yjs-firestore-provider

A Yjs Connection and Database Provider backed by Google Firestore.

Installation

The yjs-firestore-provider library has peer dependencies on yjs and firebase.

Make sure you have installed these dependencies in your project:

npm install yjs firebase

If you plan to enable awareness, you must also install two additional dependencies:

npm install simple-peer y-protocols

Some editor bindings, including y-prosemirror, TipTap and y-quill, have an explicit dependency on the y-protocols module. If you are using one of these bindings, you don't need to install y-protocols separately.

Once you have installed all the dependencies, you can install the yjs-firestore-provider library:

npm install @gmcfall/yjs-firestore-provider

Usage

The following example shows how to use the Yjs Firestore Provider with a Tiptap rich text editor. Usage with other Yjs editor bindings is similar.

This example assumes that the app allows users to edit articles stored in Firestore under a collection named "articles".

import { Editor } from '@tiptap/core'
import StarterKit from '@tiptap/starter-kit'
import Collaboration from '@tiptap/extension-collaboration'
import CollaborationCursor from '@tiptap/extension-collaboration-cursor'
import * as Y from 'yjs'
import { WebrtcProvider } from 'y-webrtc'
import { FirestoreProvider, getColor } from '@gmcfall/yjs-firestore-provider'

// The app must provide a unique identifier for each article.
const articleId = getArticleIdSomehow();

// To create a FirebaseProvider, you must have a FirebaseApp
const firebaseApp = getFirebaseAppSomehow();

// Get the name or alias for the user if using Awareness
const userName = getUserNameOrAliasSomehow();

const ydoc = new Y.Doc();
const basePath = ["articles", articleId];
const provider = new FirestoreProvider(firebaseApp, ydoc, basePath);

const editor = new Editor({
  extensions: [
    StarterKit.configure({
      // The Collaboration extension comes with its own history handling
      history: false,
    }),
    // Register the document with Tiptap
    Collaboration.configure({
      document: provider.doc,
    })
    // Register the collaboration cursor extension
    CollaborationCursor.configure({
        provider,
        user: {
            name: userName,
            color: getColor(userName)
        }
    })
  ],
})

Firestore paths

The example above uses a base path given by

const basePath = ["articles", articleId];

where "articles" is the name of a Firestore collection and articleId is the id for a Firestore document within the collection.

That Firestore document might store metadata about the article. Or it might contain sharing options. Or there might not be any Firestore document at that path at all. The Yjs Firestore Provider does not care whether there is a document at the given base path, and it never tries to read or modify that document. It stores all of the Yjs data under a nested collection at

    /articles/{articleId}/yjs

See How it works for details about the data stored under the yjs collection.

The Yjs Firestore Provider does not care how long the base path might be. Consider a learning application that hosts flashcards collected within decks.
The application might store metadata for a given flashard at a path of the form:

const basePath = ["decks", deckId, "cards", cardId];

In this case, the Yjs data for a given card would be stored under a collection at

    /decks/{deckId}/cards/{cardId}/yjs

Awareness

Awareness and presence is often implemented by displaying the cursor and text selection of each user in a different color.

As we saw in the Usage section, the TipTap editor enables awareness via the CollaborationCursor extension:

CollaborationCursor.configure({
    provider,
    user: {
        name: userName,
        color: getColor(userName)
    }
})

Other editors will expose a different API to enable awareness.

The getColor function returns one of 20 visually distinct colors from a list published by Sasha Trubestskoy. It always returns the same hex color for a given userName.

The getColor function is provided for convenience. You are certainly free to use a different method to assign a color to the user.

When awareness is enabled, FirestoreProvider persists transient documents in collections at the following paths:

    {basePath}/yjs/aware/announce/
    {basePath}/yjs/aware/signal/${to}/sig_messages/

FirestoreProvider makes a best effort to remove these transient documents when they are no longer needed. However, there are some cases (typically on mobile devices) where FirestoreProvider is unable to remove obsolete documents because the beforeunload event never fires. To ensure that obsolete documents are eventually deleted, you should create data retention policies for the announce and sig_messages collection groups by following the steps listed below:

1. Go to the Cloud Firestore Time-to-live page in the Google Cloud Platform Console.
2. Click Create Policy
3. Enter "announce" for the collection group name, and "expiresAt" for the timestamp field name.
4. Click Create
5. Click Create Policy again.
6. Enter "sig_messages" for the collection group name, and "expiresAt" for the timestamp field name.
7. Click Create

If you are NOT using awareness, you should disable it in the FirestoreProvider by passing a configuration object to the constructor...

new FirestoreProvider(firebaseApp, ydoc, basePath, {disableAwareness: true});

Deleting Yjs data

The FirestoreProvider stores data under the yjs collection to support collaborative editing of documents. If you permanently delete a given document such as an article or flashcard in the above examples, you must also delete the associated Yjs data.

Assuming provider is a FirestoreProvider, you can use the following line of code to delete the Yjs data:

    provider.deleteYjsData();

Alternatively, you can use the following snippet:

import { deleteYjsData } from '@gmcfall/yjs-firestore-provider'

const firebaseApp = getFirebaseAppSomehow();
const articleId = getArticleIdSomehow();
const basePath = ["articles", articleId];

deleteYjsData(firebaseApp, basePath);

Security Rules

Your application must define security rules for the Yjs data.

For instance, early in the development lifecycle, the app might define lax security rules of the form:

match /articles/{articleId}/yjs/{document=**} {
    allow read, write: if true;
}
match /decks/{deckId}/cards/{cardId}/yjs/{document=**} {
    allow read, write: if true;
}

These rules are probably not appropriate for use in production. The application should define whatever security rules are appropriate.

How it works

This section explains how the Yjs Firestore Provider works in gory detail. You don't need to read this section if you plan to use the default configuration, but it is helpful if you want to use a custom configuration.

Key Concepts

1. Each client creates a FirestoreProvider for a given Yjs document as illustrated in the Usage section.
2. The FirestoreProvider subscribes to update events published by the Yjs document.
3. When the FirestoreProvider receives an update event, it does NOT persist the event to Firestore immediately. Instead, if the user makes a series of changes in rapid succession, those update events are merged into a composite blob and cached temporarily.
4. The composite blob is persisted when one of the following conditions is satisifed:
   • The number of changes reaches a threshold, set to 20 by default.
   • The user pauses and makes no more changes for a certain duration, set to 600 ms by default.
5. The FirestoreProvider use a Firestore listener to receive blobs persisted by other clients in near real-time.
6. When the FirestoreProvider receives a blob from another client, it updates the local Yjs document.
7. Periodically, the FirestoreProvider removes individual blobs from Firestore and merges them into a consolidated history that contains all the changes accumulated up to a given point in time. That history is stored in Firestore, and it grows over the lifetime of the Yjs document.
8. When awareness is enabled, FirestoreProvider persists a Firestore document announcing the presence of the current user, and it invokes a listener to receive notifications about other users that have announced their presence. The document announcing the presence of the current user is deleted from Firestore when the FirestoreProvider is destroyed, or when the browser window unloads.
9. When FirestoreProvider is notified about the presence of another user editing the same Yjs document, it establishes a WebRTC connection to a peer associated with that user. Signaling is accomplished by writing and reading documents in certain Firebase collections.
   Once a connection is established, all Awareness state changes are transmitted via the WebRTC connection (not Firestore). Signaling messages are deleted from Firestore as soon as they are read.

The FirestoreProvider manages data at the following paths in Firestore:

• {basePath}/yjs/history : The consolidated history
• {basePath}/yjs/history/updates/{blobId}: A composite blob of update events
• {basePath}/yjs/time: A transient document used to compute the differece between time on the Firestore server and time on the client. For details, see How does FirebaseProvider know when a blob has exceeded its time-to-live?
• {basePath}/yjs/aware/announce/{peerId}: A transient document announcing the presence of a given user.
• {basePath}/yjs/aware/signal/{to}/sig_messages/{msg_id}: A transient signaling message sent to a particular peer.

Frequently asked questions

Why merge update events into a composite blob?

Google charges for each write operation. By merging update events into a composite blob, the number of writes is reduced, and hence the operational costs are reduced.

Don't composite blobs increase latency?

Yes. Because update events are merged into a blob and cached temporarily, other clients won't see those update events until the blob is written to Firestore. You can tune the latency with the following configuration parameters:

• maxUpdatesPerBlob: The maximum number of update events allowed in a blob, set to 20 by default. You can decrease latency by setting this parameter to a lower value. Setting it to 1 will cause the FirestoreProvider to emit every single update event immediately, at the penalty of increased cost due to more frequent writes.
• maxUpdatePause: The maximum amount of time in milliseconds that the user may pause in making changes before a blob is emitted, set to 600 ms by default. Setting this parameter to a smaller value will reduce latency, but again, at the penalty of increased cost due to more frequent writes. Setting it to a higher value will increase latency and reduce cost.

Where are composite blobs stored?

The FirebaseProvider writes each composite blob to Firestore at a path of the form

{basePath}/yjs/history/updates/{blobId}

Thus, for the examples from the Usage section, we would have:

/articles/{articleId}/yjs/history/updates/{blobId}
/decks/{deckId}/cards/{cardId}/yjs/history/updates/{blobId}

Why merge blobs into a consolidated history?

The Key Concepts section included the following observation:

Periodically, the FirestoreProvider removes individual blobs from Firestore and merges them into a consolidated history that contains all the changes accumulated up to a given point in time. That history is stored in Firestore, and it grows over the lifetime of the Yjs document.

This design helps to reduce operational costs, and it simplifies the overall solution.

When a FirestoreProvider is created, it must load the entire set of updates to the Yjs document. It is more efficient to read a single, consolidated history plus a small number of recent update blobs than to read hundreds or even thousands of individual blobs.

By reducing the number of read operations, the overall cost of the solution is reduced. Moreover, the number of bytes in a consolidated history is smaller than the sum of bytes in the individual blobs, so there are cost savings here as well.

How long do individual blobs live in Firestore?

The lifetime of a blob in Firestore is controlled by the blobTimeToLive configuration parameter which is set to 10 seconds by default.

How does FirestoreProvider know when a blob has exceeded its time-to-live?

Each blob is stored in a Firestore document that includes a timestamp which records the date/time when the blob was written to Firestore.

The FirestoreProvider uses a listener to watch all blob documents in Firestore. It maintains an in-memory cache of those blob documents, and hence it can inspect the timestamps. The Firestore provider compares the timestamp with the current time to determine if a given blob has exceeded its time-to-live.

Each timestamp records the UTC time according to the Firestore server which has a guarantee on the accuracy of its clock. But there is no guarantee that the clock on the local client is accurate.

Consequently, FirestoreProvider estimates the difference between the local clock and the server's clock by executing a block of code like this snippet:

const db = getFirestore(firebaseApp);
const timePath = basePath + "/yjs/time";
const timeRef = doc(db, timePath);

// Create a transient Firestore document that holds the current server time.
// Capture timestamps before and after the write operation, according to
// the client's local clock.

const before = Date.now();
await setDoc(timeRef, {now: serverTimestamp()});
const after = Date.now();

// Compute an estimate for the local time when the document was created in 
// Firestore. This is given by the midpoint between the `before` and `after` 
// timestamps.

const clientTime = Math.floor((before + after)/2);

// Get the server timestamp from the document that we just saved to Firestore.

const timeDoc = await getDoc(timeRef);
if (timeDoc.exists()) {
    // Compute the difference between the server and client time
    const serverNow = timeDoc.data().now as Timestamp;
    const serverTime = serverNow.seconds*1000 + 
        Math.floor(serverNow.nanoseconds/1000000);

    timeDelta = serverTime - clientTime;
}

// Cleanup by deleting the transient Firestore document.
await deleteDoc(timeRef);

In this snippet, basePath is the path passed to the FirestoreProvider constructor. For example, the transient "time" document might be created at

/articles/{articleId}/yjs/time

The code snippet above runs only once during the lifetime of the application, no matter how many FirebaseProvider instances are created. The timeDelta parameter is stored in a module variable and made available to all FirebaseProvider instances.

Where is the consolidated history for a given Yjs document stored?

The consolidated history for a given Yjs document is stored at a path of the form:

{basePath}/yjs/history

Thus, for the examples from the Usage section, we would have:

/articles/{articleId}/yjs/history
/decks/{deckId}/cards/{cardId}/yjs/history

Custom Configuration

The FirestoreProvider constructor takes an optional configuration object with one or more of the following parameters:

• maxUpdatesPerBlob: The maximum number of update events allowed in a blob, set to 20 by default. You can decrease latency by setting this parameter to a lower value. Setting it to 1 will cause the FirestoreProvider to emit every single update event immediately, at the penalty of increased cost due to more frequent writes.
• maxUpdatePause: The maximum amount of time in milliseconds that the user may pause in making changes before a blob is emitted, set to 600 ms by default. Setting this parameter to a smaller value will reduce latency, but again, at the penalty of increased cost due to more frequent writes. Setting it to a higher value will increase latency and reduce cost. For comparison, the average person types at the rate of one character every 300 ms.
• blobTimeToLive: The maximum amount of time in milliseconds that a blob of updates can live in Firestore before it is removed and merged into the consolidated history. By default, this parameter is set to 10000 (i.e. 10 seconds). As a best practice, applications should stick with this default.
• disableAwareness: A flag used to disable the Awareness feature, set to false by default.

Thus, one may create a FirestoreProvider with a custom configuration like this:

const ydoc = new Y.Doc();
const basePath = ["articles", articleId];
const provider = new FirestoreProvider(firebaseApp, ydoc, basePath, {
    maxUpdatesPerBlob: 10,
    maxUpdatePause: 500,
    disableAwareness: true
});

Licensing and Attribution

This module uses a customized version of the y-webrtc library published by Kevin Jahns.

This module is licensed under the MIT License. You are generally free to reuse or extend upon this code as you see fit. Just include copies of

• The yjs-firestore-provider license
• The y-webrtc license