@automerge/automerge-repo

Automerge Repo

This is a wrapper for the Automerge CRDT library which provides facilities to support working with many documents at once, as well as pluggable networking and storage.

This is the core library. It handles dispatch of events and provides shared functionality such as deciding which peers to connect to or when to write data out to storage.

Other packages in this monorepo include:

• @automerge/automerge-repo-demo-counter: A React-based demonstration application.
• @automerge/automerge-repo-react-hooks: Example hooks for use with React.
• @automerge/automerge-repo-sync-server: A small synchronization server that facilitates asynchronous communication between peers

Storage adapters

• @automerge/automerge-repo-storage-indexeddb: A storage adapter to persist data in a browser
• @automerge/automerge-repo-storage-nodefs: A storage adapter to write changes to the filesystem

Network adapters

• @automerge/automerge-repo-network-websocket: Network adapters for both sides of a client/server configuration over websocket
• @automerge/automerge-repo-network-messagechannel: A network adapter that uses the MessageChannel API to communicate between tabs
• @automerge/automerge-repo-network-broadcastchannel: Likely only useful for experimentation, but allows simple (inefficient) tab-to-tab data synchronization

Usage

This library provides two main components: the Repo itself, and the DocHandles it contains.

A Repo exposes these methods:

• create<T>()
  Creates a new, empty Automerge.Doc and returns a DocHandle for it.
• find<T>(docId: DocumentId)
  Looks up a given document either on the local machine or (if necessary) over any configured networks.
• delete(docId: DocumentId)
  Deletes the local copy of a document from the local cache and local storage. This does not currently delete the document from any other peers.
• .on("document", ({handle: DocHandle}) => void)
  Registers a callback to be fired each time a new document is loaded or created.
• .on("delete-document", ({handle: DocHandle}) => void)
  Registers a callback to be fired each time a new document is loaded or created.

A DocHandle is a wrapper around an Automerge.Doc. Its primary function is to dispatch changes to the document.

• handle.change((doc: T) => void)
  Calls the provided callback with an instrumented mutable object representing the document. Any changes made to the document will be recorded and distributed to other nodes.
• handle.value()
  Returns a Promise<Doc<T>> that will contain the current value of the document. it waits until the document has finished loading and/or synchronizing over the network before returning a value.

When required, you can also access the underlying document directly, but only after the handle is ready:

if (handle.ready()) {
  doc = handle.doc
} else {
  handle.value().then(d => {
    doc = d
  })
}

A DocHandle also emits these events:

• change({handle: DocHandle, doc: Doc<T>})
  Called any time changes are created or received on the document. Request the value() from the handle.
• patch({handle: DocHandle, patches: Patch[], patchInfo: PatchInfo}) Useful for manual increment maintenance of a video, most notably for text editors.
• delete
  Called when the document is deleted locally.

Creating a repo

The repo needs to be configured with storage and network adapters. If you give it neither, it will still work, but you won't be able to find any data and data created won't outlast the process.

Multiple network adapters (even of the same type) can be added to a repo, even after it is created.

A repo currently only supports a single storage adapter, and it must be provided at creation.

Here is an example of creating a repo with a indexeddb storage adapter and a broadcast channel network adapter:

const repo = new Repo({
  network: [new BroadcastChannelNetworkAdapter()],
  storage: new IndexedDBStorageAdapter(),
  sharePolicy: async (peerId: PeerId, documentId: DocumentId) => true // this is the default
})

Share Policy

The share policy is used to determine which document in your repo should be automatically shared with other peers. The default setting is to share all documents with all peers.

Warning If your local repo has deleted a document, a connecting peer with the default share policy will still share that document with you.

You can override this by providing a custom share policy. The function should return a promise resolving to a boolean value indicating whether the document should be shared with the peer.

The share policy will not stop a document being requested by another peer by its DocumentId.

## Starting the demo app

```bash
yarn
yarn dev

Quickstart

The following instructions will get you a working React app running in a browser.

yarn create vite
# Project name: hello-automerge-repo
# Select a framework: React
# Select a variant: TypeScript

cd hello-automerge-repo
yarn
yarn add @automerge/automerge @automerge/automerge-repo-react-hooks @automerge/automerge-repo-network-broadcastchannel @automerge/automerge-repo-storage-indexeddb vite-plugin-wasm vite-plugin-top-level-await

Edit the vite.config.ts. (This is all need to work around packaging hiccups due to WASM. We look forward to the day that we can delete this step entirely.)

// vite.config.ts
import { defineConfig } from "vite"
import react from "@vitejs/plugin-react"
import wasm from "vite-plugin-wasm"
import topLevelAwait from "vite-plugin-top-level-await"

export default defineConfig({
  plugins: [wasm(), topLevelAwait(), react()],

  worker: {
    format: "es",
    plugins: [wasm(), topLevelAwait()],
  },

  optimizeDeps: {
    // This is necessary because otherwise `vite dev` includes two separate
    // versions of the JS wrapper. This causes problems because the JS
    // wrapper has a module level variable to track JS side heap
    // allocations, and initializing this twice causes horrible breakage
    exclude: [
      "@automerge/automerge-wasm",
      "@automerge/automerge-wasm/bundler/bindgen_bg.wasm",
      "@syntect/wasm",
    ],
  },

  server: {
    fs: {
      strict: false,
    },
  },
})

Now set up the repo in src/main.tsx by importing the bits, creating the repo, and passing down a RepoContext. We also create a document and store its documentId in localStorage.

// src/main.tsx
import React from "react"
import ReactDOM from "react-dom/client"
import App from "./App.js"
import { Repo } from "@automerge/automerge-repo"
import { BroadcastChannelNetworkAdapter } from "@automerge/automerge-repo-network-broadcastchannel"
import { IndexedDBStorageAdapter } from "@automerge/automerge-repo-storage-indexeddb"
import { RepoContext } from "@automerge/automerge-repo-react-hooks"

const repo = new Repo({
  network: [new BroadcastChannelNetworkAdapter()],
  storage: new IndexedDBStorageAdapter(),
})

let rootDocId = localStorage.rootDocId
if (!rootDocId) {
  const handle = repo.create()
  localStorage.rootDocId = rootDocId = handle.documentId
}

ReactDOM.createRoot(document.getElementById("root") as HTMLElement).render(
  <RepoContext.Provider value={repo}>
    <React.StrictMode>
      <App documentId={rootDocId} />
    </React.StrictMode>
  </RepoContext.Provider>
)

Now update App.tsx to load the document from the Repo based on the documentId passed in. Then, use the document to render a button that increments the count.

// App.tsx
import { useDocument } from "@automerge/automerge-repo-react-hooks"
import { DocumentId } from "@automerge/automerge-repo"

interface Doc {
  count: number
}

export default function App(props: { documentId: DocumentId }) {
  const [doc, changeDoc] = useDocument<Doc>(props.documentId)

  return (
    <button
      onClick={() => {
        changeDoc((d: any) => {
          d.count = (d.count || 0) + 1
        })
      }}
    >
      count is: {doc?.count ?? 0}
    </button>
  )
}

You should now have a working React application using Automerge. Try running it with yarn dev, and open it in two browser windows. You should see the count increment in both windows.

This application is also available as a package in this repo in automerge-repo-demo-counter. You can run it with yarn dev:demo.

Adding a sync server

First, get a sync-server running locally, following the instructions for the automerge-repo-sync-server package.

Next, update your application to synchronize with it:

Install the websocket network adapter:

yarn add automerge-repo-network-websocket

Now import it and add it to your list of network adapters:

// main.tsx
import { BrowserWebSocketClientAdapter } from "@automerge/automerge-repo-network-websocket" // <-- add this line

// ...

const repo = new Repo({
  network: [
    new BroadcastChannelNetworkAdapter(),
    new BrowserWebSocketClientAdapter("ws://localhost:3030"), // <-- add this line
  ],
  storage: new IndexedDBStorageAdapter(),
})

// ...

And you're finished! You can test that your sync server is opening the same document in two different browsers (e.g. Chrome and Firefox). (Note that with our current trivial implementation you'll need to manually copy the rootDocId value between the browsers.)

Acknowledgements

Originally authored by Peter van Hardenberg.

With gratitude for contributions by:

• Herb Caudill
• Jeremy Rose
• Alex Currie-Clark
• Dylan Mackenzie