sharedb

ShareDB

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.

Features

• 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

Reconnection

TLDR

const WebSocket = require('reconnecting-websocket');
var socket = new WebSocket('ws://' + window.location.host);
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 reconnecting-websocket.

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

Initialization

First, create a ShareDB server instance:

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

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().
• options.milestoneDb (instance of ShareDB.MilestoneDB`) Store snapshots of documents at a specified interval of versions

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

• wsbus

Milestone Adapters

• sharedb-milestone-mongo, backed by Mongo

Listening to WebSocket connections

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

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

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

Middlewares

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.
  • 'afterWrite': An operation was successfully written to the database.
  • 'receive': Received a message from a client
  • 'reply': About to send a non-error reply to a client message
• fn (Function(context, callback)) Call this function at the time specified by action.
  • context will always have the following properties:
    • action: The action this middleware is hanlding
    • agent: A reference to the server agent handling this client
    • backend: A reference to this ShareDB backend instance
  • context can also have additional properties, as relevant for the action:
    • collection: The collection name being handled
    • id: The document id being handled
    • op: The op being handled
    • req: HTTP request being handled, if provided to share.listen (for 'connect')
    • stream: The duplex Stream provided to share.listen (for 'connect')
    • query: The query object being handled (for 'query')
    • snapshots: Array of retrieved snapshots (for 'readSnapshots')
    • data: Received client message (for 'receive')
    • request: Client message being replied to (for 'reply')
    • reply: Reply to be sent to the client (for 'reply')

Projections

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.

Logging

By default, ShareDB logs to console. This can be overridden if you wish to silence logs, or to log to your own logging driver or alert service.

Methods can be overridden by passing a console-like object to logger.setMethods:

var ShareDB = require('sharedb');
ShareDB.logger.setMethods({
  info: () => {},                         // Silence info
  warn: () => alerts.warn(arguments),     // Forward warnings to alerting service
  error: () => alerts.critical(arguments) // Remap errors to critical alerts
});

ShareDB only supports the following logger methods:

• info
• warn
• error

Shutdown

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. If null, the latest version is fetched.

• 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
  }
  

connection.fetchSnapshotByTimestamp(collection, id, timestamp, 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

• timestamp (number) [optional] The timestamp of the desired snapshot. The returned snapshot will be the latest snapshot before the provided timestamp. If null, the latest version is fetched.

• 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

doc.id (String) Unique document ID

doc.data (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.unsubscribe(function (err) {...}) Stop listening for document updates. The document data at the time of unsubscribing remains in memory, but no longer stays up-to-date. Resubscribe with doc.subscribe.

doc.ingestSnapshot(snapshot, callback) Ingest snapshot data. The snapshot param must include the fields v (doc version), data, and type (OT type). This method is generally called interally as a result of fetch or subscribe and not directly from user code. However, it may still be called directly from user code 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.

Class: ShareDB.Backend

Backend represents the server-side instance of ShareDB. It is primarily responsible for connecting to clients, and sending requests to the database adapters. It is also responsible for some configuration, such as setting up middleware and projections.

constructor

var Backend = require('sharedb');
var backend = new Backend(options);

Constructs a new Backend instance, with the provided options:

• db DB (optional): an instance of a ShareDB database adapter that provides the data store for ShareDB. If omitted, a new, non-persistent, in-memory adapter will be created, which should not be used in production, but may be useful for testing
• pubsub PubSub (optional): an instance of a ShareDB Pub/Sub adapter that provides a channel for notifying other ShareDB instances of changes to data. If omitted, a new, in-memory adapter will be created. Unlike the database adapter, the in-memory instance may be used in a production environment where pub/sub state need only persist across a single, stand-alone server
• milestoneDb MilestoneDB (optional): an instance of a ShareDB milestone adapter that provides the data store for milestone snapshots, which are historical snapshots of documents stored at a specified version interval. If omitted, this functionality will not be enabled
• extraDbs Object (optional): an object whose values are extra DB instances which can be queried. The keys are the names that can be passed into the query options db field
• suppressPublish boolean (optional): if set to true, any changes committed will not be published on pubsub
• maxSubmitRetries number (optional): the number of times to allow a submit to be retried. If omitted, the request will retry an unlimited number of times

connect

var connection = backend.connect();

Connects to ShareDB and returns an instance of a Connection. This is the server-side equivalent of new ShareDBClient.Connection(socket) in the browser.

This method also supports infrequently used optional arguments:

var connection = backend.connect(connection, req);

• connection Connection (optional): a Connection instance to bind to the Backend
• req Object (optional): a connection context object that can contain information such as cookies or session data that will be available in the middleware

Returns a Connection.

listen

var agent = backend.listen(stream, req);

Registers a Stream with the backend. This should be called when the server receives a new connection from a client.

• stream Stream: a Stream (or Stream-like object) that will be used to communicate between the new Agent and the Backend
• req Object (optional): a connection context object that can contain information such as cookies or session data that will be available in the middleware

Returns an Agent, which is also available in the middleware.

close

backend.close(callback);

Disconnects ShareDB and all of its underlying services (database, pubsub, etc.).

• callback Function: a callback with the signature function (error: Error): void that will be called once the services have stopped, or with an error if at least one of them could not be stopped

use

backend.use(action, middleware);

Adds middleware to the Backend.

• action string | string[]: an action, or array of action names defining when to apply the middleware
• middleware Function: a middleware function with the signature function (context: Object, callback: Function): void;. See middleware for more details

Returns the Backend instance, which allows for multiple chained calls.

addProjection

backend.addProjection(name, collection, fields);

Adds a projection.

• name string: the name of the projection
• collection string: the name of the collection on which to apply the projection
• fields Object: a declaration of which fields to include in the projection, such as { field1: true }. Defining sub-field projections is not supported.

submit

backend.submit(agent, index, id, op, options, callback);

Submits an operation to the Backend.

• agent Agent: connection agent to pass to the middleware
• index string: the name of the target collection or projection
• id string: the document ID
• op Object: the operation to submit
• options Object: these options are passed through to the database adapter's commit method, so any options that are valid there can be used here
• callback Function: a callback with the signature function (error: Error, ops: Object[]): void;, where ops are the ops committed by other clients between the submitted op being submitted and committed

getOps

backend.getOps(agent, index, id, from, to, options, callback);

Fetches the ops for a document between the requested version numbers, where the from value is inclusive, but the to value is non-inclusive.

• agent Agent: connection agent to pass to the middleware
• index string: the name of the target collection or projection
• id string: the document ID
• from number: the first op version to fetch. If set to null, then ops will be fetched from the earliest version
• to number: The last op version. This version will not be fetched (ie to is non-inclusive). If set to null, then ops will be fetched up to the latest version
• options: Object (optional): options can be passed directly to the database driver's getOps inside the opsOptions property: {opsOptions: {metadata: true}}
• callback: Function: a callback with the signature function (error: Error, ops: Object[]): void;, where ops is an array of the requested ops

getOpsBulk

backend.getOpsBulk(agent, index, fromMap, toMap, options, callback);

Fetches the ops for multiple documents in a collection between the requested version numbers, where the from value is inclusive, but the to value is non-inclusive.

• agent Agent: connection agent to pass to the middleware
• index string: the name of the target collection or projection
• id string: the document ID
• fromMap Object: an object whose keys are the IDs of the target documents. The values are the first versions requested of each document. For example, {abc: 3} will fetch ops for document with ID abc from version 3 (inclusive)
• toMap Object: an object whose keys are the IDs of the target documents. The values are the last versions requested of each document (non-inclusive). For example, {abc: 3} will fetch ops for document with ID abc up to version 3 (not inclusive)
• options: Object (optional): options can be passed directly to the database driver's getOpsBulk inside the opsOptions property: {opsOptions: {metadata: true}}
• callback: Function: a callback with the signature function (error: Error, opsMap: Object): void;, where opsMap is an object whose keys are the IDs of the requested documents, and their values are the arrays of requested ops, eg {abc: []}

fetch

backend.fetch(agent, index, id, options, callback);

Fetch the current snapshot of a document.

• agent Agent: connection agent to pass to the middleware
• index string: the name of the target collection or projection
• id string: the document ID
• options: Object (optional): options can be passed directly to the database driver's fetch inside the snapshotOptions property: {snapshotOptions: {metadata: true}}
• callback: Function: a callback with the signature function (error: Error, snapshot: Snapshot): void;, where snapshot is the requested snapshot

fetchBulk

backend.fetchBulk(agent, index, ids, options, callback);

Fetch multiple document snapshots from a collection.

• agent Agent: connection agent to pass to the middleware
• index string: the name of the target collection or projection
• ids string[]: array of document IDs
• options: Object (optional): options can be passed directly to the database driver's fetchBulk inside the snapshotOptions property: {snapshotOptions: {metadata: true}}
• callback: Function: a callback with the signature function (error: Error, snapshotMap: Object): void;, where snapshotMap is an object whose keys are the requested IDs, and the values are the requested Snapshots

queryFetch

backend.queryFetch(agent, index, query, options, callback);

Fetch snapshots that match the provided query. In most cases, querying the backing database directly should be preferred, but queryFetch can be used in order to apply middleware, whilst avoiding the overheads associated with using a Doc instance.

• agent Agent: connection agent to pass to the middleware
• index string: the name of the target collection or projection
• query Object: a query object, whose format will depend on the database adapter being used
• options Object: an object that may contain a db property, which specifies which database to run the query against. These extra databases can be attached via the extraDbs option in the Backend constructor
• callback Function: a callback with the signature function (error: Error, snapshots: Snapshot[], extra: Object): void;, where snapshots is an array of the snapshots matching the query, and extra is an (optional) object that the database adapter might return with more information about the results (such as counts)

Class: ShareDB.Agent

An Agent is the representation of a client's Connection state on the server. If the Connection was created through backend.connect (ie the client is running on the server), then the Agent associated with a Connection can be accessed through a direct reference: connection.agent.

The Agent will be made available in all middleware requests. The agent.custom field is an object that can be used for storing arbitrary information for use in middleware. For example:

backend.useMiddleware('connect', function (request, callback) {
  // Best practice to clone to prevent mutating the object after connection.
  // You may also want to consider a deep clone, depending on the shape of request.req.
  Object.assign(request.agent.custom, request.req);
  callback();
});

backend.useMiddleware('readSnapshots', function (request, callback) {
  var connectionInfo = request.agent.custom;
  var snapshots = request.snapshots;

  // Use the information provided at connection to determine if a user can access snapshots.
  // This should also be checked when fetching and submitting ops.
  if (!userCanAccessSnapshots(connectionInfo, snapshots)) {
    return callback(new Error('Authentication error'));
  }

  callback();
});

// Here you should determine what permissions a user has, probably by reading a cookie and
// potentially making some database request to check which documents they can access, or which
// roles they have, etc. If doing this asynchronously, make sure you call backend.connect
// after the permissions have been fetched.
var connectionInfo = getUserPermissions();
// Pass info in as the second argument. This will be made available as request.req in the
// 'connection' middleware.
var connection = backend.connect(null, connectionInfo);

Logging

By default, ShareDB logs to console. This can be overridden if you wish to silence logs, or to log to your own logging driver or alert service.

Methods can be overridden by passing a console-like object to logger.setMethods

var ShareDB = require('sharedb/lib/client');
ShareDB.logger.setMethods({
  info: () => {},                         // Silence info
  warn: () => alerts.warn(arguments),     // Forward warnings to alerting service
  error: () => alerts.critical(arguments) // Remap errors to critical alerts
});

ShareDB only supports the following logger methods:

• info
• warn
• error

Errors

ShareDB returns errors as an instance of ShareDBError, with a machine-parsable code, as well as more details in the human-readable message.

Common error codes

ERR_OP_SUBMIT_REJECTED

The op submitted by the client has been rejected by the server for a non-critical reason.

When the client receives this code, it will attempt to roll back the rejected op, leaving the client in a usable state.

This error might be used as part of standard control flow. For example, consumers may define a middleware that validates document structure, and rejects operations that do not conform to this schema using this error code to reset the client to a valid state.

ERR_OP_ALREADY_SUBMITTED

The same op has been received by the server twice.

This is non-critical, and part of normal control flow, and is sent as an error in order to short-circuit the op processing. It is eventually swallowed by the server, and shouldn't need further handling.

ERR_SUBMIT_TRANSFORM_OPS_NOT_FOUND

The ops needed to transform the submitted op up to the current version of the snapshot could not be found.

If a client on an old version of a document submits an op, that op needs to be transformed by all the ops that have been applied to the document in the meantime. If the server cannot fetch these ops from the database, then this error is returned.

The most common case of this would be ops being deleted from the database. For example, let's assume we have a TTL set up on the ops in our database. Let's also say we have a client that is so old that the op corresponding to its version has been deleted by the TTL policy. If this client then attempts to submit an op, the server will not be able to find the ops required to transform the op to apply to the current version of the snapshot.

Other causes of this error may be dropping the ops collection all together, or having the database corrupted in some other way.

ERR_MAX_SUBMIT_RETRIES_EXCEEDED

The number of retries defined by the maxSubmitRetries option has been exceeded by a submission.

ERR_DOC_ALREADY_CREATED

The creation request has failed, because the document was already created by another client.

This can happen when two clients happen to simultaneously try to create the same document, and is potentially recoverable by simply fetching the already-created document.

ERR_DOC_WAS_DELETED

The deletion request has failed, because the document was already deleted by another client.

This can happen when two clients happen to simultaneously try to delete the same document. Given that the end result is the same, this error can potentially just be ignored.

ERR_DOC_TYPE_NOT_RECOGNIZED

The specified document type has not been registered with ShareDB.

This error can usually be remedied by remembering to register any types you need:

var ShareDB = require('sharedb');
var richText = require('rich-text');

ShareDB.types.register(richText.type);

ERR_DEFAULT_TYPE_MISMATCH

The default type being used by the client does not match the default type expected by the server.

This will typically only happen when using a different default type to the built-in json0 used by ShareDB by default (eg if using a fork). The exact same type must be used by both the client and the server, and should be registered as the default type:

var ShareDB = require('sharedb');
var forkedJson0 = require('forked-json0');

// Make sure to also do this on your client
ShareDB.types.defaultType = forkedJson0.type;

ERR_OP_NOT_ALLOWED_IN_PROJECTION

The submitted op is not valid when applied to the projection.

This may happen if the op targets some property that is not included in the projection.

ERR_TYPE_CANNOT_BE_PROJECTED

The document's type cannot be projected. json0 is currently the only type that supports projections.