@just-be/automerge-cloudflare

Automerge storage and network primitives for Cloudflare Workers.

Designed around a one-Durable-Object-per-document architecture with full hibernation support — clients stay connected while idle DOs sleep, with no billing for inactive time.

Exports

Architecture

The root export, AutomergeDO, is a per-document Durable Object that hosts an Automerge Repo, accepts client WebSockets (with hibernation), and persists chunks through the storage layer. Subclass it to customize the repo-store id or peer id.

Storage is split across two Durable Object classes plus a small client-side library:

• RepoStoreDO — top-level router. Implements automerge-repo's storage RPC surface; reads the documentId from each StorageKey (always key[0] per the automerge-repo contract) and forwards every call to that document's DocStoreDO. Stateless — no chunk data lives here.
• DocStoreDO — per-document store. One instance per documentId, named via idFromName(docId). Owns the chunks for that doc. The store tier is the DO's own SQLite storage; the archive tier is an optional R2 bucket bound via env. Writes go to the store only; reads fall through store → archive; removals propagate to both; tiering lifecycle (hydrate, flushToArchive, clearAll) is exposed for use from an alarm or external coordinator.
• RepoStoreAdapter — the library wrapper. Implements automerge-repo's StorageAdapterInterface by delegating each call to a RepoStoreDO stub over DO RPC. This is what you hand to new Repo({ storage }).

Why two DO layers? The router gives the repo a single addressable stub (one binding for the consumer), while per-doc DOs keep each document's storage in its own DO — letting writes/reads stay strongly consistent with that document's writers and allowing per-document lifecycle without cross-doc coordination.

Quick start

1. Wire up your Worker

AutomergeDO is ready to use as-is — re-export the three DO classes so wrangler can bind them, and route WebSocket upgrades with routeWebSocket:

// src/index.ts
import { AutomergeDO } from "@just-be/automerge-cloudflare"
import { routeWebSocket } from "@just-be/automerge-cloudflare/network"
import {
  RepoStoreDO,
  DocStoreDO,
} from "@just-be/automerge-cloudflare/storage"

// Cloudflare requires DO classes to be exported from the entry module.
export { AutomergeDO, RepoStoreDO, DocStoreDO }

interface Env {
  AUTOMERGE_DO: DurableObjectNamespace<AutomergeDO>
}

export default {
  async fetch(request: Request, env: Env): Promise<Response> {
    return routeWebSocket({ request, namespace: env.AUTOMERGE_DO })
  },
}

Subclass AutomergeDO to customize the repo-store DO name or peer id:

export class MyAutomergeDO extends AutomergeDO {
  protected override repoStoreId() {
    return this.ctx.id.toString() // one repo store per document
  }
}

routeWebSocket uses the last URL path segment as the document ID (e.g. /doc/abc123 routes to the DO named abc123). Pass a custom getDocumentId function to change this:

routeWebSocket({
  request,
  namespace: env.AUTOMERGE_DO,
  getDocumentId: (req) => new URL(req.url).searchParams.get("docId")!,
})

2. Configure wrangler

# wrangler.toml
name = "automerge-sync"
main = "src/index.ts"
compatibility_date = "2024-01-01"

[[durable_objects.bindings]]
name = "AUTOMERGE_DO"
class_name = "AutomergeDO"

[[durable_objects.bindings]]
name = "AUTOMERGE_REPO_STORE"
class_name = "RepoStoreDO"

[[durable_objects.bindings]]
name = "AUTOMERGE_DOC_STORE"
class_name = "DocStoreDO"

# Optional cold tier for the per-doc DOs.
[[r2_buckets]]
binding = "AUTOMERGE_R2"
bucket_name = "automerge-cold"

[[migrations]]
tag = "v1"
new_sqlite_classes = ["AutomergeDO", "RepoStoreDO", "DocStoreDO"]

3. Connect from a client

Use the standard @automerge/automerge-repo-network-websocket client adapter, pointed at your Worker URL with the document ID in the path:

import { Repo } from "@automerge/automerge-repo"
import { BrowserWebSocketClientAdapter } from "@automerge/automerge-repo-network-websocket"

const repo = new Repo({
  network: [new BrowserWebSocketClientAdapter("wss://your-worker.workers.dev/doc/abc123")],
})

Advanced: build your own DO

AutomergeDO is a thin composition — if you need a different storage layer or extra behavior beyond what subclassing offers, assemble the pieces yourself: construct a DONetworkAdapter with the DO's ctx, hand it to new Repo(...) alongside a RepoStoreAdapter (or any StorageAdapterInterface), and forward the DO's webSocketMessage/webSocketClose/webSocketError handlers to the adapter's receiveMessage/handleClose. The AutomergeDO source is the reference implementation.

Naming a root document: loadOrInit

RepoStoreDO exposes one RPC beyond the storage interface: loadOrInit(key, value) — an atomic set-if-absent for single-segment meta keys. Use it to name a well-known root document URL exactly once across racing clients:

const stub = env.AUTOMERGE_REPO_STORE.get(
  env.AUTOMERGE_REPO_STORE.idFromName("default")
)
const winner = await stub.loadOrInit(
  ["default-root"],
  new TextEncoder().encode(handle.url)
)

Atomicity relies on the router DO handling meta keys synchronously in its own SQLite, so doc-scoped (multi-segment) keys are rejected — routing those would open the DO's input gate mid-operation.

Archive tier (R2)

Bind AUTOMERGE_R2 in the env of DocStoreDO to enable an archive tier. When present:

• Writes go to the store (DO SQLite) only.
• Reads check the store first, then fall through to the archive. Archive hits are promoted into the store so subsequent reads stay hot (lazy hydration).
• Removes propagate to both tiers so the read fallback can't resurrect deleted keys.

Idle-flush alarm

When an archive is bound, DocStoreDO runs a per-doc alarm that flushes the whole store to the archive once the doc has been idle (no writes) for AUTOMERGE_IDLE_FLUSH_MS (default 7 days). After a flush the chunks live only in R2; if the doc wakes up again, reads pull them back into the store on demand.

Configure via [vars] in wrangler.toml:

[vars]
AUTOMERGE_IDLE_FLUSH_MS = "604800000"  # 7 days (default)

Lifecycle methods on DocStoreDO (callable over DO RPC):

To drop chunks without archiving them, call removeRange(prefix) — that's the normal StorageAdapterInterface op and it propagates to both tiers.

For most workloads the built-in idle-flush alarm is sufficient; the explicit flushToArchive / hydrate / clearAll RPCs are there for cases where you want to drive the lifecycle from outside the DO.

Hibernation

The network adapter fully supports Durable Object hibernation. When a DO hibernates:

• Client WebSocket connections are maintained by Cloudflare's infrastructure
• Peer identity is persisted on each WebSocket via serializeAttachment
• On wake-up, the adapter restores peer mappings from ctx.getWebSockets() and re-announces peers to the Repo so syncing resumes automatically

This means you only pay for compute time when messages are actually being exchanged.