treaty-outbox

@superjackfruitlabs/treaty-outbox makes events exactly once in effect on top of a transport that is at least once. Use it when a service writes a change and must send the event for it, or receives events and must not act on one twice.

• Outbox: enqueue() writes the event inside the transaction that makes the change, so the two commit or fail together. drain() sends due events through your send function, at least once, and schedules a retry when it throws.
• Inbox: handleOnce() runs your handler once per CloudEvents (source, id), in one transaction with the record that it ran. A replay returns "duplicate" and changes nothing; a throw rolls back, so a later delivery can try again.

Two implementations with the same functions: sqlite, shaped for a Durable Object’s ctx.storage, and postgres, for any client with query(text, params).

import { sqlite } from "@superjackfruitlabs/treaty-outbox";

sqlite.migrate(ctx.storage.sql);
ctx.storage.transactionSync(() => {
  // ... write the change ...
  sqlite.enqueue(ctx.storage.sql, event, subject);
});
await sqlite.drain(ctx.storage.sql, async ({ event, subject }) => send(subject, event));

const outcome = sqlite.handleOnce(ctx.storage, incoming, () => {
  // ... apply it ...
});

What it exports

Export	What
sqlite, postgres	Each has migrate(), enqueue(), drain() and handleOnce(), and its schema (SQLITE_SCHEMA, POSTGRES_SCHEMA).
OutboxEvent	An event as stored: the same shape as CloudEvent from treaty-cloudevents, so either passes for the other. It needs id and source.
PendingEvent	What drain() hands to send: the event, its subject and how many attempts it has had.
Delivery	What handleOnce() did: "handled" or "duplicate".
Backoff, DEFAULT_BACKOFF	When to retry a failed send. The default waits 1 minute after the first failure, doubling to at most 1 hour, and parks an event as dead after 12 attempts.
DrainOptions	nowMs, limit (default 100 events), leaseMs (default 60 s: how long a claimed event is hidden from another drain) and backoff.

Enqueuing the same (source, id) twice does nothing, so a retried request does not send its event twice.