A CRDT framework with a powerful abstraction of shared data
Yjs is a CRDT implementation that exposes its internal
data structure as shared types. Shared types are common data types like Map
or Array
with superpowers: changes are automatically distributed to other
peers and merged without merge conflicts.
Yjs is network agnostic (p2p!), supports many existing rich text editors, offline editing, version snapshots, undo/redo and shared cursors. It scales well with an unlimited number of users and is well suited for even large documents.
- Demos: https://github.com/yjs/yjs-demos
- Discuss: https://discuss.yjs.dev
- Benchmark Yjs vs. Automerge: https://github.com/dmonad/crdt-benchmarks
- Podcast "Yjs Deep Dive into real time collaborative editing solutions":
- Podcast "Google Docs-style editing in Gutenberg with the YJS framework":
👷♀️ If you are looking for professional (paid) support to build collaborative or distributed applications ping us at yjs@tag1consulting.com. Otherwise you can find help on our discussion board.
Sponsors
I'm currently looking for sponsors that allow me to be less dependent on contracting work. These awesome backers already fund further development of Yjs:
Sponsorship also comes with special perks!
Who is using Yjs
- Serenity Notes End-to-end encrypted collaborative notes app.
- Relm A collaborative gameworld for teamwork and
community.
🌟 - Input A collaborative note taking app.
🌟 - Room.sh A meeting application with integrated
collaborative drawing, editing, and coding tools.
⭐ - https://coronavirustechhandbook.com/
A collaborative wiki that is edited by thousands of different people to work
on a rapid and sophisticated response to the coronavirus outbreak and
subsequent impacts.
⭐ - Nimbus Note A note-taking app designed by Nimbus Web.
- JoeDocs An open collaborative wiki.
- Pluxbox RadioManager A web-based app to collaboratively organize radio broadcasts.
- Alldone A next-gen project management and collaboration platform.
Table of Contents
Overview
This repository contains a collection of shared types that can be observed for changes and manipulated concurrently. Network functionality and two-way-bindings are implemented in separate modules.
Bindings
Name | Cursors | Binding | Demo |
---|---|---|---|
ProseMirror | ✔ | y-prosemirror | demo |
Quill | y-quill | demo | |
CodeMirror | y-codemirror | demo | |
Monaco | y-monaco | demo | |
Slate | slate-yjs | demo | |
valtio | valtio-yjs | demo | |
React / Vue / MobX | SyncedStore | demo |
Providers
Setting up the communication between clients, managing awareness information, and storing shared data for offline usage is quite a hassle. Providers manage all that for you and are the perfect starting point for your collaborative app.
- y-webrtc
- Propagates document updates peer-to-peer using WebRTC. The peers exchange signaling data over signaling servers. Publically available signaling servers are available. Communication over the signaling servers can be encrypted by providing a shared secret, keeping the connection information and the shared document private.
- y-websocket
- A module that contains a simple websocket backend and a websocket client that connects to that backend. The backend can be extended to persist updates in a leveldb database.
- y-indexeddb
- Efficiently persists document updates to the browsers indexeddb database. The document is immediately available and only diffs need to be synced through the network provider.
- y-libp2p
- Uses libp2p to propagate updates via GossipSub. Also includes a peer-sync mechanism to catch up on missed updates.
- y-dat
- [WIP] Write document updates efficiently to the dat network using multifeed. Each client has an append-only log of CRDT local updates (hypercore). Multifeed manages and sync hypercores and y-dat listens to changes and applies them to the Yjs document.
Getting Started
Install Yjs and a provider with your favorite package manager:
npm i yjs y-websocket
Start the y-websocket server:
PORT=1234 node ./node_modules/y-websocket/bin/server.js
Example: Observe types
const yarray = doc.getArray('my-array')
yarray.observe(event => {
console.log('yarray was modified')
})
// every time a local or remote client modifies yarray, the observer is called
yarray.insert(0, ['val']) // => "yarray was modified"
Example: Nest types
Remember, shared types are just plain old data types. The only limitation is that a shared type must exist only once in the shared document.
const ymap = doc.getMap('map')
const foodArray = new Y.Array()
foodArray.insert(0, ['apple', 'banana'])
ymap.set('food', foodArray)
ymap.get('food') === foodArray // => true
ymap.set('fruit', foodArray) // => Error! foodArray is already defined
Now you understand how types are defined on a shared document. Next you can jump to the demo repository or continue reading the API docs.
Example: Using and combining providers
Any of the Yjs providers can be combined with each other. So you can sync data over different network technologies.
In most cases you want to use a network provider (like y-websocket or y-webrtc) in combination with a persistence provider (y-indexeddb in the browser). Persistence allows you to load the document faster and to persist data that is created while offline.
For the sake of this demo we combine two different network providers with a persistence provider.
import * as Y from 'yjs'
import { WebrtcProvider } from 'y-webrtc'
import { WebsocketProvider } from 'y-websocket'
import { IndexeddbPersistence } from 'y-indexeddb'
const ydoc = new Y.Doc()
// this allows you to instantly get the (cached) documents data
const indexeddbProvider = new IndexeddbPersistence('count-demo', ydoc)
indexeddbProvider.whenSynced.then(() => {
console.log('loaded data from indexed db')
})
// Sync clients with the y-webrtc provider.
const webrtcProvider = new WebrtcProvider('count-demo', ydoc)
// Sync clients with the y-websocket provider
const websocketProvider = new WebsocketProvider(
'wss://demos.yjs.dev', 'count-demo', ydoc
)
// array of numbers which produce a sum
const yarray = ydoc.getArray('count')
// observe changes of the sum
yarray.observe(event => {
// print updates when the data changes
console.log('new sum: ' + yarray.toArray().reduce((a,b) => a + b))
})
// add 1 to the sum
yarray.push([1]) // => "new sum: 1"
API
import * as Y from 'yjs'
Shared Types
Y.Array
A shareable Array-like type that supports efficient insert/delete of elements at any position. Internally it uses a linked list of Arrays that is split when necessary.
const yarray = new Y.Array()
-
Insert content at index. Note that content is an array of elements.
I.e.
array.insert(0, [1])
splices the list and inserts 1 at position 0. - Retrieve a range of content
- Copies the content of this YArray to a new Array.
-
Copies the content of this YArray to a new Array. It transforms all child types
to JSON using their
toJSON
method. -
Returns an YArray Iterator that contains the values for each index in the array.
for (let value of yarray) { .. }
- Adds an event listener to this type that will be called synchronously every time this type is modified. In the case this type is modified in the event listener, the event listener will be called again after the current event listener returns.
-
Removes an
observe
event listener from this type. - Adds an event listener to this type that will be called synchronously every time this type or any of its children is modified. In the case this type is modified in the event listener, the event listener will be called again after the current event listener returns. The event listener receives all Events created by itself or any of its children.
-
Removes an
observeDeep
event listener from this type.
parent:Y.AbstractType|null
insert(index:number, content:Array<object|boolean|Array|string|number|null|Uint8Array|Y.Type>)
push(Array<Object|boolean|Array|string|number|null|Uint8Array|Y.Type>)
unshift(Array<Object|boolean|Array|string|number|null|Uint8Array|Y.Type>)
delete(index:number, length:number)
get(index:number)
slice(start:number, end:number):Array<Object|boolean|Array|string|number|null|Uint8Array|Y.Type>
length:number
forEach(function(value:object|boolean|Array|string|number|null|Uint8Array|Y.Type,
index:number, array: Y.Array))
map(function(T, number, YArray):M):Array<M>
toArray():Array<object|boolean|Array|string|number|null|Uint8Array|Y.Type>
toJSON():Array<Object|boolean|Array|string|number|null>
[Symbol.Iterator]
observe(function(YArrayEvent, Transaction):void)
unobserve(function(YArrayEvent, Transaction):void)
observeDeep(function(Array<YEvent>, Transaction):void)
unobserveDeep(function(Array<YEvent>, Transaction):void)
Y.Map
A shareable Map type.
const ymap = new Y.Map()
- Total number of key/value pairs.
- Removes all elements from this YMap.
- Clone this type into a fresh Yjs type.
-
Copies the
[key,value]
pairs of this YMap to a new Object.It transforms all child types to JSON using theirtoJSON
method. - Execute the provided function once for every key-value pair.
-
Returns an Iterator of
[key, value]
pairs.for (let [key, value] of ymap) { .. }
-
Returns an Iterator of
[key, value]
pairs. - Returns an Iterator of all values.
- Returns an Iterator of all keys.
- Adds an event listener to this type that will be called synchronously every time this type is modified. In the case this type is modified in the event listener, the event listener will be called again after the current event listener returns.
-
Removes an
observe
event listener from this type. - Adds an event listener to this type that will be called synchronously every time this type or any of its children is modified. In the case this type is modified in the event listener, the event listener will be called again after the current event listener returns. The event listener receives all Events created by itself or any of its children.
-
Removes an
observeDeep
event listener from this type.
parent:Y.AbstractType|null
size: number
get(key:string):object|boolean|string|number|null|Uint8Array|Y.Type
set(key:string, value:object|boolean|string|number|null|Uint8Array|Y.Type)
delete(key:string)
has(key:string):boolean
get(index:number)
clear()
clone():Y.Map
toJSON():Object<string, Object|boolean|Array|string|number|null|Uint8Array>
forEach(function(value:object|boolean|Array|string|number|null|Uint8Array|Y.Type,
key:string, map: Y.Map))
[Symbol.Iterator]
entries()
values()
keys()
observe(function(YMapEvent, Transaction):void)
unobserve(function(YMapEvent, Transaction):void)
observeDeep(function(Array<YEvent>, Transaction):void)
unobserveDeep(function(Array<YEvent>, Transaction):void)
Y.Text
A shareable type that is optimized for shared editing on text. It allows to assign properties to ranges in the text. This makes it possible to implement rich-text bindings to this type.
This type can also be transformed to the delta format. Similarly the YTextEvents compute changes as deltas.
const ytext = new Y.Text()
-
Insert a string at index and assign formatting attributes to it.
ytext.insert(0, 'bold text', { bold: true })
- Assign formatting attributes to a range in the text
-
See Quill Delta
Can set options for preventing remove ending newLines, default is true.
ytext.applyDelta(delta, { sanitize: false })
- Transforms this type, without formatting options, into a string.
- See
toString
- Transforms this type to a Quill Delta
- Adds an event listener to this type that will be called synchronously every time this type is modified. In the case this type is modified in the event listener, the event listener will be called again after the current event listener returns.
-
Removes an
observe
event listener from this type. - Adds an event listener to this type that will be called synchronously every time this type or any of its children is modified. In the case this type is modified in the event listener, the event listener will be called again after the current event listener returns. The event listener receives all Events created by itself or any of its children.
-
Removes an
observeDeep
event listener from this type.
parent:Y.AbstractType|null
insert(index:number, content:string, [formattingAttributes:Object<string,string>])
delete(index:number, length:number)
format(index:number, length:number, formattingAttributes:Object<string,string>)
applyDelta(delta: Delta, opts:Object<string,any>)
length:number
toString():string
toJSON():string
toDelta():Delta
observe(function(YTextEvent, Transaction):void)
unobserve(function(YTextEvent, Transaction):void)
observeDeep(function(Array<YEvent>, Transaction):void)
unobserveDeep(function(Array<YEvent>, Transaction):void)
Y.XmlFragment
A container that holds an Array of Y.XmlElements.
const yxml = new Y.XmlFragment()
- Retrieve a range of content
- Clone this type into a fresh Yjs type.
- Copies the children to a new Array.
- Transforms this type and all children to new DOM elements.
- Get the XML serialization of all descendants.
- See
toString
. - Create an Iterable that walks through the children.
- Adds an event listener to this type that will be called synchronously every time this type is modified. In the case this type is modified in the event listener, the event listener will be called again after the current event listener returns.
-
Removes an
observe
event listener from this type. - Adds an event listener to this type that will be called synchronously every time this type or any of its children is modified. In the case this type is modified in the event listener, the event listener will be called again after the current event listener returns. The event listener receives all Events created by itself or any of its children.
-
Removes an
observeDeep
event listener from this type.
parent:Y.AbstractType|null
firstChild:Y.XmlElement|Y.XmlText|null
insert(index:number, content:Array<Y.XmlElement|Y.XmlText>)
delete(index:number, length:number)
get(index:number)
slice(start:number, end:number):Array<Y.XmlElement|Y.XmlText>
length:number
clone():Y.XmlFragment
toArray():Array<Y.XmlElement|Y.XmlText>
toDOM():DocumentFragment
toString():string
toJSON():string
createTreeWalker(filter: function(AbstractType<any>):boolean):Iterable
observe(function(YXmlEvent, Transaction):void)
unobserve(function(YXmlEvent, Transaction):void)
observeDeep(function(Array<YEvent>, Transaction):void)
unobserveDeep(function(Array<YEvent>, Transaction):void)
Y.XmlElement
A shareable type that represents an XML Element. It has a nodeName
,
attributes, and a list of children. But it makes no effort to validate its
content and be actually XML compliant.
const yxml = new Y.XmlElement()
- Retrieve the i-th element.
- Retrieve a range of content
- Clone this type into a fresh Yjs type.
- Copies the children to a new Array.
- Transforms this type and all children to a new DOM element.
- Get the XML serialization of all descendants.
- See
toString
. - Adds an event listener to this type that will be called synchronously every time this type is modified. In the case this type is modified in the event listener, the event listener will be called again after the current event listener returns.
-
Removes an
observe
event listener from this type. - Adds an event listener to this type that will be called synchronously every time this type or any of its children is modified. In the case this type is modified in the event listener, the event listener will be called again after the current event listener returns. The event listener receives all Events created by itself or any of its children.
-
Removes an
observeDeep
event listener from this type.
parent:Y.AbstractType|null
firstChild:Y.XmlElement|Y.XmlText|null
nextSibling:Y.XmlElement|Y.XmlText|null
prevSibling:Y.XmlElement|Y.XmlText|null
insert(index:number, content:Array<Y.XmlElement|Y.XmlText>)
delete(index:number, length:number)
get(index:number)
length:number
setAttribute(attributeName:string, attributeValue:string)
removeAttribute(attributeName:string)
getAttribute(attributeName:string):string
getAttributes():Object<string,string>
get(i:number):Y.XmlElement|Y.XmlText
slice(start:number, end:number):Array<Y.XmlElement|Y.XmlText>
clone():Y.XmlElement
toArray():Array<Y.XmlElement|Y.XmlText>
toDOM():Element
toString():string
toJSON():string
observe(function(YXmlEvent, Transaction):void)
unobserve(function(YXmlEvent, Transaction):void)
observeDeep(function(Array<YEvent>, Transaction):void)
unobserveDeep(function(Array<YEvent>, Transaction):void)
Y.Doc
const doc = new Y.Doc()
- A unique id that identifies this client. (readonly)
- Whether garbage collection is enabled on this doc instance. Set `doc.gc = false` in order to disable gc and be able to restore old content. See https://github.com/yjs/yjs#yjs-crdt-algorithm for more information about gc in Yjs.
-
Every change on the shared document happens in a transaction. Observer calls and
the
update
event are called after each transaction. You should bundle changes into a single transaction to reduce the amount of event calls. I.e.doc.transact(() => { yarray.insert(..); ymap.set(..) })
triggers a single change event.
You can specify an optionalorigin
parameter that is stored ontransaction.origin
andon('update', (update, origin) => ..)
. -
Deprecated: It is recommended to call toJSON directly on the shared types.
Converts the entire document into a js object, recursively traversing each yjs
type. Doesn't log types that have not been defined (using
ydoc.getType(..)
). - Define a shared type.
- Define a shared Y.Array type. Is equivalent to
y.get(string, Y.Array)
. - Define a shared Y.Map type. Is equivalent to
y.get(string, Y.Map)
. - Define a shared Y.XmlFragment type. Is equivalent to
y.get(string, Y.XmlFragment)
. - Register an event listener on the shared type
- Unregister an event listener from the shared type
clientID
gc
transact(function(Transaction):void [, origin:any])
toJSON():any
get(string, Y.[TypeClass]):[Type]
getArray(string):Y.Array
getMap(string):Y.Map
getXmlFragment(string):Y.XmlFragment
on(string, function)
off(string, function)
Y.Doc Events
- Listen to document updates. Document updates must be transmitted to all other peers. You can apply document updates in any order and multiple times.
- Emitted before each transaction.
- Emitted after each transaction.
- Transactions can be nested (e.g. when an event within a transaction calls another transaction). Emitted before the first transaction.
- Emitted after the last transaction is cleaned up.
on('update', function(updateMessage:Uint8Array, origin:any, Y.Doc):void)
on('beforeTransaction', function(Y.Transaction, Y.Doc):void)
on('afterTransaction', function(Y.Transaction, Y.Doc):void)
on('beforeAllTransactions', function(Y.Doc):void)
on('afterAllTransactions', function(Y.Doc, Array<Y.Transaction>):void)
Document Updates
Changes on the shared document are encoded into document updates. Document updates are commutative and idempotent. This means that they can be applied in any order and multiple times.
Example: Listen to update events and apply them on remote client
const doc1 = new Y.Doc()
const doc2 = new Y.Doc()
doc1.on('update', update => {
Y.applyUpdate(doc2, update)
})
doc2.on('update', update => {
Y.applyUpdate(doc1, update)
})
// All changes are also applied to the other document
doc1.getArray('myarray').insert(0, ['Hello doc2, you got this?'])
doc2.getArray('myarray').get(0) // => 'Hello doc2, you got this?'
Yjs internally maintains a state vector that denotes the next expected clock from each client. In a different interpretation it holds the number of structs created by each client. When two clients sync, you can either exchange the complete document structure or only the differences by sending the state vector to compute the differences.
Example: Sync two clients by exchanging the complete document structure
const state1 = Y.encodeStateAsUpdate(ydoc1)
const state2 = Y.encodeStateAsUpdate(ydoc2)
Y.applyUpdate(ydoc1, state2)
Y.applyUpdate(ydoc2, state1)
Example: Sync two clients by computing the differences
This example shows how to sync two clients with the minimal amount of exchanged data by computing only the differences using the state vector of the remote client. Syncing clients using the state vector requires another roundtrip, but can save a lot of bandwidth.
const stateVector1 = Y.encodeStateVector(ydoc1)
const stateVector2 = Y.encodeStateVector(ydoc2)
const diff1 = Y.encodeStateAsUpdate(ydoc1, stateVector2)
const diff2 = Y.encodeStateAsUpdate(ydoc2, stateVector1)
Y.applyUpdate(ydoc1, diff2)
Y.applyUpdate(ydoc2, diff1)
Example: Syncing clients without loading the Y.Doc
It is possible to sync clients and compute delta updates without loading the Yjs document to memory. Yjs exposes an API to compute the differences directly on the binary document updates.
// encode the current state as a binary buffer
let currentState1 = Y.encodeStateAsUpdate(ydoc1)
let currentState2 = Y.encodeStateAsUpdate(ydoc2)
// now we can continue syncing clients using state vectors without using the Y.Doc
ydoc1.destroy()
ydoc2.destroy()
const stateVector1 = Y.encodeStateVectorFromUpdate(currentState1)
const stateVector2 = Y.encodeStateVectorFromUpdate(currentState2)
const diff1 = Y.diffUpdate(currentState1, stateVector2)
const diff2 = Y.diffUpdate(currentState2, stateVector1)
// sync clients
currentState1 = Y.mergeUpdates([currentState1, diff2])
currentState1 = Y.mergeUpdates([currentState1, diff1])
Using V2 update format
Yjs implements two update formats. By default you are using the V1 update format.
You can opt-in into the V2 update format wich provides much better compression.
It is not yet used by all providers. However, you can already use it if
you are building your own provider. All below functions are available with the
suffix "V2". E.g. Y.applyUpdate
⇒ Y.applyUpdateV2
. We also support conversion
functions between both formats: Y.convertUpdateFormatV1ToV2
& Y.convertUpdateFormatV2ToV1
.
Update API
-
Apply a document update on the shared document. Optionally you can specify
transactionOrigin
that will be stored ontransaction.origin
andydoc.on('update', (update, origin) => ..)
. - Encode the document state as a single update message that can be applied on the remote document. Optionally specify the target state vector to only write the differences to the update message.
- Computes the state vector and encodes it into an Uint8Array.
- Merge several document updates into a single document update while removing duplicate information. The merged document update is always smaller than the separate updates because of the compressed encoding.
- Computes the state vector from a document update and encodes it into an Uint8Array.
-
Encode the missing differences to another update message. This function works
similarly to
Y.encodeStateAsUpdate(ydoc, stateVector)
but works on updates instead. - Convert V1 update format to the V2 update format.
- Convert V2 update format to the V1 update format.
Y.applyUpdate(Y.Doc, update:Uint8Array, [transactionOrigin:any])
Y.encodeStateAsUpdate(Y.Doc, [encodedTargetStateVector:Uint8Array]):Uint8Array
Y.encodeStateVector(Y.Doc):Uint8Array
Y.mergeUpdates(Array<Uint8Array>)
Y.encodeStateVectorFromUpdate(Uint8Array): Uint8Array
Y.diffUpdate(update: Uint8Array, stateVector: Uint8Array): Uint8Array
convertUpdateFormatV1ToV2
convertUpdateFormatV2ToV1
Relative Positions
Example: Transform to RelativePosition and back
const relPos = Y.createRelativePositionFromTypeIndex(ytext, 2)
const pos = Y.createAbsolutePositionFromRelativePosition(relPos, doc)
pos.type === ytext // => true
pos.index === 2 // => true
Example: Send relative position to remote client (json)
const relPos = Y.createRelativePositionFromTypeIndex(ytext, 2)
const encodedRelPos = JSON.stringify(relPos)
// send encodedRelPos to remote client..
const parsedRelPos = JSON.parse(encodedRelPos)
const pos = Y.createAbsolutePositionFromRelativePosition(parsedRelPos, remoteDoc)
pos.type === remoteytext // => true
pos.index === 2 // => true
Example: Send relative position to remote client (Uint8Array)
const relPos = Y.createRelativePositionFromTypeIndex(ytext, 2)
const encodedRelPos = Y.encodeRelativePosition(relPos)
// send encodedRelPos to remote client..
const parsedRelPos = Y.decodeRelativePosition(encodedRelPos)
const pos = Y.createAbsolutePositionFromRelativePosition(parsedRelPos, remoteDoc)
pos.type === remoteytext // => true
pos.index === 2 // => true
Y.createRelativePositionFromTypeIndex(Uint8Array|Y.Type, number)
Y.createAbsolutePositionFromRelativePosition(RelativePosition, Y.Doc)
Y.encodeRelativePosition(RelativePosition):Uint8Array
Y.decodeRelativePosition(Uint8Array):RelativePosition
Y.UndoManager
Yjs ships with an Undo/Redo manager for selective undo/redo of changes on a Yjs type. The changes can be optionally scoped to transaction origins.
const ytext = doc.getText('text')
const undoManager = new Y.UndoManager(ytext)
ytext.insert(0, 'abc')
undoManager.undo()
ytext.toString() // => ''
undoManager.redo()
ytext.toString() // => 'abc'
- Accepts either single type as scope or an array of types.
-
Register an event that is called when a
StackItem
is added to the undo- or the redo-stack. -
Register an event that is called when a
StackItem
is popped from the undo- or the redo-stack.
constructor(scope:Y.AbstractType|Array<Y.AbstractType>
[, {captureTimeout:number,trackedOrigins:Set<any>,deleteFilter:function(item):boolean}])
undo()
redo()
stopCapturing()
on('stack-item-added', { stackItem: { meta: Map<any,any> }, type: 'undo'
| 'redo' })
on('stack-item-popped', { stackItem: { meta: Map<any,any> }, type: 'undo'
| 'redo' })
Example: Stop Capturing
UndoManager merges Undo-StackItems if they are created within time-gap
smaller than options.captureTimeout
. Call um.stopCapturing()
so that the next
StackItem won't be merged.
// without stopCapturing
ytext.insert(0, 'a')
ytext.insert(1, 'b')
undoManager.undo()
ytext.toString() // => '' (note that 'ab' was removed)
// with stopCapturing
ytext.insert(0, 'a')
undoManager.stopCapturing()
ytext.insert(0, 'b')
undoManager.undo()
ytext.toString() // => 'a' (note that only 'b' was removed)
Example: Specify tracked origins
Every change on the shared document has an origin. If no origin was specified,
it defaults to null
. By specifying trackedOrigins
you can
selectively specify which changes should be tracked by UndoManager
. The
UndoManager instance is always added to trackedOrigins
.
class CustomBinding {}
const ytext = doc.getText('text')
const undoManager = new Y.UndoManager(ytext, {
trackedOrigins: new Set([42, CustomBinding])
})
ytext.insert(0, 'abc')
undoManager.undo()
ytext.toString() // => 'abc' (does not track because origin `null` and not part
// of `trackedTransactionOrigins`)
ytext.delete(0, 3) // revert change
doc.transact(() => {
ytext.insert(0, 'abc')
}, 42)
undoManager.undo()
ytext.toString() // => '' (tracked because origin is an instance of `trackedTransactionorigins`)
doc.transact(() => {
ytext.insert(0, 'abc')
}, 41)
undoManager.undo()
ytext.toString() // => '' (not tracked because 41 is not an instance of
// `trackedTransactionorigins`)
ytext.delete(0, 3) // revert change
doc.transact(() => {
ytext.insert(0, 'abc')
}, new CustomBinding())
undoManager.undo()
ytext.toString() // => '' (tracked because origin is a `CustomBinding` and
// `CustomBinding` is in `trackedTransactionorigins`)
Example: Add additional information to the StackItems
When undoing or redoing a previous action, it is often expected to restore additional meta information like the cursor location or the view on the document. You can assign meta-information to Undo-/Redo-StackItems.
const ytext = doc.getText('text')
const undoManager = new Y.UndoManager(ytext, {
trackedOrigins: new Set([42, CustomBinding])
})
undoManager.on('stack-item-added', event => {
// save the current cursor location on the stack-item
event.stackItem.meta.set('cursor-location', getRelativeCursorLocation())
})
undoManager.on('stack-item-popped', event => {
// restore the current cursor location on the stack-item
restoreCursorLocation(event.stackItem.meta.get('cursor-location'))
})
Yjs CRDT Algorithm
Conflict-free replicated data types (CRDT) for collaborative editing are an alternative approach to operational transformation (OT). A very simple differenciation between the two approaches is that OT attempts to transform index positions to ensure convergence (all clients end up with the same content), while CRDTs use mathematical models that usually do not involve index transformations, like linked lists. OT is currently the de-facto standard for shared editing on text. OT approaches that support shared editing without a central source of truth (a central server) require too much bookkeeping to be viable in practice. CRDTs are better suited for distributed systems, provide additional guarantees that the document can be synced with remote clients, and do not require a central source of truth.
Yjs implements a modified version of the algorithm described in this paper. This article explains a simple optimization on the CRDT model and gives more insight about the performance characteristics in Yjs. More information about the specific implementation is available in INTERNALS.md and in this walkthrough of the Yjs codebase.
CRDTs that suitable for shared text editing suffer from the fact that they only grow in size. There are CRDTs that do not grow in size, but they do not have the characteristics that are benificial for shared text editing (like intention preservation). Yjs implements many improvements to the original algorithm that diminish the trade-off that the document only grows in size. We can't garbage collect deleted structs (tombstones) while ensuring a unique order of the structs. But we can 1. merge preceeding structs into a single struct to reduce the amount of meta information, 2. we can delete content from the struct if it is deleted, and 3. we can garbage collect tombstones if we don't care about the order of the structs anymore (e.g. if the parent was deleted).
Examples:
- If a user inserts elements in sequence, the struct will be merged into a
single struct. E.g.
text.insert(0, 'a'), text.insert(1, 'b');
is first represented as two structs ([{id: {client, clock: 0}, content: 'a'}, {id: {client, clock: 1}, content: 'b'}
) and then merged into a single struct:[{id: {client, clock: 0}, content: 'ab'}]
. - When a struct that contains content (e.g.
ItemString
) is deleted, the struct will be replaced with anItemDeleted
that does not contain content anymore. - When a type is deleted, all child elements are transformed to
GC
structs. AGC
struct only denotes the existence of a struct and that it is deleted.GC
structs can always be merged with otherGC
structs if the id's are adjacent.
Especially when working on structured content (e.g. shared editing on ProseMirror), these improvements yield very good results when benchmarking random document edits. In practice they show even better results, because users usually edit text in sequence, resulting in structs that can easily be merged. The benchmarks show that even in the worst case scenario that a user edits text from right to left, Yjs achieves good performance even for huge documents.
State Vector
Yjs has the ability to exchange only the differences when syncing two clients.
We use lamport timestamps to identify structs and to track in which order a
client created them. Each struct has an struct.id = { client: number, clock: number}
that uniquely identifies a struct. We define the next expected clock
by each client as the state vector. This data structure is similar to the
version vectors data structure.
But we use state vectors only to describe the state of the local document, so we
can compute the missing struct of the remote client. We do not use it to track
causality.
License and Author
Yjs and all related projects are MIT licensed.
Yjs is based on my research as a student at the RWTH i5. Now I am working on Yjs in my spare time.
Fund this project by donating on GitHub Sponsors or hiring me as a contractor for your collaborative app.