Observe state

Subscribers see committed values, never speculative in-memory mutations.

const unsubscribe = workspace.subscribe((value, event) => {
  console.log(event.revision, event.source, value);
});

The event object carries:

• revision — the monotonic revision number after the commit.
• source — where the commit came from:
  • local — a set() or update() in this tab.
  • external — another tab's commit reconciled from storage.
  • migration — a schema migration ran on this open.
  • reset — reset() was called.

Call unsubscribe() to detach. A subscriber that throws does not silence the others.

Cross-tab

When another tab commits a slot, this tab's subscribers are notified. The mechanism is:

1. Tab A commits and increments the revision.
2. Tab A broadcasts { slot, revision, source } on a shared BroadcastChannel.
3. Tab B receives the message, sees the revision is higher than what it has, and reads the current envelope from IndexedDB.
4. Tab B's subscribers observe the new value with source: "external".

Storage is the source of truth. Cross-context messaging is a poke that says "look again". A dropped message does not corrupt state — the next open() or the next pageshow reconciliation recovers it.

The bfcache trap

WebKit silently drops BroadcastChannel messages to bfcached (frozen) pages. When your page returns from bfcache, subscribers may have missed notifications. durable-local handles this: on pageshow with event.persisted === true, every subscriber re-reads from storage and fires if the revision moved.

You do not need to call anything for this — it is automatic. But it is why subscribers see events labelled external shortly after a bfcache restore even when they thought they had been quiet.

The library never broadcasts values

Only notices — never the value itself. The value comes from storage. This keeps the notification channel small, the source of truth in one place, and the correctness story simple: a lost message can never be the reason a subscriber sees stale data.