Realtime database backend based on Operational Transformation (OT)
ericyhwang Merge pull request #245 from share/snapshot-request-test
Improve test coverage for `SnapshotRequest.onConnectionStateChanged`
Latest commit e45010c Sep 6, 2018


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ShareDB is a realtime database backend based on Operational Transformation (OT) of JSON documents. It is the realtime backend for the DerbyJS web application framework.

For questions, discussion and announcements, join the ShareJS mailing list or check the FAQ.

Please report any bugs you find to the issue tracker.


  • Realtime synchronization of any JSON document
  • Concurrent multi-user collaboration
  • Synchronous editing API with asynchronous eventual consistency
  • Realtime query subscriptions
  • Simple integration with any database - MongoDB, PostgresQL (experimental)
  • Horizontally scalable with pub/sub integration
  • Projections to select desired fields from documents and operations
  • Middleware for implementing access control and custom extensions
  • Ideal for use in browsers or on the server
  • Offline change syncing upon reconnection
  • In-memory implementations of database and pub/sub for unit testing



const WebSocket = require('reconnecting-websocket');
var socket = new WebSocket('ws://' +;
var connection = new sharedb.Connection(socket);

The native Websocket object that you feed to ShareDB's Connection constructor does not handle reconnections.

The easiest way is to give it a WebSocket object that does reconnect. There are plenty of example on the web. The most important thing is that the custom reconnecting websocket, must have the same API as the native rfc6455 version.

In the "textarea" example we show this off using a Reconnecting Websocket implementation from

Example apps

Simple app demonstrating realtime sync

Leaderboard app demonstrating live queries

Data model

In ShareDB's view of the world, every document has 3 properties:

  • version - An incrementing number starting at 0
  • type - An OT type. OT types are defined in share/ottypes. Documents which don't exist implicitly have a type of null.
  • data - The actual data that the document contains. This must be pure acyclic JSON. Its also type-specific. (JSON type uses raw JSON, text documents use a string, etc).

ShareDB implicitly has a record for every document you can access. New documents have version 0, a null type and no data. To use a document, you must first submit a create operation, which will set the document's type and give it initial data. Then you can submit editing operations on the document (using OT). Finally you can delete the document with a delete operation. By default, ShareDB stores all operations forever - nothing is truly deleted.

Server API


First, create a ShareDB server instance:

var ShareDB = require('sharedb');
var share = new ShareDB(options);


  • options.db (instance of ShareDB.DB) Store documents and ops with this database adapter. Defaults to ShareDB.MemoryDB().
  • options.pubsub (instance of ShareDB.PubSub) Notify other ShareDB processes when data changes through this pub/sub adapter. Defaults to ShareDB.MemoryPubSub().

Database Adapters

  • ShareDB.MemoryDB, backed by a non-persistent database with no queries
  • ShareDBMongo, backed by a real Mongo database and full query support
  • ShareDBMingoMemory, backed by a non-persistent database supporting most Mongo queries. Useful for faster testing of a Mongo-based app.
  • ShareDBPostgres, backed by PostgresQL. No query support.

Pub/Sub Adapters

  • ShareDB.MemoryPubSub can be used with a single process
  • ShareDBRedisPubSub can be used with multiple processes using Redis' pub/sub mechanism

Community Provided Pub/Sub Adapters

Listening to WebSocket connections

var WebSocketJSONStream = require('websocket-json-stream');

// 'ws' is a websocket server connection, as passed into
// new (require('ws').Server).on('connection', ...)
var stream = new WebSocketJSONStream(ws);

For transports other than WebSockets, expose a duplex stream that writes and reads JavaScript objects. Then pass that stream directly into share.listen.


Middlewares let you hook into the ShareDB server pipeline. In middleware code you can read and also modify objects as they flow through ShareDB. For example, sharedb-access uses middlewares to implement access control.

share.use(action, fn) Register a new middleware.

  • action (String) One of:
    • 'connect': A new client connected to the server.
    • 'op': An operation was loaded from the database.
    • 'readSnapshots': Snapshot(s) were loaded from the database for a fetch or subscribe of a query or document
    • 'query': A query is about to be sent to the database
    • 'submit': An operation is about to be submitted to the database
    • 'apply': An operation is about to be applied to a snapshot before being committed to the database
    • 'commit': An operation was applied to a snapshot; The operation and new snapshot are about to be written to the database.
    • 'afterSubmit': An operation was successfully submitted to the database.
    • 'receive': Received a message from a client
  • fn (Function(request, callback)) Call this function at the time specified by action. request contains a subset of the following properties, as relevant for the action:
    • action: The action this middleware is handing
    • agent: An object corresponding to the server agent handing this client
    • req: The HTTP request being handled
    • collection: The collection name being handled
    • id: The document id being handled
    • snapshots: The retrieved snapshots for the readSnapshots action
    • query: The query object being handled
    • op: The op being handled


ShareDB supports exposing a projection of a real collection, with a specified (limited) set of allowed fields. Once configured, the projected collection looks just like a real collection - except documents only have the fields you've requested. Operations (gets, queries, sets, etc) on the fake collection work, but you only see a small portion of the data.

addProjection(name, collection, fields) Configure a projection.

  • name The name of the projected collection.
  • collection The name of the existing collection.
  • fields A map (object) of the allowed fields in documents.
    • Keys are field names.
    • Values should be true.

For example, you could make a users_limited projection which lets users view each other's names and profile pictures, but not password hashes. You would configure this by calling:

share.addProjection('users_limited', 'users', { name:true, profileUrl:true });

Note that only the JSON0 OT type is supported for projections.


share.close(callback) Closes connections to the database and pub/sub adapters.

Client API

The client API can be used from either Node or a browser. First, get a ShareDB.Connection object by connecting to the ShareDB server instance:

From Node:

// `share` should be a ShareDB server instance
var connection = share.connect();

To use ShareDB from a browser, use a client bundler like Browserify or Webpack. The following code connects to the ShareDB server instance over WebSockets:

var ShareDB = require('sharedb/lib/client');
var socket = new WebSocket('ws://localhost:8080');
var connection = new ShareDB.Connection(socket);

For transports other than WebSockets, create an object implementing the WebSocket specification and pass it into the ShareDB.Connection constructor.

Class: ShareDB.Connection

connection.get(collectionName, documentId) Get a ShareDB.Doc instance on a given collection and document ID.

connection.createFetchQuery(collectionName, query, options, callback) connection.createSubscribeQuery(collectionName, query, options, callback) Get query results from the server. createSubscribeQuery also subscribes to changes. Returns a ShareDB.Query instance.

  • query (Object) A descriptor of a database query with structure defined by the database adapter.
  • callback (Function) Called with (err, results) when server responds, or on error.
  • options.results (Array) Prior query results if available, such as from server rendering.
  • options.* All other options are passed through to the database adapter.

connection.fetchSnapshot(collection, id, version, callback): void; Get a read-only snapshot of a document at the requested version.

  • collection (String) Collection name of the snapshot

  • id (String) ID of the snapshot

  • version (number) [optional] The version number of the desired snapshot

  • callback (Function) Called with (error, snapshot), where snapshot takes the following form:

      id: string;         // ID of the snapshot
      v: number;          // version number of the snapshot
      type: string;       // the OT type of the snapshot, or null if it doesn't exist or is deleted
      data: any;          // the snapshot

Class: ShareDB.Doc

doc.type (String) The OT type of this document (String) Unique document ID (Object) Document contents. Available after document is fetched or subscribed to.

doc.fetch(function(err) {...}) Populate the fields on doc with a snapshot of the document from the server.

doc.subscribe(function(err) {...}) Populate the fields on doc with a snapshot of the document from the server, and fire events on subsequent changes.

doc.ingestSnapshot(snapshot, callback) Ingest snapshot data. This data must include a version, snapshot and type. This method is generally called interally as a result of fetch or subscribe and not directly. However, it may be called directly to pass data that was transferred to the client external to the client's ShareDB connection, such as snapshot data sent along with server rendering of a webpage.

doc.destroy() Unsubscribe and stop firing events.

doc.on('load', function() {...}) The initial snapshot of the document was loaded from the server. Fires at the same time as callbacks to fetch and subscribe.

doc.on('create', function(source) {...}) The document was created. Technically, this means it has a type. source will be false for ops received from the server and defaults to true for ops generated locally.

doc.on('before op'), function(op, source) {...}) An operation is about to be applied to the data. source will be false for ops received from the server and defaults to true for ops generated locally.

doc.on('op', function(op, source) {...}) An operation was applied to the data. source will be false for ops received from the server and defaults to true for ops generated locally.

doc.on('del', function(data, source) {...}) The document was deleted. Document contents before deletion are passed in as an argument. source will be false for ops received from the server and defaults to true for ops generated locally.

doc.on('error', function(err) {...}) There was an error fetching the document or applying an operation.

doc.removeListener(eventName, listener) Removes any listener you added with doc.on. eventName should be one of 'load', 'create', 'before op', 'op', 'del', or 'error'. listener should be the function you passed in as the second argument to on. Note that both on and removeListener are inherited from EventEmitter.

doc.create(data[, type][, options][, function(err) {...}]) Create the document locally and send create operation to the server.

  • data Initial document contents
  • type (OT type) Defaults to 'ot-json0', for which data is an Object
  • options.source Argument passed to the 'create' event locally. This is not sent to the server or other clients. Defaults to true.

doc.submitOp(op, [, options][, function(err) {...}]) Apply operation to document and send it to the server. op structure depends on the document type. See the operations for the default 'ot-json0' type. Call this after you've either fetched or subscribed to the document.

  • options.source Argument passed to the 'op' event locally. This is not sent to the server or other clients. Defaults to true.

doc.del([options][, function(err) {...}]) Delete the document locally and send delete operation to the server. Call this after you've either fetched or subscribed to the document.

  • options.source Argument passed to the 'del' event locally. This is not sent to the server or other clients. Defaults to true.

doc.whenNothingPending(function(err) {...}) Invokes the given callback function after

  • all ops submitted via doc.submitOp have been sent to the server, and
  • all pending fetch, subscribe, and unsubscribe requests have been resolved.

Note that whenNothingPending does NOT wait for pending model.query() calls.

Class: ShareDB.Query

query.ready (Boolean) True if query results are ready and available on query.results

query.results (Array) Query results, as an array of ShareDB.Doc instances.

query.extra (Type depends on database adapter and query) Extra query results that aren't an array of documents. Available for certain database adapters and queries.

query.on('ready', function() {...})) The initial query results were loaded from the server. Fires at the same time as the callbacks to createFetchQuery and createSubscribeQuery.

query.on('error', function(err) {...})) There was an error receiving updates to a subscription.

query.destroy() Unsubscribe and stop firing events.

query.on('changed', function(results) {...})) (Only fires on subscription queries) The query results changed. Fires only once after a sequence of diffs are handled.

query.on('insert', function(docs, atIndex) {...})) (Only fires on subscription queries) A contiguous sequence of documents were added to the query result array.

query.on('move', function(docs, from, to) {...})) (Only fires on subscription queries) A contiguous sequence of documents moved position in the query result array.

query.on('remove', function(docs, atIndex) {...})) (Only fires on subscription queries) A contiguous sequence of documents were removed from the query result array.

query.on('extra', function() {...})) (Only fires on subscription queries) query.extra changed.

Error codes

ShareDB returns errors as plain JavaScript objects with the format:

  code: 5000,
  message: 'ShareDB internal error'

Additional fields may be added to the error object for debugging context depending on the error. Common additional fields include collection, id, and op.

4000 - Bad request

  • 4001 - Unknown error type
  • 4002 - Database adapter does not support subscribe
  • 4003 - Database adapter not found
  • 4004 - Missing op
  • 4005 - Op must be an array
  • 4006 - Create data in op must be an object
  • 4007 - Create op missing type
  • 4008 - Unknown type
  • 4009 - del value must be true
  • 4010 - Missing op, create or del
  • 4011 - Invalid src
  • 4012 - Invalid seq
  • 4013 - Found seq but not src
  • 4014 - op.m invalid
  • 4015 - Document does not exist
  • 4016 - Document already exists
  • 4017 - Document was deleted
  • 4018 - Document was created remotely
  • 4019 - Invalid protocol version
  • 4020 - Invalid default type
  • 4021 - Invalid client id
  • 4022 - Database adapter does not support queries
  • 4023 - Cannot project snapshots of this type
  • 4024 - Invalid version

5000 - Internal error

The 41xx and 51xx codes are reserved for use by ShareDB DB adapters, and the 42xx and 52xx codes are reserved for use by ShareDB PubSub adapters.

  • 5001 - No new ops returned when retrying unsuccessful submit
  • 5002 - Missing snapshot
  • 5003 - Snapshot and op version don't match
  • 5004 - Missing op
  • 5005 - Missing document
  • 5006 - Version mismatch
  • 5007 - Invalid state transition
  • 5008 - Missing version in snapshot
  • 5009 - Cannot ingest snapshot with null version
  • 5010 - No op to send
  • 5011 - Commit DB method unimplemented
  • 5012 - getSnapshot DB method unimplemented
  • 5013 - getOps DB method unimplemented
  • 5014 - queryPollDoc DB method unimplemented
  • 5015 - _subscribe PubSub method unimplemented
  • 5016 - _unsubscribe PubSub method unimplemented
  • 5017 - _publish PubSub method unimplemented
  • 5018 - Required QueryEmitter listener not assigned
  • 5019 - getMilestoneSnapshot MilestoneDB method unimplemented
  • 5020 - saveMilestoneSnapshot MilestoneDB method unimplemented