journal that has already-persisted raw events:

import { migrateInMemoryJournal } from 'actor-ts';


// One-shot: wrap every raw event as a v1 envelope before recovery.
await migrateInMemoryJournal(journal, (e) => `${e.kind}`);

This rewrites the raw events into v1 envelopes; from then on your normal adapter upcasts them like any other v1 event. See Wrap legacy for details (and the per-row wrapEventAsEnvelope primitive for non-in-memory backends).

Persistence formats

Section titled “Persistence formats”

The envelope is a regular JSON object — stored through the tagged JSON tree format every store writes (or through a per-store custom serializer, when configured).

This means:

  • Rich types round-trip inside _e — Date, Map, Set, bigint and Uint8Array survive via type tags; functions, symbols and circular references throw at persist time. See what events and state may contain.
  • Deeply-nested objects work — the envelope is one level deep; _e itself can be arbitrarily complex.
  • Binary-friendly via a custom serializer — a store configured with withSerializer(...) frames the whole envelope through that serializer (more compact, binary-friendly).

Pitfalls

Section titled “Pitfalls”

Where to next

Section titled “Where to next”