peer-lease

@loro-dev/peer-lease is a TypeScript library for safely reusing CRDT peer IDs without collisions.

Installation

pnpm add @loro-dev/peer-lease
# or
npm install @loro-dev/peer-lease

Usage

import { LoroDoc } from "loro-crdt";
import { acquirePeerId, tryReuseLoroPeerId } from "@loro-dev/peer-lease";

const doc = new LoroDoc();
// ... Import local data into doc first
const lease = await acquirePeerId(
  "doc-123",
  () => new LoroDoc().peerIdStr,
  JSON.stringify(doc.frontiers()),
  (a, b) => {
    const fA = JSON.parse(a);
    const fB = JSON.parse(b);
    return doc.cmpFrontiers(fA, fB);
  },
);

try {
  console.log("Using peer", lease.value);
  doc.setPeerId(lease.value);
  // use doc here...
} finally {
  await lease.release(JSON.stringify(doc.frontiers()));
  // Or use FinalizeRegistry to release the lease
  // Note: release can be invoked exactly once; a second call throws.
}

// Later, when you reopen the same document, try to reuse the cached peer id
const release = await tryReuseLoroPeerId("doc-123", doc);
try {
  // doc.peerIdStr now matches the previously leased id when the cache is still valid
} finally {
  await release();
}

The first argument is the document identifier that scopes locking and cache entries, ensuring leases only coordinate with peers working on the same document.

acquirePeerId first tries to coordinate through the Web Locks API. When that API is unavailable it falls back to a localStorage-backed mutex with a TTL, heartbeat refresh, and release notifications. A released ID is cached together with the document version that produced it and is only handed out when the caller proves their version has advanced, preventing stale edits from reusing a peer ID.

tryReuseLoroPeerId(docId, doc) wraps the caching flow so you can reopen a document and automatically load the most recent peer ID if the stored frontiers prove the local state is up to date. The returned release handle is callable (await release()) and is also safe to invoke in synchronous lifecycle handlers—release() stages the result synchronously and finishes the mutex flush in the background:

window.addEventListener("pagehide", () => {
  // Only synchronous work is allowed here; release() stages the data right away.
  release(JSON.stringify(doc.frontiers()));
});

window.addEventListener("pageshow", () => {
  if (!release.isReleased()) {
    return;
  }
  // Optionally restart peer work if the page returned from BFCache.
});

release() writes the lease outcome to synchronous storage before returning, so browsers terminating the page (e.g. during pagehide on mobile) still mark the peer ID available even if the returned promise never resolves. If the page survives the lifecycle event, you can still await release() later; repeated calls reuse the same promise and do not restage.

To wire these lifecycle hooks without repeating boilerplate, use the helper exported as attachPeerLeaseLifecycle:

import { attachPeerLeaseLifecycle } from "@loro-dev/peer-lease";

const detachLifecycle = attachPeerLeaseLifecycle({
  release,
  doc,
  onResume: async () => {
    // Re-acquire a lease or restart transports when the tab resumes from BFCache.
  }
});

// Later, when tearing down the document entirely
detachLifecycle();

The helper stages the latest frontiers while the page is visible, calls release() during pagehide, and invokes onResume after pageshow if the handle was released. Provide an onFreeze callback if you need to pause background work when a BFCache transition is detected.

Coordination strategy

• Lock negotiation – Calls use navigator.locks.request in supporting browsers so the lease state is mutated under an exclusive Web Lock. Fallback tabs use a fencing localStorage record with TTL heartbeats, and wake waiters via storage events plus a BroadcastChannel.
• Version gating – Every lease carries document metadata. We only recycle a peer ID after the releasing tab supplies the version it used, and a future caller provides a strictly newer version according to the supplied comparator. This stops pre-load editing sessions from replaying IDs once the real document snapshot arrives.
• Explicit release – A lease is only recycled when the releasing tab provides its final version metadata. If a tab crashes or never releases, the ID stays reserved so it cannot be handed out again accidentally; any lease left active for 24 hours is simply discarded instead of being returned to the available pool.

Lock implementation details

When Web Locks are available the mutex is just a thin wrapper around navigator.locks.request, enforcing an acquire timeout. In browsers without that API we fall back to a localStorage-backed mutex that writes a JSON record containing a token, fence, and expiry. The holder extends the expiry with a heartbeat (a setInterval that calls refresh) so long tasks don’t lose the lock, while waiters observe the fence value and storage/BroadcastChannel notifications to wake up promptly. If the tab crashes the record expires after lockTtlMs, letting another peer take over without manual cleanup.

The mutex implementation is exported so advanced users can coordinate other shared state:

import { createMutex, type AsyncMutex } from "@loro-dev/peer-lease";

const mutex: AsyncMutex = createMutex({
  storage: window.localStorage,
  lockKey: "my-lock",
  fenceKey: "my-lock:fence",
  channelName: "my-lock:channel",
  webLockName: "my-lock:web",
  options: {
    lockTtlMs: 10_000,
    acquireTimeoutMs: 5_000,
    retryDelayMs: 40,
    retryJitterMs: 60,
  },
});

await mutex.runExclusive(async () => {
  // critical section
});

You can reuse the same mutex that acquirePeerId does by passing the document id to keep coordination scoped per document.

Development

• pnpm install – install dependencies
• pnpm build – produce ESM/CJS/d.ts bundles via tsdown
• pnpm dev – run tsdown in watch mode
• pnpm test – run Vitest
• pnpm lint – run oxlint
• pnpm typecheck – run the TypeScript compiler without emitting files
• pnpm check – type check, lint, update snapshots, and test

Release workflow

• Push Conventional Commits to main; Release Please opens or updates a release PR with the changelog and semver bump.
• Merging that PR tags the release and triggers .github/workflows/publish-on-tag.yml, which publishes to npm using NODE_AUTH_TOKEN derived from the NPM_TOKEN secret.
• Publish provenance is enabled via .npmrc and publishConfig.provenance.

Continuous integration

The CI workflow installs dependencies, lints, type-checks, runs Vitest in run mode, and builds the library on pushes and pull requests.