ShareDB

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

ShareDB lets submit operations (edit documents) and subscribe to documents. Subscribing gives you a stream of all operations applied to the given document, as they happen. You can also make live bound queries, which give you the results of your query and a feed of changes to the result set over time.

To use it, you need a database to actually store your data in. A database wrapper for mongo is available in share/sharedb-mongo. I hope to add more over time.

If you want to mess about, sharedb also has an in-memory database backend you can use. The in-memory database stores all documents and operations in memory forever (or at least, until you restart your server - at which point all documents and operations are lost.)

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

Please report any bugs you find to the issue tracker.

Quick tour

var sharedb = require('sharedb');
var db = require('sharedb-mongo')('localhost:27017/test?auto_reconnect', {safe:true});

backend = sharedb.client(db);

backend.fetchAndSubscribe('users', 'fred', function(err, data, stream) {
  // Data is simply {v:0} because the fred document doesn't exist yet.

  stream.on('data', function(op) {
    // We'll see all changes to the fred document as they happen
    console.log('Fred was edited by the operation', op);
  });
});


// This could happen from a different process / server but only if you use the
// redis driver. (Otherwise they won't see each other's changes and everything
// breaks)
backend.submit('users', 'fred', {v:0, create:{type:'json0', data:{name:'Fred'}}}, function(err) {
  // Created with data {name:'Fred'}

  // Other concurrent edits can happen too, and they'll all be merged using OT.
  // This operations says at doc['name'][4], insert characters ' Flintstone'.
  backend.submit('users', 'fred', {v:1, op:[{p:['name', 4], si:' Flintstone'}]}, function(err) {
    // users.fred now has data {name:'Fred Flintstone'}
  });
});

Data Model

In LiveDB'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. Types are referenced using their URIs (even though those URIs don't actually mean anything). 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).

LiveDB 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.

Using ShareDB

ShareDB requires 3 puzzle pieces to operate:

• A snapshot database, to store actual documents.
• An oplog to store historical operations. We currently require that operations are stored forever, but I want to change this before 1.0. (It might work today, but we're missing tests).
• A sharedb driver. If you have multiple servers, the driver manages communication between them all. The driver also makes commits atomic (so servers won't clobber each other's changes) and publishes operations. If you only have one frontend server, you can use the inprocess driver. (This is the default if you do not specify a driver.)

You can put operations and snapshot data in different places if you want, but its easier to put all of your data in the same database.

The backend database(s) needs to implement a simple API which has documentation and a sample implementation here. Currently the only database binding is sharedb-mongo.

A sharedb client is created using either an options object or a database backend. If you specify a database backend, its used as both oplog and snapshot.

db = require('sharedb-mongo')('localhost:27017/test?auto_reconnect', {safe:true});
backend = sharedb.client(db);

This is the equivalent to this:

db = require('sharedb-mongo')('localhost:27017/test?auto_reconnect', {safe:true});
backend = sharedb.client({db:db});
// Also equivalent to sharedb.client({db:db, oplog:db});

You can use a different database for both snapshots and operations if you want:

db = require('sharedb-mongo')('localhost:27017/test?auto_reconnect', {safe:true});
oplog = {writeOp:..., getVersion:..., getOps:...};
backend = sharedb.client({db:db, oplog:oplog});

All of the above examples will use the in-process driver by default. If you want to scale across multiple frontend servers, you should use the redis driver:

var redis = require('redis');
client1 = redis.createClient(6379, '192.168.1.123', auth_pass:'secret');
client2 = redis.createClient(6379, '192.168.1.123', auth_pass:'secret');

driver = sharedb.redisDriver(oplog, client1, client2);
backend = sharedb.client({db:db, driver:driver});

The redis driver needs 2 redis clients because redis can't use the same connection for commands and pubsub. See node-redis documentation for help configuring redis.

The options object can also be passed:

• extraDbs: {name:query db} This is used to register extra database backends which will be notified whenever operations are submitted. They can also be used in queries.

Creating documents

To create a document, you need to submit a create operation to the document to set its type. In sharedb's world, a document doesn't exist until it has a type set.

A create operation looks like this: {create:{type:TYPE, [data:INITIAL DATA]}, [v:VERSION]}. The type should be something accessible in the map returned by require('ottypes'), for example json0 or http://sharejs.org/types/textv1. Specifying initial data is optional. If provided, it is passed to the type's create() method. This does what you expect - for JSON documents, pass your initial object here. For text documents, pass a string containing the document's contents. As with all operations, the version is optional. You probably don't want to specify the version for a create message.

To submit any changes to documents, you use sharedb.submit(cName, docName, opData, callback).

For example:

sharedb.submit('users', 'fred', {create:{type:'json0', data:[1,2,3]}}, function(err, version, transformedByOps, snapshot) {
  // I made a document, ma!
});

Since documents implicitly exist with no type at version 0, usually the create message will increment the version from 0 to 1. Not all documents you want to delete have a version of 0 - if a document is deleted, it will retain its version.

Deleting documents

Deleting documents is similar to creating them. A deleted document has no type and no data, but will retain its version (actually, the delete operation will bump the document's version). A delete operation looks like this: {del:true, [v:VERSION]}.

You use the same submit function as above to delete documents:

sharedb.submit('users', 'fred', {del:true}, function(err) {
  //goneskies! Kapow!
});

Editing documents

You edit a document by submitting an operation. Operations are OT type-specific JSON blobs. Refer to the documentation on the particular OT type for details. For example, text documents are documented here. If we had a text document stored in LiveDB and wanted to edit it, it might look like this:

sharedb.submit('love letters', 'dear fred', {op:[6, "You never return my calls!"], v:1002}, function(err) {
  // ...
});

You should always specify the version when submitting edit operations. The version is technically optional - if its missing, your operation will be submitted against the most recent version of the document in the server. This is useful for creating a document which may already exist, but for normal edits you should always specify the expected current version of the document.

Getting a document

You can fetch the most recent version of a document using sharedb.fetch(cName, docName, callback) or sharedb.bulkFetch(request, callback). This will fetch the document(s) from the snapshot database and fetch all operations which may or may not have been committed.

Fetch returns a snapshot data object via its callback. The snapshot data object has the following fields:

• v: version. This is an integer (starting at 0) containing the version of the document
• type: Document type, if set. This field is missing if the document does not exist.
• data: The document's actual data. For JSON documents this is a JSON tree. For text documents this is a string. This field is missing if the document does not exist.

sharedb.fetch('users', 'fred', function(err, snapshot) {
  // snapshot has {v:123, type:'...', data:{name:'Fred Flintstone'}}
  // If the document doesn't exist, only the v:version field will exist in the data.
});

If you need to get many documents, its more efficient to issue bulk fetches. To pass the set of requested documents to bulkFetch, you need to make a request object which maps collection names to lists of documents you want in that collection. For example, to get 'red', 'green' and 'blue' from the colors collection, you would make a bulkFetch request of {colors:['red', 'green', 'blue']}.

The response maps each collection name to a set of snapshots. Each set of snapshots maps document names to snapshot data objects. Continuing our colors example above, the response could be {colors:{red:{v:0}, green:{v:10, type:..., data:"emerald"}, blue:{v:1, type:..., data:{favorite:true}}}}.

For example:

sharedb.bulkFetch({users:['fred', 'wilma', 'homer'], admins:['zerocool']}, function(err, results) {
  // results will be {users:{fred:..., wilma:..., homer:...}, admins:{zerocool:...}}.
  // Each document has v and optional type and data fields like fetch (above).
});

Getting historic changes to a document

You can get old versions of a document (for playback or catching up a client) using sharedb.getOps(cName, docName, from, to, callback). This will return all operations which have been applied to the named document in the requested range. The range is open, so getOps('users', 'fred', 0, 3, ..) will return all operations up to (but not including) 3. (Ie, operations 0, 1 and 2).

If you set the to field to null, getOps will get all operations up to the current version.

ShareDB documents always start at version 0, so you can get a document's entire history using getOps('users', fred', 0, null, callback);.

If you set to to a version in the future, behaviour is not defined.

Example usage:

sharedb.submit('users', 'fred', {create:{type:'json0', data:{name:'Fred'}}}, function(err) {
  sharedb.submit('users', 'fred', {v:1, op:[{p:['name', 4], si:' Flintstone'}]}, function(err) {
    // ...
  });
});

// ---- Sometime later...

sharedb.getOps('users', 'fred', 0, null, function(err, ops) {
  // ops contains the two operations which were submitted above:
  // [{v:0, create:{...}, {v:1, op:[...]}]
});

Streaming changes to a document in realtime

You can subscribe to changes from a document using sharedb.subscribe(cName, docName, v, callback) or sharedb.bulkSubscribe(request, callback). When you subscribe, you get an operation stream which gets packed with operations as they happen.

When you subscribe to a document, you need to specify which version you're subscribing to the document from. The version cannot be in the future.

The stream will be populated with each operation from the requested version onwards (to infinity and beyond). Each operation will appear in the stream exactly once. If you subscribe and request an old document version, all operations from that version to the current version will be buffered in the stream before the stream is returned to the callback.

You usually want to call subscribe after fetching a document. Pass the document version that you got from calling fetch into your call to subscribe.

For example:

sharedb.fetch('users', 'fred', function(err, data) {
  if (err) { ... }
  var version = data.v;

  // ... Any amount of time later (literally).
  sharedb.subscribe('users', 'fred', version, function(err, stream) {
    if (err) { ... }

    // stream is a nodejs ReadableStream with all operations that happen to
    // users.fred.

    stream.on('data', function(opData) {
      // The opData is a JSON object, the same object you can pass to submit().
      // It always has a v: field.

      // ShareDB exports a helper function to apply the operation to some
      // snapshot data:
      var err = ldb.ot.apply(data, opData);
      if (err) { ... }
    });
  });
});

Important! To avoid leaking memory, when you're done with a stream call stream.destroy() to clean it up.

There is a helper method which will both fetch and subscribe for you (cleverly called fetchAndSubscribe(cName, docName, callback)). It is defined like this:

ShareDB.prototype.fetchAndSubscribe = function(cName, docName, callback) {
  var self = this;
  this.fetch(cName, docName, function(err, data) {
    if (err) return callback(err);
    self.subscribe(cName, docName, data.v, function(err, stream) {
      callback(err, data, stream);
    });
  });
};

It calls your callback with (err, snapshot, stream), giving you both the current document snapshot and the stream of operations from the current version.

Bulk Subscribe

If you want to subscribe to multiple documents at once, you should call bulkSubscribe(request, callback). The bulk subscribe request is a map from cName -> map from docName -> version. For example, {colors: {red:5, blue:6, green:0}}. The response is a map from cName -> map from docName -> stream. For example, {colors: {red:<stream>, blue:<stream>, green:<stream>}}. bulkSubscribe will either return a stream for all requested objects or (if there was an error), none of them.

Again, remember to call stream.destroy() on all streams returned by bulk subscribe when you're done with them.

Queries

ShareDB supports running live queries against the database. It can re-run queries when it suspects that a query's results might have changed - and notify the caller with any changes to the result set.

This is incredibly inefficient and I want to completely rewrite / rework them. For now, I recommend against using live bound queries in a production app with a decent load. I'll document them when I'm happier with them.

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. You can use this to drop server & db load dramatically and speed up page times. Its similar to SQL VIEWs. For now, this only works on JSON documents. (I don't know what it would look like for text documents).

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:

sharedb.addProjection('users_limited', 'users', 'json0', {name:true, profileUrl:true});

However, be aware that on its own this is not sufficient for access control. If users are still allowed to make arbitrary mongo queries against the projected collection, they can find out any data in the hidden fields.

Configure a projection by calling addProjection(projCName, realCName, type, fields).

• projCName: The projected collection name. (Eg, users_limited)
• realCName: The underlying collection name
• type: The OT type. Only JSON0 is supported for now.
• fields: A map of the allowed fields in documents. The keys in this map represent the field names, and the values should be true.

Limitations:

• You can only whitelist fields (not blacklist them).
• The third parameter must be 'json0'.
• Projections can only limit / allow fields at the top level of the document

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 --

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