backend.js

var async = require('async');
var Agent = require('./agent');
var Connection = require('./client/connection');
var emitter = require('./emitter');
var MemoryDB = require('./db/memory');
var NoOpMilestoneDB = require('./milestone-db/no-op');
var MemoryPubSub = require('./pubsub/memory');
var ot = require('./ot');
var projections = require('./projections');
var QueryEmitter = require('./query-emitter');
var ShareDBError = require('./error');
var Snapshot = require('./snapshot');
var StreamSocket = require('./stream-socket');
var SubmitRequest = require('./submit-request');
var ReadSnapshotsRequest = require('./read-snapshots-request');
var util = require('./util');
var logger = require('./logger');

var ERROR_CODE = ShareDBError.CODES;

function Backend(options) {
  if (!(this instanceof Backend)) return new Backend(options);
  emitter.EventEmitter.call(this);

  if (!options) options = {};
  this.db = options.db || new MemoryDB();
  this.pubsub = options.pubsub || new MemoryPubSub();
  // This contains any extra databases that can be queried
  this.extraDbs = options.extraDbs || {};
  this.milestoneDb = options.milestoneDb || new NoOpMilestoneDB();

  // Map from projected collection -> {type, fields}
  this.projections = Object.create(null);

  this.suppressPublish = !!options.suppressPublish;
  this.maxSubmitRetries = options.maxSubmitRetries || null;
  this.presenceEnabled = !!options.presence;
  this.doNotForwardSendPresenceErrorsToClient = !!options.doNotForwardSendPresenceErrorsToClient;
  if (this.presenceEnabled && !this.doNotForwardSendPresenceErrorsToClient) {
    logger.warn(
      'Broadcasting "sendPresence" middleware errors to clients is deprecated ' +
      'and will be removed in a future release. Disable this behaviour with:\n\n' +
      'new Backend({doNotForwardSendPresenceErrorsToClient: true})\n\n'
    );
  }
  this.doNotCommitNoOps = !!options.doNotCommitNoOps;

  // Map from event name to a list of middleware
  this.middleware = Object.create(null);

  // The number of open agents for monitoring and testing memory leaks
  this.agentsCount = 0;
  this.remoteAgentsCount = 0;

  this.errorHandler = typeof options.errorHandler === 'function' ?
    options.errorHandler :
    // eslint-disable-next-line no-unused-vars
    function(error, context) {
      logger.error(error);
    };
}
module.exports = Backend;
emitter.mixin(Backend);

Backend.prototype.MIDDLEWARE_ACTIONS = {
  // An operation was successfully written to the database.
  afterWrite: 'afterWrite',
  // An operation is about to be applied to a snapshot before being committed to the database
  apply: 'apply',
  // An operation was applied to a snapshot; The operation and new snapshot are about to be written to the database.
  commit: 'commit',
  // A new client connected to the server.
  connect: 'connect',
  // An operation was loaded from the database
  op: 'op',
  // A query is about to be sent to the database
  query: 'query',
  // Snapshot(s) were received from the database and are about to be returned to a client
  readSnapshots: 'readSnapshots',
  // Received a message from a client
  receive: 'receive',
  // About to send a non-error reply to a client message.
  // WARNING: This gets passed a direct reference to the reply object, so
  // be cautious with it. While modifications to the reply message are possible
  // by design, changing existing reply properties can cause weird bugs, since
  // the rest of ShareDB would be unaware of those changes.
  reply: 'reply',
  // The server received presence information
  receivePresence: 'receivePresence',
  // About to send presence information to a client
  sendPresence: 'sendPresence',
  // An operation is about to be submitted to the database
  submit: 'submit'
};

Backend.prototype.SNAPSHOT_TYPES = {
  // The current snapshot is being fetched (eg through backend.fetch)
  current: 'current',
  // A specific snapshot is being fetched by version (eg through backend.fetchSnapshot)
  byVersion: 'byVersion',
  // A specific snapshot is being fetch by timestamp (eg through backend.fetchSnapshotByTimestamp)
  byTimestamp: 'byTimestamp'
};

Backend.prototype.close = function(callback) {
  var wait = 4;
  var backend = this;
  function finish(err) {
    if (err) {
      if (callback) return callback(err);
      return backend.emit('error', err);
    }
    if (--wait) return;
    if (callback) callback();
  }
  this.pubsub.close(finish);
  this.db.close(finish);
  this.milestoneDb.close(finish);
  for (var name in this.extraDbs) {
    wait++;
    this.extraDbs[name].close(finish);
  }
  finish();
};

Backend.prototype.connect = function(connection, req, callback) {
  var socket = new StreamSocket();
  if (connection) {
    connection.bindToSocket(socket);
  } else {
    connection = new Connection(socket);
  }
  socket._open();
  var agent = this.listen(socket.stream, req);
  // Store a reference to the agent on the connection for convenience. This is
  // not used internal to ShareDB, but it is handy for server-side only user
  // code that may cache state on the agent and read it in middleware
  connection.agent = agent;

  if (typeof callback === 'function') {
    connection.once('connected', function() {
      callback(connection);
    });
  }

  return connection;
};

/** A client has connected through the specified stream. Listen for messages.
 *
 * The optional second argument (req) is an initial request which is passed
 * through to any connect() middleware. This is useful for inspecting cookies
 * or an express session or whatever on the request object in your middleware.
 *
 * (The agent is available through all middleware)
 */
Backend.prototype.listen = function(stream, req) {
  var agent = new Agent(this, stream);
  this.trigger(this.MIDDLEWARE_ACTIONS.connect, agent, {stream: stream, req: req}, function(err) {
    if (err) return agent.close(err);
    agent._open();
  });
  return agent;
};

Backend.prototype.addProjection = function(name, collection, fields) {
  if (this.projections[name]) {
    throw new Error('Projection ' + name + ' already exists');
  }

  for (var key in fields) {
    if (fields[key] !== true) {
      throw new Error('Invalid field ' + key + ' - fields must be {somekey: true}. Subfields not currently supported.');
    }
  }

  this.projections[name] = {
    target: collection,
    fields: fields
  };
};

/**
 * Add middleware to an action or array of actions
 */
Backend.prototype.use = function(action, fn) {
  if (Array.isArray(action)) {
    for (var i = 0; i < action.length; i++) {
      this.use(action[i], fn);
    }
    return this;
  }
  var fns = this.middleware[action] || (this.middleware[action] = []);
  fns.push(fn);
  return this;
};

/**
 * Passes request through the middleware stack
 *
 * Middleware may modify the request object. After all middleware have been
 * invoked we call `callback` with `null` and the modified request. If one of
 * the middleware resturns an error the callback is called with that error.
 */
Backend.prototype.trigger = function(action, agent, request, callback) {
  request.action = action;
  request.agent = agent;
  request.backend = this;

  var fns = this.middleware[action];
  if (!fns) return callback();

  // Copying the triggers we'll fire so they don't get edited while we iterate.
  fns = fns.slice();
  var next = function(err) {
    if (err) return callback(err);
    var fn = fns.shift();
    if (!fn) return callback();
    fn(request, next);
  };
  next();
};

// Submit an operation on the named collection/docname. op should contain a
// {op:}, {create:} or {del:} field. It should probably contain a v: field (if
// it doesn't, it defaults to the current version).
Backend.prototype.submit = function(agent, index, id, op, options, originalCallback) {
  var backend = this;
  var request = new SubmitRequest(this, agent, index, id, op, options);

  var callback = function(error, ops) {
    backend.emit('submitRequestEnd', error, request);
    originalCallback(error, ops, request);
  };

  var err = ot.checkOp(op);
  if (err) return callback(err);
  backend.trigger(backend.MIDDLEWARE_ACTIONS.submit, agent, request, function(err) {
    if (err) return callback(err);
    request.submit(function(err) {
      if (err) return callback(err);
      backend.trigger(backend.MIDDLEWARE_ACTIONS.afterWrite, agent, request, function(err) {
        if (err) return callback(err);
        backend._sanitizeOps(agent, request.projection, request.collection, id, request.ops, function(err) {
          if (err) return callback(err);
          backend.emit('timing', 'submit.total', Date.now() - request.start, request);
          callback(err, request.ops);
        });
      });
    });
  });
};

Backend.prototype.sanitizeOp = function(agent, index, id, op, callback) {
  var projection = this.projections[index];
  var collection = (projection) ? projection.target : index;
  this._sanitizeOp(agent, projection, collection, id, op, callback);
};

Backend.prototype._sanitizeOp = function(agent, projection, collection, id, op, callback) {
  if (projection) {
    try {
      projections.projectOp(projection.fields, op);
    } catch (err) {
      return callback(err);
    }
  }
  this.trigger(this.MIDDLEWARE_ACTIONS.op, agent, {collection: collection, id: id, op: op}, callback);
};
Backend.prototype._sanitizeOps = function(agent, projection, collection, id, ops, callback) {
  var backend = this;
  async.each(ops, function(op, eachCb) {
    backend._sanitizeOp(agent, projection, collection, id, op, function(err) {
      util.nextTick(eachCb, err);
    });
  }, callback);
};
Backend.prototype._sanitizeOpsBulk = function(agent, projection, collection, opsMap, callback) {
  var backend = this;
  async.forEachOf(opsMap, function(ops, id, eachCb) {
    backend._sanitizeOps(agent, projection, collection, id, ops, eachCb);
  }, callback);
};

Backend.prototype._sanitizeSnapshots = function(agent, projection, collection, snapshots, snapshotType, callback) {
  if (projection) {
    try {
      projections.projectSnapshots(projection.fields, snapshots);
    } catch (err) {
      return callback(err);
    }
  }

  var request = new ReadSnapshotsRequest(collection, snapshots, snapshotType);

  this.trigger(this.MIDDLEWARE_ACTIONS.readSnapshots, agent, request, function(err) {
    if (err) return callback(err);
    // Handle "partial rejection" - "readSnapshots" middleware functions can use
    // `request.rejectSnapshotRead(snapshot, error)` to reject the read of a specific snapshot.
    if (request.hasSnapshotRejection()) {
      err = request.getReadSnapshotsError();
    }
    if (err) {
      callback(err);
    } else {
      callback();
    }
  });
};

Backend.prototype._getSnapshotProjection = function(db, projection) {
  return (db.projectsSnapshots) ? null : projection;
};

Backend.prototype._getSnapshotsFromMap = function(ids, snapshotMap) {
  var snapshots = new Array(ids.length);
  for (var i = 0; i < ids.length; i++) {
    var id = ids[i];
    snapshots[i] = snapshotMap[id];
  }
  return snapshots;
};

Backend.prototype._getSanitizedOps = function(agent, projection, collection, id, from, to, opsOptions, callback) {
  var backend = this;
  if (!opsOptions) opsOptions = {};
  if (agent) opsOptions.agentCustom = agent.custom;
  backend.db.getOps(collection, id, from, to, opsOptions, function(err, ops) {
    if (err) return callback(err);
    backend._sanitizeOps(agent, projection, collection, id, ops, function(err) {
      if (err) return callback(err);
      callback(null, ops);
    });
  });
};

Backend.prototype._getSanitizedOpsBulk = function(agent, projection, collection, fromMap, toMap, opsOptions, callback) {
  var backend = this;
  if (!opsOptions) opsOptions = {};
  if (agent) opsOptions.agentCustom = agent.custom;
  backend.db.getOpsBulk(collection, fromMap, toMap, opsOptions, function(err, opsMap) {
    if (err) return callback(err);
    backend._sanitizeOpsBulk(agent, projection, collection, opsMap, function(err) {
      if (err) return callback(err);
      callback(null, opsMap);
    });
  });
};

// Non inclusive - gets ops from [from, to). Ie, all relevant ops. If to is
// not defined (null or undefined) then it returns all ops.
Backend.prototype.getOps = function(agent, index, id, from, to, options, callback) {
  if (typeof options === 'function') {
    callback = options;
    options = null;
  }
  var start = Date.now();
  var projection = this.projections[index];
  var collection = (projection) ? projection.target : index;
  var backend = this;
  var request = {
    agent: agent,
    index: index,
    collection: collection,
    id: id,
    from: from,
    to: to
  };
  var opsOptions = options && options.opsOptions;
  backend._getSanitizedOps(agent, projection, collection, id, from, to, opsOptions, function(err, ops) {
    if (err) return callback(err);
    backend.emit('timing', 'getOps', Date.now() - start, request);
    callback(null, ops);
  });
};

Backend.prototype.getOpsBulk = function(agent, index, fromMap, toMap, options, callback) {
  if (typeof options === 'function') {
    callback = options;
    options = null;
  }
  var start = Date.now();
  var projection = this.projections[index];
  var collection = (projection) ? projection.target : index;
  var backend = this;
  var request = {
    agent: agent,
    index: index,
    collection: collection,
    fromMap: fromMap,
    toMap: toMap
  };
  var opsOptions = options && options.opsOptions;
  backend._getSanitizedOpsBulk(agent, projection, collection, fromMap, toMap, opsOptions, function(err, opsMap) {
    if (err) return callback(err);
    backend.emit('timing', 'getOpsBulk', Date.now() - start, request);
    callback(null, opsMap);
  });
};

Backend.prototype.fetch = function(agent, index, id, options, callback) {
  if (typeof options === 'function') {
    callback = options;
    options = null;
  }
  var start = Date.now();
  var projection = this.projections[index];
  var collection = (projection) ? projection.target : index;
  var fields = projection && projection.fields;
  var backend = this;
  var request = {
    agent: agent,
    index: index,
    collection: collection,
    id: id
  };
  var snapshotOptions = (options && options.snapshotOptions) || {};
  snapshotOptions.agentCustom = agent.custom;
  backend.db.getSnapshot(collection, id, fields, snapshotOptions, function(err, snapshot) {
    if (err) return callback(err);
    var snapshotProjection = backend._getSnapshotProjection(backend.db, projection);
    var snapshots = [snapshot];
    backend._sanitizeSnapshots(
      agent,
      snapshotProjection,
      collection,
      snapshots,
      backend.SNAPSHOT_TYPES.current,
      function(err) {
        if (err) return callback(err);
        backend.emit('timing', 'fetch', Date.now() - start, request);
        callback(null, snapshot);
      });
  });
};

/**
 * Map of document id to Snapshot or error object.
 * @typedef {{ [id: string]: Snapshot | { error: Error | string } }} SnapshotMap
 */

/**
 * @param {Agent} agent
 * @param {string} index
 * @param {string[]} ids
 * @param {*} options
 * @param {(err?: Error | string, snapshotMap?: SnapshotMap) => void} callback
 */
Backend.prototype.fetchBulk = function(agent, index, ids, options, callback) {
  if (typeof options === 'function') {
    callback = options;
    options = null;
  }
  var start = Date.now();
  var projection = this.projections[index];
  var collection = (projection) ? projection.target : index;
  var fields = projection && projection.fields;
  var backend = this;
  var request = {
    agent: agent,
    index: index,
    collection: collection,
    ids: ids
  };
  var snapshotOptions = (options && options.snapshotOptions) || {};
  snapshotOptions.agentCustom = agent.custom;
  backend.db.getSnapshotBulk(collection, ids, fields, snapshotOptions, function(err, snapshotMap) {
    if (err) return callback(err);
    var snapshotProjection = backend._getSnapshotProjection(backend.db, projection);
    var snapshots = backend._getSnapshotsFromMap(ids, snapshotMap);
    backend._sanitizeSnapshots(
      agent,
      snapshotProjection,
      collection,
      snapshots,
      backend.SNAPSHOT_TYPES.current,
      function(err) {
        if (err) {
          if (err.code === ERROR_CODE.ERR_SNAPSHOT_READS_REJECTED) {
            for (var docId in err.idToError) {
              snapshotMap[docId] = {error: err.idToError[docId]};
            }
            err = undefined;
          } else {
            snapshotMap = undefined;
          }
        }
        backend.emit('timing', 'fetchBulk', Date.now() - start, request);
        callback(err, snapshotMap);
      });
  });
};

// Subscribe to the document from the specified version or null version
Backend.prototype.subscribe = function(agent, index, id, version, options, callback) {
  if (typeof options === 'function') {
    callback = options;
    options = null;
  }
  if (options) {
    // We haven't yet implemented the ability to pass options to subscribe. This is because we need to
    // add the ability to SubmitRequest.commit to optionally pass the metadata to other clients on
    // PubSub. This behaviour is not needed right now, but we have added an options object to the
    // subscribe() signature so that it remains consistent with getOps() and fetch().
    return callback(new ShareDBError(
      ERROR_CODE.ERR_DATABASE_METHOD_NOT_IMPLEMENTED,
      'Passing options to subscribe has not been implemented'
    ));
  }
  var start = Date.now();
  var projection = this.projections[index];
  var collection = (projection) ? projection.target : index;
  var channel = this.getDocChannel(collection, id);
  var backend = this;
  var request = {
    agent: agent,
    index: index,
    collection: collection,
    id: id,
    version: version
  };
  backend.pubsub.subscribe(channel, function(err, stream) {
    if (err) return callback(err);
    if (version == null) {
      // Subscribing from null means that the agent doesn't have a document
      // and needs to fetch it as well as subscribing
      backend.fetch(agent, index, id, function(err, snapshot) {
        if (err) {
          stream.destroy();
          return callback(err);
        }
        backend.emit('timing', 'subscribe.snapshot', Date.now() - start, request);
        callback(null, stream, snapshot);
      });
    } else {
      backend._getSanitizedOps(agent, projection, collection, id, version, null, null, function(err, ops) {
        if (err) {
          stream.destroy();
          return callback(err);
        }
        backend.emit('timing', 'subscribe.ops', Date.now() - start, request);
        callback(null, stream, null, ops);
      });
    }
  });
};

/**
 * Map of document id to pubsub stream.
 * @typedef {{ [id: string]: Stream }} StreamMap
 */
/**
 * Map of document id to array of ops for the doc.
 * @typedef {{ [id: string]: Op[] }} OpsMap
 */

/**
 * @param {Agent} agent
 * @param {string} index
 * @param {string[]} versions
 * @param {(
 *   err?: Error | string | null,
 *   streams?: StreamMap,
 *   snapshotMap?: SnapshotMap | null
 *   opsMap?: OpsMap
 * ) => void} callback
 */
Backend.prototype.subscribeBulk = function(agent, index, versions, callback) {
  var start = Date.now();
  var projection = this.projections[index];
  var collection = (projection) ? projection.target : index;
  var backend = this;
  var streams = Object.create(null);
  var doFetch = Array.isArray(versions);
  var ids = (doFetch) ? versions : Object.keys(versions);
  var request = {
    agent: agent,
    index: index,
    collection: collection,
    versions: versions
  };
  async.each(ids, function(id, eachCb) {
    var channel = backend.getDocChannel(collection, id);
    backend.pubsub.subscribe(channel, function(err, stream) {
      if (err) return eachCb(err);
      streams[id] = stream;
      eachCb();
    });
  }, function(err) {
    if (err) {
      destroyStreams(streams);
      return callback(err);
    }
    if (doFetch) {
      // If an array of ids, get current snapshots
      backend.fetchBulk(agent, index, ids, function(err, snapshotMap) {
        if (err) {
          // Full error, destroy all streams.
          destroyStreams(streams);
          streams = undefined;
          snapshotMap = undefined;
        }
        for (var docId in snapshotMap) {
          // The doc id could map to an object `{error: Error | string}`, which indicates that
          // particular snapshot's read was rejected. Destroy the streams fur such docs.
          if (snapshotMap[docId].error) {
            streams[docId].destroy();
            delete streams[docId];
          }
        }
        backend.emit('timing', 'subscribeBulk.snapshot', Date.now() - start, request);
        callback(err, streams, snapshotMap);
      });
    } else {
      // If a versions map, get ops since requested versions
      backend._getSanitizedOpsBulk(agent, projection, collection, versions, null, null, function(err, opsMap) {
        if (err) {
          destroyStreams(streams);
          return callback(err);
        }
        backend.emit('timing', 'subscribeBulk.ops', Date.now() - start, request);
        callback(null, streams, null, opsMap);
      });
    }
  });
};
function destroyStreams(streams) {
  for (var id in streams) {
    streams[id].destroy();
  }
}

Backend.prototype.queryFetch = function(agent, index, query, options, callback) {
  var start = Date.now();
  var backend = this;
  backend._triggerQuery(agent, index, query, options, function(err, request) {
    if (err) return callback(err);
    backend._query(agent, request, function(err, snapshots, extra) {
      if (err) return callback(err);
      backend.emit('timing', 'queryFetch', Date.now() - start, request);
      callback(null, snapshots, extra);
    });
  });
};

// Options can contain:
// db: The name of the DB (if the DB is specified in the otherDbs when the backend instance is created)
// skipPoll: function(collection, id, op, query) {return true or false; }
//  this is a synchronous function which can be used as an early filter for
//  operations going through the system to reduce the load on the DB.
// pollDebounce: Minimum delay between subsequent database polls. This is
//  used to batch updates to reduce load on the database at the expense of
//  liveness
Backend.prototype.querySubscribe = function(agent, index, query, options, callback) {
  var start = Date.now();
  var backend = this;
  backend._triggerQuery(agent, index, query, options, function(err, request) {
    if (err) return callback(err);
    if (request.db.disableSubscribe) {
      return callback(new ShareDBError(
        ERROR_CODE.ERR_DATABASE_DOES_NOT_SUPPORT_SUBSCRIBE,
        'DB does not support subscribe'
      ));
    }

    var channels = request.channels;

    if (request.channel) {
      logger.warn(
        '[DEPRECATED] "query" middleware\'s context.channel is deprecated, use context.channels instead. ' +
        'Read more: https://share.github.io/sharedb/middleware/actions#query'
      );
      channels = [request.channel];
    }

    if (!channels || !channels.length) {
      return callback(new ShareDBError(ERROR_CODE.ERR_QUERY_CHANNEL_MISSING, 'Required minimum one query channel.'));
    }

    var streams = [];

    function destroyStreams() {
      streams.forEach(function(stream) {
        stream.destroy();
      });
    }

    function createQueryEmitter() {
      if (options.ids) {
        var queryEmitter = new QueryEmitter(request, streams, options.ids);
        backend.emit('timing', 'querySubscribe.reconnect', Date.now() - start, request);
        callback(null, queryEmitter);
        return;
      }
      // Issue query on db to get our initial results
      backend._query(agent, request, function(err, snapshots, extra) {
        if (err) {
          destroyStreams();
          return callback(err);
        }
        var ids = pluckIds(snapshots);
        var queryEmitter = new QueryEmitter(request, streams, ids, extra);
        backend.emit('timing', 'querySubscribe.initial', Date.now() - start, request);
        callback(null, queryEmitter, snapshots, extra);
      });
    }

    channels.forEach(function(channel) {
      backend.pubsub.subscribe(channel, function(err, stream) {
        if (err) {
          destroyStreams();
          return callback(err);
        }
        streams.push(stream);

        var subscribedToAllChannels = streams.length === channels.length;
        if (subscribedToAllChannels) {
          createQueryEmitter();
        }
      });
    });
  });
};

Backend.prototype._triggerQuery = function(agent, index, query, options, callback) {
  var projection = this.projections[index];
  var collection = (projection) ? projection.target : index;
  var fields = projection && projection.fields;
  var request = {
    index: index,
    collection: collection,
    projection: projection,
    fields: fields,
    channels: [this.getCollectionChannel(collection)],
    query: query,
    options: options,
    db: null,
    snapshotProjection: null
  };
  var backend = this;
  backend.trigger(backend.MIDDLEWARE_ACTIONS.query, agent, request, function(err) {
    if (err) return callback(err);
    // Set the DB reference for the request after the middleware trigger so
    // that the db option can be changed in middleware
    request.db = (options.db) ? backend.extraDbs[options.db] : backend.db;
    if (!request.db) return callback(new ShareDBError(ERROR_CODE.ERR_DATABASE_ADAPTER_NOT_FOUND, 'DB not found'));
    request.snapshotProjection = backend._getSnapshotProjection(request.db, projection);
    callback(null, request);
  });
};

Backend.prototype._query = function(agent, request, callback) {
  var backend = this;
  request.db.query(request.collection, request.query, request.fields, request.options, function(err, snapshots, extra) {
    if (err) return callback(err);
    backend._sanitizeSnapshots(
      agent,
      request.snapshotProjection,
      request.collection,
      snapshots,
      backend.SNAPSHOT_TYPES.current,
      function(err) {
        callback(err, snapshots, extra);
      });
  });
};

Backend.prototype.getCollectionChannel = function(collection) {
  return collection;
};

Backend.prototype.getDocChannel = function(collection, id) {
  return collection + '.' + id;
};

Backend.prototype.getChannels = function(collection, id) {
  return [
    this.getCollectionChannel(collection),
    this.getDocChannel(collection, id)
  ];
};

Backend.prototype.fetchSnapshot = function(agent, index, id, version, callback) {
  var start = Date.now();
  var backend = this;
  var projection = this.projections[index];
  var collection = projection ? projection.target : index;
  var request = {
    agent: agent,
    index: index,
    collection: collection,
    id: id,
    version: version
  };

  this._fetchSnapshot(collection, id, version, function(error, snapshot) {
    if (error) return callback(error);
    var snapshotProjection = backend._getSnapshotProjection(backend.db, projection);
    var snapshots = [snapshot];
    var snapshotType = backend.SNAPSHOT_TYPES.byVersion;
    backend._sanitizeSnapshots(agent, snapshotProjection, collection, snapshots, snapshotType, function(error) {
      if (error) return callback(error);
      backend.emit('timing', 'fetchSnapshot', Date.now() - start, request);
      callback(null, snapshot);
    });
  });
};

Backend.prototype._fetchSnapshot = function(collection, id, version, callback) {
  var db = this.db;
  var backend = this;

  var shouldGetLatestSnapshot = version === null;
  if (shouldGetLatestSnapshot) {
    return backend.db.getSnapshot(collection, id, null, null, function(error, snapshot) {
      if (error) return callback(error);

      callback(null, snapshot);
    });
  }


  this.milestoneDb.getMilestoneSnapshot(collection, id, version, function(error, milestoneSnapshot) {
    if (error) return callback(error);

    // Bypass backend.getOps so that we don't call _sanitizeOps. We want to avoid this, because:
    // - we want to avoid the 'op' middleware, because we later use the 'readSnapshots' middleware in _sanitizeSnapshots
    // - we handle the projection in _sanitizeSnapshots
    var from = milestoneSnapshot ? milestoneSnapshot.v : 0;
    db.getOps(collection, id, from, version, null, function(error, ops) {
      if (error) return callback(error);

      backend._buildSnapshotFromOps(id, milestoneSnapshot, ops, function(error, snapshot) {
        if (error) return callback(error);

        if (version > snapshot.v) {
          return callback(new ShareDBError(
            ERROR_CODE.ERR_OP_VERSION_NEWER_THAN_CURRENT_SNAPSHOT,
            'Requested version exceeds latest snapshot version'
          ));
        }

        callback(null, snapshot);
      });
    });
  });
};

Backend.prototype.fetchSnapshotByTimestamp = function(agent, index, id, timestamp, callback) {
  var start = Date.now();
  var backend = this;
  var projection = this.projections[index];
  var collection = projection ? projection.target : index;
  var request = {
    agent: agent,
    index: index,
    collection: collection,
    id: id,
    timestamp: timestamp
  };

  this._fetchSnapshotByTimestamp(collection, id, timestamp, function(error, snapshot) {
    if (error) return callback(error);
    var snapshotProjection = backend._getSnapshotProjection(backend.db, projection);
    var snapshots = [snapshot];
    var snapshotType = backend.SNAPSHOT_TYPES.byTimestamp;
    backend._sanitizeSnapshots(agent, snapshotProjection, collection, snapshots, snapshotType, function(error) {
      if (error) return callback(error);
      backend.emit('timing', 'fetchSnapshot', Date.now() - start, request);
      callback(null, snapshot);
    });
  });
};

Backend.prototype._fetchSnapshotByTimestamp = function(collection, id, timestamp, callback) {
  var db = this.db;
  var milestoneDb = this.milestoneDb;
  var backend = this;

  var milestoneSnapshot;
  var from = 0;
  var to = null;

  var shouldGetLatestSnapshot = timestamp === null;
  if (shouldGetLatestSnapshot) {
    return backend.db.getSnapshot(collection, id, null, null, function(error, snapshot) {
      if (error) return callback(error);

      callback(null, snapshot);
    });
  }

  milestoneDb.getMilestoneSnapshotAtOrBeforeTime(collection, id, timestamp, function(error, snapshot) {
    if (error) return callback(error);
    milestoneSnapshot = snapshot;
    if (snapshot) from = snapshot.v;

    milestoneDb.getMilestoneSnapshotAtOrAfterTime(collection, id, timestamp, function(error, snapshot) {
      if (error) return callback(error);
      if (snapshot) to = snapshot.v;

      var options = {metadata: true};
      db.getOps(collection, id, from, to, options, function(error, ops) {
        if (error) return callback(error);
        filterOpsInPlaceBeforeTimestamp(ops, timestamp);
        backend._buildSnapshotFromOps(id, milestoneSnapshot, ops, callback);
      });
    });
  });
};

Backend.prototype._buildSnapshotFromOps = function(id, startingSnapshot, ops, callback) {
  var snapshot = util.clone(startingSnapshot) || new Snapshot(id, 0, null, undefined, null);
  var error = ot.applyOps(snapshot, ops, {_normalizeLegacyJson0Ops: true});
  callback(error, snapshot);
};

Backend.prototype.transformPresenceToLatestVersion = function(agent, presence, callback) {
  if (!presence.c || !presence.d) return callback(null, presence);
  this.getOps(agent, presence.c, presence.d, presence.v, null, function(error, ops) {
    if (error) return callback(error);
    for (var i = 0; i < ops.length; i++) {
      var op = ops[i];
      var isOwnOp = op.src === presence.src;
      var transformError = ot.transformPresence(presence, op, isOwnOp);
      if (transformError) {
        return callback(transformError);
      }
    }
    callback(null, presence);
  });
};

function pluckIds(snapshots) {
  var ids = [];
  for (var i = 0; i < snapshots.length; i++) {
    ids.push(snapshots[i].id);
  }
  return ids;
}

function filterOpsInPlaceBeforeTimestamp(ops, timestamp) {
  for (var i = 0; i < ops.length; i++) {
    var op = ops[i];
    var opTimestamp = op.m && op.m.ts;
    if (opTimestamp > timestamp) {
      ops.length = i;
      return;
    }
  }
}