doc.js

var emitter = require('../emitter');
var logger = require('../logger');
var ShareDBError = require('../error');
var types = require('../types');
var util = require('../util');
var clone = util.clone;
var deepEqual = require('fast-deep-equal');
var ACTIONS = require('../message-actions').ACTIONS;

var ERROR_CODE = ShareDBError.CODES;

/**
 * A Doc is a client's view on a sharejs document.
 *
 * It is is uniquely identified by its `id` and `collection`.  Documents
 * should not be created directly. Create them with connection.get()
 *
 *
 * Subscriptions
 * -------------
 *
 * We can subscribe a document to stay in sync with the server.
 *   doc.subscribe(function(error) {
 *     doc.subscribed // = true
 *   })
 * The server now sends us all changes concerning this document and these are
 * applied to our data. If the subscription was successful the initial
 * data and version sent by the server are loaded into the document.
 *
 * To stop listening to the changes we call `doc.unsubscribe()`.
 *
 * If we just want to load the data but not stay up-to-date, we call
 *   doc.fetch(function(error) {
 *     doc.data // sent by server
 *   })
 *
 *
 * Events
 * ------
 *
 * You can use doc.on(eventName, callback) to subscribe to the following events:
 * - `before op (op, source)` Fired before a partial operation is applied to the data.
 *   It may be used to read the old data just before applying an operation
 * - `op (op, source)` Fired after every partial operation with this operation as the
 *   first argument
 * - `create (source)` The document was created. That means its type was
 *   set and it has some initial data.
 * - `del (data, source)` Fired after the document is deleted, that is
 *   the data is null. It is passed the data before deletion as an
 *   argument
 * - `load ()` Fired when a new snapshot is ingested from a fetch, subscribe, or query
 */

module.exports = Doc;
function Doc(connection, collection, id) {
  emitter.EventEmitter.call(this);

  this.connection = connection;

  this.collection = collection;
  this.id = id;

  this.version = null;
  // The OT type of this document. An uncreated document has type `null`
  this.type = null;
  this.data = undefined;

  // Array of callbacks or nulls as placeholders
  this.inflightFetch = [];
  this.inflightSubscribe = null;
  this.pendingFetch = [];
  this.pendingSubscribe = [];

  this._isInHardRollback = false;

  // Whether we think we are subscribed on the server. Synchronously set to
  // false on calls to unsubscribe and disconnect. Should never be true when
  // this.wantSubscribe is false
  this.subscribed = false;
  // Whether to re-establish the subscription on reconnect
  this.wantSubscribe = false;

  this._wantsDestroy = false;

  // The op that is currently roundtripping to the server, or null.
  //
  // When the connection reconnects, the inflight op is resubmitted.
  //
  // This has the same format as an entry in pendingOps
  this.inflightOp = null;

  // All ops that are waiting for the server to acknowledge this.inflightOp
  // This used to just be a single operation, but creates & deletes can't be
  // composed with regular operations.
  //
  // This is a list of {[create:{...}], [del:true], [op:...], callbacks:[...]}
  this.pendingOps = [];

  // The applyStack enables us to track any ops submitted while we are
  // applying an op incrementally. This value is an array when we are
  // performing an incremental apply and null otherwise. When it is an array,
  // all submitted ops should be pushed onto it. The `_otApply` method will
  // reset it back to null when all incremental apply loops are complete.
  this.applyStack = null;

  // Disable the default behavior of composing submitted ops. This is read at
  // the time of op submit, so it may be toggled on before submitting a
  // specifc op and toggled off afterward
  this.preventCompose = false;

  // If set to true, the source will be submitted over the connection. This
  // will also have the side-effect of only composing ops whose sources are
  // equal
  this.submitSource = false;

  // Prevent own ops being submitted to the server. If subscribed, remote
  // ops are still received. Should be toggled through the pause() and
  // resume() methods to correctly flush on resume.
  this.paused = false;

  // Internal counter that gets incremented every time doc.data is updated.
  // Used as a cheap way to check if doc.data has changed.
  this._dataStateVersion = 0;
}
emitter.mixin(Doc);

Doc.prototype.destroy = function(callback) {
  this._wantsDestroy = true;
  var doc = this;
  doc.whenNothingPending(function() {
    if (doc.wantSubscribe) {
      doc.unsubscribe(function(err) {
        if (err) {
          if (callback) return callback(err);
          return doc.emit('error', err);
        }
        doc.connection._destroyDoc(doc);
        if (callback) callback();
      });
    } else {
      doc.connection._destroyDoc(doc);
      if (callback) callback();
    }
  });
};


// ****** Manipulating the document data, version and type.

// Set the document's type, and associated properties. Most of the logic in
// this function exists to update the document based on any added & removed API
// methods.
//
// @param newType OT type provided by the ottypes library or its name or uri
Doc.prototype._setType = function(newType) {
  if (typeof newType === 'string') {
    newType = types.map[newType];
  }

  if (newType) {
    this.type = newType;
  } else if (newType === null) {
    this.type = newType;
    // If we removed the type from the object, also remove its data
    this._setData(undefined);
  } else {
    var err = new ShareDBError(ERROR_CODE.ERR_DOC_TYPE_NOT_RECOGNIZED, 'Missing type ' + newType);
    return this.emit('error', err);
  }
};

Doc.prototype._setData = function(data) {
  this.data = data;
  this._dataStateVersion++;
};

// Ingest snapshot data. This data must include a version, snapshot and type.
// This is used both to ingest data that was exported with a webpage and data
// that was received from the server during a fetch.
//
// @param snapshot.v    version
// @param snapshot.data
// @param snapshot.type
// @param callback
Doc.prototype.ingestSnapshot = function(snapshot, callback) {
  if (!snapshot) return callback && callback();

  if (typeof snapshot.v !== 'number') {
    var err = new ShareDBError(
      ERROR_CODE.ERR_INGESTED_SNAPSHOT_HAS_NO_VERSION,
      'Missing version in ingested snapshot. ' + this.collection + '.' + this.id
    );
    if (callback) return callback(err);
    return this.emit('error', err);
  }

  // If the doc is already created or there are ops pending, we cannot use the
  // ingested snapshot and need ops in order to update the document
  if (this.type || this.hasWritePending()) {
    // The version should only be null on a created document when it was
    // created locally without fetching
    if (this.version == null) {
      if (this.hasWritePending()) {
        // If we have pending ops and we get a snapshot for a locally created
        // document, we have to wait for the pending ops to complete, because
        // we don't know what version to fetch ops from. It is possible that
        // the snapshot came from our local op, but it is also possible that
        // the doc was created remotely (which would conflict and be an error)
        return callback && this.once('no write pending', callback);
      }
      // Otherwise, we've encounted an error state
      var err = new ShareDBError(
        ERROR_CODE.ERR_DOC_MISSING_VERSION,
        'Cannot ingest snapshot in doc with null version. ' + this.collection + '.' + this.id
      );
      if (callback) return callback(err);
      return this.emit('error', err);
    }
    // If we got a snapshot for a version further along than the document is
    // currently, issue a fetch to get the latest ops and catch us up
    if (snapshot.v > this.version) return this.fetch(callback);
    return callback && callback();
  }

  // Ignore the snapshot if we are already at a newer version. Under no
  // circumstance should we ever set the current version backward
  if (this.version > snapshot.v) return callback && callback();

  this.version = snapshot.v;
  var type = (snapshot.type === undefined) ? types.defaultType : snapshot.type;
  this._setType(type);
  this._setData(
    (this.type && this.type.deserialize) ?
      this.type.deserialize(snapshot.data) :
      snapshot.data
  );
  this.emit('load');
  callback && callback();
};

Doc.prototype.whenNothingPending = function(callback) {
  var doc = this;
  util.nextTick(function() {
    if (doc.hasPending()) {
      doc.once('nothing pending', callback);
      return;
    }
    callback();
  });
};

Doc.prototype.hasPending = function() {
  return !!(
    this.inflightOp ||
    this.pendingOps.length ||
    this.inflightFetch.length ||
    this.inflightSubscribe ||
    this.pendingFetch.length ||
    this.pendingSubscribe.length
  );
};

Doc.prototype.hasWritePending = function() {
  return !!(this.inflightOp || this.pendingOps.length);
};

Doc.prototype._emitNothingPending = function() {
  if (this.hasWritePending()) return;
  this.emit('no write pending');
  if (this.hasPending()) return;
  this.emit('nothing pending');
};

// **** Helpers for network messages

Doc.prototype._emitResponseError = function(err, callback) {
  if (err && err.code === ERROR_CODE.ERR_SNAPSHOT_READ_SILENT_REJECTION) {
    this.wantSubscribe = false;
    if (callback) {
      callback();
    }
    this._emitNothingPending();
    return;
  }
  if (callback) {
    callback(err);
    this._emitNothingPending();
    return;
  }
  this._emitNothingPending();
  this.emit('error', err);
};

Doc.prototype._handleFetch = function(error, snapshot) {
  var callbacks = this.pendingFetch;
  this.pendingFetch = [];
  var callback = this.inflightFetch.shift();
  if (callback) callbacks.unshift(callback);
  if (callbacks.length) {
    callback = function(error) {
      util.callEach(callbacks, error);
    };
  }
  if (error) return this._emitResponseError(error, callback);
  this.ingestSnapshot(snapshot, callback);
  this._emitNothingPending();
};

Doc.prototype._handleSubscribe = function(error, snapshot) {
  var request = this.inflightSubscribe;
  this.inflightSubscribe = null;
  var callbacks = this.pendingFetch;
  this.pendingFetch = [];
  if (request.callback) callbacks.push(request.callback);
  var callback;
  if (callbacks.length) {
    callback = function(error) {
      util.callEach(callbacks, error);
    };
  }
  if (error) return this._emitResponseError(error, callback);
  this.subscribed = request.wantSubscribe;
  if (this.subscribed) this.ingestSnapshot(snapshot, callback);
  else if (callback) callback();
  this._emitNothingPending();
  this._flushSubscribe();
};

Doc.prototype._handleOp = function(err, message) {
  if (err) {
    if (err.code === ERROR_CODE.ERR_NO_OP && message.seq === this.inflightOp.seq) {
      // Our op was a no-op, either because we submitted a no-op, or - more
      // likely - because our op was transformed into a no-op by the server
      // because of a similar remote op. In this case, the server has avoided
      // committing the op to the database, and we should just clear the in-flight
      // op and call the callbacks. However, let's first catch ourselves up to
      // the remote, so that we're in a nice consistent state
      return this.fetch(this._clearInflightOp.bind(this));
    }
    if (this.inflightOp) {
      return this._rollback(err);
    }
    return this.emit('error', err);
  }

  if (this.inflightOp &&
      message.src === this.inflightOp.src &&
      message.seq === this.inflightOp.seq) {
    // The op has already been applied locally. Just update the version
    // and pending state appropriately
    this._opAcknowledged(message);
    return;
  }

  if (this.version == null || message.v > this.version) {
    // This will happen in normal operation if we become subscribed to a
    // new document via a query. It can also happen if we get an op for
    // a future version beyond the version we are expecting next. This
    // could happen if the server doesn't publish an op for whatever reason
    // or because of a race condition. In any case, we can send a fetch
    // command to catch back up.
    //
    // Fetch only sends a new fetch command if no fetches are inflight, which
    // will act as a natural debouncing so we don't send multiple fetch
    // requests for many ops received at once.
    this.fetch();
    return;
  }

  if (message.v < this.version) {
    // We can safely ignore the old (duplicate) operation.
    return;
  }

  if (this.inflightOp) {
    var transformErr = transformX(this.inflightOp, message);
    if (transformErr) return this._hardRollback(transformErr);
  }

  for (var i = 0; i < this.pendingOps.length; i++) {
    var transformErr = transformX(this.pendingOps[i], message);
    if (transformErr) return this._hardRollback(transformErr);
  }

  this.version++;
  try {
    this._otApply(message, false);
  } catch (error) {
    return this._hardRollback(error);
  }
};

// Called whenever (you guessed it!) the connection state changes. This will
// happen when we get disconnected & reconnect.
Doc.prototype._onConnectionStateChanged = function() {
  if (this.connection.canSend) {
    this.flush();
    this._resubscribe();
  } else {
    if (this.inflightOp) {
      this.pendingOps.unshift(this.inflightOp);
      this.inflightOp = null;
    }
    this.subscribed = false;
    if (this.inflightSubscribe) {
      if (this.inflightSubscribe.wantSubscribe) {
        this.pendingSubscribe.unshift(this.inflightSubscribe);
        this.inflightSubscribe = null;
      } else {
        this._handleSubscribe();
      }
    }
    if (this.inflightFetch.length) {
      this.pendingFetch = this.pendingFetch.concat(this.inflightFetch);
      this.inflightFetch.length = 0;
    }
  }
};

Doc.prototype._resubscribe = function() {
  if (!this.pendingSubscribe.length && this.wantSubscribe) {
    return this.subscribe();
  }
  var willFetch = this.pendingSubscribe.some(function(request) {
    return request.wantSubscribe;
  });
  if (!willFetch && this.pendingFetch.length) this.fetch();
  this._flushSubscribe();
};

// Request the current document snapshot or ops that bring us up to date
Doc.prototype.fetch = function(callback) {
  this._fetch({}, callback);
};

Doc.prototype._fetch = function(options, callback) {
  this.pendingFetch.push(callback);
  var shouldSend = this.connection.canSend && (
    options.force || !this.inflightFetch.length
  );
  if (!shouldSend) return;
  this.inflightFetch.push(this.pendingFetch.shift());
  this.connection.sendFetch(this);
};

// Fetch the initial document and keep receiving updates
Doc.prototype.subscribe = function(callback) {
  var wantSubscribe = true;
  this._queueSubscribe(wantSubscribe, callback);
};

// Unsubscribe. The data will stay around in local memory, but we'll stop
// receiving updates
Doc.prototype.unsubscribe = function(callback) {
  var wantSubscribe = false;
  this._queueSubscribe(wantSubscribe, callback);
};

Doc.prototype._queueSubscribe = function(wantSubscribe, callback) {
  var lastRequest = this.pendingSubscribe[this.pendingSubscribe.length - 1] || this.inflightSubscribe;
  var isDuplicateRequest = lastRequest && lastRequest.wantSubscribe === wantSubscribe;
  if (isDuplicateRequest) {
    lastRequest.callback = combineCallbacks([lastRequest.callback, callback]);
    return;
  }
  this.pendingSubscribe.push({
    wantSubscribe: !!wantSubscribe,
    callback: callback
  });
  this._flushSubscribe();
};

Doc.prototype._flushSubscribe = function() {
  if (this.inflightSubscribe || !this.pendingSubscribe.length) return;

  if (this.connection.canSend) {
    this.inflightSubscribe = this.pendingSubscribe.shift();
    this.wantSubscribe = this.inflightSubscribe.wantSubscribe;
    if (this.wantSubscribe) {
      this.connection.sendSubscribe(this);
    } else {
      // Be conservative about our subscription state. We'll be unsubscribed
      // some time between sending this request, and receiving the callback,
      // so let's just set ourselves to unsubscribed now.
      this.subscribed = false;
      this.connection.sendUnsubscribe(this);
    }

    return;
  }

  // If we're offline, then we're already unsubscribed. Therefore, call back
  // the next request immediately if it's an unsubscribe request.
  if (!this.pendingSubscribe[0].wantSubscribe) {
    this.inflightSubscribe = this.pendingSubscribe.shift();
    var doc = this;
    util.nextTick(function() {
      doc._handleSubscribe();
    });
  }
};

function combineCallbacks(callbacks) {
  callbacks = callbacks.filter(util.truthy);
  if (!callbacks.length) return null;
  return function(error) {
    util.callEach(callbacks, error);
  };
}


// Operations //

// Send the next pending op to the server, if we can.
//
// Only one operation can be in-flight at a time. If an operation is already on
// its way, or we're not currently connected, this method does nothing.
Doc.prototype.flush = function() {
  // Ignore if we can't send or we are already sending an op
  if (!this.connection.canSend || this.inflightOp) return;

  // Send first pending op unless paused
  if (!this.paused && this.pendingOps.length) {
    this._sendOp();
  }
};

// Helper function to set op to contain a no-op.
function setNoOp(op) {
  delete op.op;
  delete op.create;
  delete op.del;
}

// Transform server op data by a client op, and vice versa. Ops are edited in place.
function transformX(client, server) {
  // Order of statements in this function matters. Be especially careful if
  // refactoring this function

  // A client delete op should dominate if both the server and the client
  // delete the document. Thus, any ops following the client delete (such as a
  // subsequent create) will be maintained, since the server op is transformed
  // to a no-op
  if (client.del) return setNoOp(server);

  if (server.del) {
    return new ShareDBError(ERROR_CODE.ERR_DOC_WAS_DELETED, 'Document was deleted');
  }
  if (server.create) {
    return new ShareDBError(ERROR_CODE.ERR_DOC_ALREADY_CREATED, 'Document already created');
  }

  // Ignore no-op coming from server
  if (!('op' in server)) return;

  // I believe that this should not occur, but check just in case
  if (client.create) {
    return new ShareDBError(ERROR_CODE.ERR_DOC_ALREADY_CREATED, 'Document already created');
  }

  // They both edited the document. This is the normal case for this function -
  // as in, most of the time we'll end up down here.
  //
  // You should be wondering why I'm using client.type instead of this.type.
  // The reason is, if we get ops at an old version of the document, this.type
  // might be undefined or a totally different type. By pinning the type to the
  // op data, we make sure the right type has its transform function called.
  if (client.type.transformX) {
    var result = client.type.transformX(client.op, server.op);
    client.op = result[0];
    server.op = result[1];
  } else {
    var clientOp = client.type.transform(client.op, server.op, 'left');
    var serverOp = client.type.transform(server.op, client.op, 'right');
    client.op = clientOp;
    server.op = serverOp;
  }
};

/**
 * Applies the operation to the snapshot
 *
 * If the operation is create or delete it emits `create` or `del`. Then the
 * operation is applied to the snapshot and `op` and `after op` are emitted.
 * If the type supports incremental updates and `this.incremental` is true we
 * fire `op` after every small operation.
 *
 * This is the only function to fire the above mentioned events.
 *
 * @private
 */
Doc.prototype._otApply = function(op, source) {
  if ('op' in op) {
    if (!this.type) {
      // Throw here, because all usage of _otApply should be wrapped with a try/catch
      throw new ShareDBError(
        ERROR_CODE.ERR_DOC_DOES_NOT_EXIST,
        'Cannot apply op to uncreated document. ' + this.collection + '.' + this.id
      );
    }

    // NB: If we need to add another argument to this event, we should consider
    // the fact that the 'op' event has op.src as its 3rd argument
    this.emit('before op batch', op.op, source);

    // Iteratively apply multi-component remote operations and rollback ops
    // (source === false) for the default JSON0 OT type. It could use
    // type.shatter(), but since this code is so specific to use cases for the
    // JSON0 type and ShareDB explicitly bundles the default type, we might as
    // well write it this way and save needing to iterate through the op
    // components twice.
    //
    // Ideally, we would not need this extra complexity. However, it is
    // helpful for implementing bindings that update DOM nodes and other
    // stateful objects by translating op events directly into corresponding
    // mutations. Such bindings are most easily written as responding to
    // individual op components one at a time in order, and it is important
    // that the snapshot only include updates from the particular op component
    // at the time of emission. Eliminating this would require rethinking how
    // such external bindings are implemented.
    if (!source && this.type === types.defaultType && op.op.length > 1) {
      if (!this.applyStack) this.applyStack = [];
      var stackLength = this.applyStack.length;
      for (var i = 0; i < op.op.length; i++) {
        var component = op.op[i];
        var componentOp = {op: [component]};
        // Apply the individual op component
        this.emit('before op', componentOp.op, source, op.src);
        // Transform componentOp against any ops that have been submitted
        // sychronously inside of an op event handler since we began apply of
        // our operation
        for (var j = stackLength; j < this.applyStack.length; j++) {
          var transformErr = transformX(this.applyStack[j], componentOp);
          if (transformErr) return this._hardRollback(transformErr);
        }
        this._setData(this.type.apply(this.data, componentOp.op));
        this.emit('op', componentOp.op, source, op.src);
      }
      this.emit('op batch', op.op, source);
      // Pop whatever was submitted since we started applying this op
      this._popApplyStack(stackLength);
      return;
    }

    // The 'before op' event enables clients to pull any necessary data out of
    // the snapshot before it gets changed
    this.emit('before op', op.op, source, op.src);
    // Apply the operation to the local data, mutating it in place
    this._setData(this.type.apply(this.data, op.op));
    // Emit an 'op' event once the local data includes the changes from the
    // op. For locally submitted ops, this will be synchronously with
    // submission and before the server or other clients have received the op.
    // For ops from other clients, this will be after the op has been
    // committed to the database and published
    this.emit('op', op.op, source, op.src);
    this.emit('op batch', op.op, source);
    return;
  }

  if (op.create) {
    this._setType(op.create.type);
    if (this.type.deserialize) {
      if (this.type.createDeserialized) {
        this._setData(this.type.createDeserialized(op.create.data));
      } else {
        this._setData(this.type.deserialize(this.type.create(op.create.data)));
      }
    } else {
      this._setData(this.type.create(op.create.data));
    }
    this.emit('create', source);
    return;
  }

  if (op.del) {
    var oldData = this.data;
    this._setType(null);
    this.emit('del', oldData, source);
    return;
  }
};


// ***** Sending operations

// Actually send op to the server.
Doc.prototype._sendOp = function() {
  if (!this.connection.canSend) return;
  var src = this.connection.id;

  // When there is no inflightOp, send the first item in pendingOps. If
  // there is inflightOp, try sending it again
  if (!this.inflightOp) {
    // Send first pending op
    this.inflightOp = this.pendingOps.shift();
  }
  var op = this.inflightOp;
  if (!op) {
    var err = new ShareDBError(ERROR_CODE.ERR_INFLIGHT_OP_MISSING, 'No op to send on call to _sendOp');
    return this.emit('error', err);
  }

  // Track data for retrying ops
  op.sentAt = Date.now();
  op.retries = (op.retries == null) ? 0 : op.retries + 1;

  // The src + seq number is a unique ID representing this operation. This tuple
  // is used on the server to detect when ops have been sent multiple times and
  // on the client to match acknowledgement of an op back to the inflightOp.
  // Note that the src could be different from this.connection.id after a
  // reconnect, since an op may still be pending after the reconnection and
  // this.connection.id will change. In case an op is sent multiple times, we
  // also need to be careful not to override the original seq value.
  if (op.seq == null) {
    if (this.connection.seq >= util.MAX_SAFE_INTEGER) {
      return this.emit('error', new ShareDBError(
        ERROR_CODE.ERR_CONNECTION_SEQ_INTEGER_OVERFLOW,
        'Connection seq has exceeded the max safe integer, maybe from being open for too long'
      ));
    }

    op.seq = this.connection.seq++;
  }

  this.connection.sendOp(this, op);

  // src isn't needed on the first try, since the server session will have the
  // same id, but it must be set on the inflightOp in case it is sent again
  // after a reconnect and the connection's id has changed by then
  if (op.src == null) op.src = src;
};


// Queues the operation for submission to the server and applies it locally.
//
// Internal method called to do the actual work for submit(), create() and del().
// @private
//
// @param op
// @param [op.op]
// @param [op.del]
// @param [op.create]
// @param [callback] called when operation is submitted
Doc.prototype._submit = function(op, source, callback) {
  // Locally submitted ops must always have a truthy source
  if (!source) source = true;

  // The op contains either op, create, delete, or none of the above (a no-op).
  if ('op' in op) {
    if (!this.type) {
      if (this._isInHardRollback) {
        var err = new ShareDBError(
          ERROR_CODE.ERR_DOC_IN_HARD_ROLLBACK,
          'Cannot submit op. Document is performing hard rollback. ' + this.collection + '.' + this.id
        );
      } else {
        var err = new ShareDBError(
          ERROR_CODE.ERR_DOC_DOES_NOT_EXIST,
          'Cannot submit op. Document has not been created. ' + this.collection + '.' + this.id
        );
      }

      if (callback) return callback(err);
      return this.emit('error', err);
    }
    // Try to normalize the op. This removes trailing skip:0's and things like that.
    if (this.type.normalize) op.op = this.type.normalize(op.op);
  }

  try {
    this._pushOp(op, source, callback);
    this._otApply(op, source);
  } catch (error) {
    return this._hardRollback(error);
  }

  // The call to flush is delayed so if submit() is called multiple times
  // synchronously, all the ops are combined before being sent to the server.
  var doc = this;
  util.nextTick(function() {
    doc.flush();
  });
};

Doc.prototype._pushOp = function(op, source, callback) {
  op.source = source;
  if (this.applyStack) {
    // If we are in the process of incrementally applying an operation, don't
    // compose the op and push it onto the applyStack so it can be transformed
    // against other components from the op or ops being applied
    this.applyStack.push(op);
  } else {
    // If the type supports composes, try to compose the operation onto the
    // end of the last pending operation.
    var composed = this._tryCompose(op);
    if (composed) {
      composed.callbacks.push(callback);
      return;
    }
  }
  // Push on to the pendingOps queue of ops to submit if we didn't compose
  op.type = this.type;
  op.callbacks = [callback];
  this.pendingOps.push(op);
};

Doc.prototype._popApplyStack = function(to) {
  if (to > 0) {
    this.applyStack.length = to;
    return;
  }
  // Once we have completed the outermost apply loop, reset to null and no
  // longer add ops to the applyStack as they are submitted
  var op = this.applyStack[0];
  this.applyStack = null;
  if (!op) return;
  // Compose the ops added since the beginning of the apply stack, since we
  // had to skip compose when they were originally pushed
  var i = this.pendingOps.indexOf(op);
  if (i === -1) return;
  var ops = this.pendingOps.splice(i);
  for (var i = 0; i < ops.length; i++) {
    var op = ops[i];
    var composed = this._tryCompose(op);
    if (composed) {
      composed.callbacks = composed.callbacks.concat(op.callbacks);
    } else {
      this.pendingOps.push(op);
    }
  }
};

// Try to compose a submitted op into the last pending op. Returns the
// composed op if it succeeds, undefined otherwise
Doc.prototype._tryCompose = function(op) {
  if (this.preventCompose) return;

  // We can only compose into the last pending op. Inflight ops have already
  // been sent to the server, so we can't modify them
  var last = this.pendingOps[this.pendingOps.length - 1];
  if (!last || last.sentAt) return;

  // If we're submitting the op source, we can only combine ops that have
  // a matching source
  if (this.submitSource && !deepEqual(op.source, last.source)) return;

  // Compose an op into a create by applying it. This effectively makes the op
  // invisible, as if the document were created including the op originally
  if (last.create && 'op' in op) {
    last.create.data = this.type.apply(last.create.data, op.op);
    return last;
  }

  // Compose two ops into a single op if supported by the type. Types that
  // support compose must be able to compose any two ops together
  if ('op' in last && 'op' in op && this.type.compose) {
    last.op = this.type.compose(last.op, op.op);
    return last;
  }
};

// *** Client OT entrypoints.

// Submit an operation to the document.
//
// @param operation handled by the OT type
// @param options  {source: ...}
// @param [callback] called after operation submitted
//
// @fires before op, op, after op
Doc.prototype.submitOp = function(component, options, callback) {
  if (typeof options === 'function') {
    callback = options;
    options = null;
  }
  var op = {op: component};
  var source = options && options.source;
  this._submit(op, source, callback);
};

// Create the document, which in ShareJS semantics means to set its type. Every
// object implicitly exists in the database but has no data and no type. Create
// sets the type of the object and can optionally set some initial data on the
// object, depending on the type.
//
// @param data  initial
// @param type  OT type
// @param options  {source: ...}
// @param callback  called when operation submitted
Doc.prototype.create = function(data, type, options, callback) {
  if (typeof type === 'function') {
    callback = type;
    options = null;
    type = null;
  } else if (typeof options === 'function') {
    callback = options;
    options = null;
  }
  if (!type) {
    type = types.defaultType.uri;
  }
  if (this.type) {
    var err = new ShareDBError(ERROR_CODE.ERR_DOC_ALREADY_CREATED, 'Document already exists');
    if (callback) return callback(err);
    return this.emit('error', err);
  }
  var op = {create: {type: type, data: data}};
  var source = options && options.source;
  this._submit(op, source, callback);
};

// Delete the document. This creates and submits a delete operation to the
// server. Deleting resets the object's type to null and deletes its data. The
// document still exists, and still has the version it used to have before you
// deleted it (well, old version +1).
//
// @param options  {source: ...}
// @param callback  called when operation submitted
Doc.prototype.del = function(options, callback) {
  if (typeof options === 'function') {
    callback = options;
    options = null;
  }
  if (!this.type) {
    var err = new ShareDBError(ERROR_CODE.ERR_DOC_DOES_NOT_EXIST, 'Document does not exist');
    if (callback) return callback(err);
    return this.emit('error', err);
  }
  var op = {del: true};
  var source = options && options.source;
  this._submit(op, source, callback);
};


// Stops the document from sending any operations to the server.
Doc.prototype.pause = function() {
  this.paused = true;
};

// Continue sending operations to the server
Doc.prototype.resume = function() {
  this.paused = false;
  this.flush();
};

// Create a snapshot that can be serialized, deserialized, and passed into `Doc.ingestSnapshot`.
Doc.prototype.toSnapshot = function() {
  return {
    v: this.version,
    data: clone(this.data),
    type: this.type.uri
  };
};

// *** Receiving operations

// This is called when the server acknowledges an operation from the client.
Doc.prototype._opAcknowledged = function(message) {
  if (this.inflightOp.create) {
    this.version = message.v;
  } else if (message.v !== this.version) {
    // We should already be at the same version, because the server should
    // have sent all the ops that have happened before acknowledging our op
    logger.warn('Invalid version from server. Expected: ' + this.version + ' Received: ' + message.v, message);

    // Fetching should get us back to a working document state
    return this.fetch();
  }

  if (message[ACTIONS.fixup]) {
    for (var i = 0; i < message[ACTIONS.fixup].length; i++) {
      var fixupOp = message[ACTIONS.fixup][i];

      for (var j = 0; j < this.pendingOps.length; j++) {
        var transformErr = transformX(this.pendingOps[i], fixupOp);
        if (transformErr) return this._hardRollback(transformErr);
      }

      try {
        this._otApply(fixupOp, false);
      } catch (error) {
        return this._hardRollback(error);
      }
    }
  }

  // The op was committed successfully. Increment the version number
  this.version++;

  this._clearInflightOp();
};

Doc.prototype._rollback = function(err) {
  // The server has rejected submission of the current operation. Invert by
  // just the inflight op if possible. If not possible to invert, cancel all
  // pending ops and fetch the latest from the server to get us back into a
  // working state, then call back
  var op = this.inflightOp;

  if (!('op' in op && op.type.invert)) {
    return this._hardRollback(err);
  }

  try {
    op.op = op.type.invert(op.op);
  } catch (error) {
    // If the op doesn't support `.invert()`, we just reload the doc
    // instead of trying to locally revert it.
    return this._hardRollback(err);
  }

  // Transform the undo operation by any pending ops.
  for (var i = 0; i < this.pendingOps.length; i++) {
    var transformErr = transformX(this.pendingOps[i], op);
    if (transformErr) return this._hardRollback(transformErr);
  }

  // ... and apply it locally, reverting the changes.
  //
  // This operation is applied to look like it comes from a remote source.
  // I'm still not 100% sure about this functionality, because its really a
  // local op. Basically, the problem is that if the client's op is rejected
  // by the server, the editor window should update to reflect the undo.
  try {
    this._otApply(op, false);
  } catch (error) {
    return this._hardRollback(error);
  }

  // The server has rejected submission of the current operation. If we get
  // an "Op submit rejected" error, this was done intentionally
  // and we should roll back but not return an error to the user.
  if (err.code === ERROR_CODE.ERR_OP_SUBMIT_REJECTED) {
    return this._clearInflightOp(null);
  }

  this._clearInflightOp(err);
};

Doc.prototype._hardRollback = function(err) {
  this._isInHardRollback = true;
  // Store pending ops so that we can notify their callbacks of the error.
  // We combine the inflight op and the pending ops, because it's possible
  // to hit a condition where we have no inflight op, but we do have pending
  // ops. This can happen when an invalid op is submitted, which causes us
  // to hard rollback before the pending op was flushed.
  var pendingOps = this.pendingOps;
  var inflightOp = this.inflightOp;

  // Cancel all pending ops and reset if we can't invert
  this._setType(null);
  this.version = null;
  this.inflightOp = null;
  this.pendingOps = [];

  // Fetch the latest version from the server to get us back into a working state
  var doc = this;
  this._fetch({force: true}, function(fetchError) {
    doc._isInHardRollback = false;

    // We want to check that no errors are swallowed, so we check that:
    // - there are callbacks to call, and
    // - that every single pending op called a callback
    // If there are no ops queued, or one of them didn't handle the error,
    // then we emit the error.

    if (fetchError) {
      // This is critical error as it means that our doc is not in usable state
      // anymore, we should throw doc error.
      logger.error('Hard rollback doc fetch failed.', fetchError, inflightOp);

      doc.emit('error', new ShareDBError(
        ERROR_CODE.ERR_HARD_ROLLBACK_FETCH_FAILED,
        'Hard rollback fetch failed: ' + fetchError.message
      ));
    }

    if (err.code === ERROR_CODE.ERR_OP_SUBMIT_REJECTED) {
      /**
       * Handle special case of ERR_OP_SUBMIT_REJECTED
       * This ensures that we resolve the main op callback and reject
       * all the pending ops. This is hard rollback so all the pending ops will be
       * discarded. This will ensure that the user is at least informed about it.
       * more info: https://github.com/share/sharedb/pull/626
       */
      if (inflightOp) {
        util.callEach(inflightOp.callbacks);
        inflightOp = null;
      }

      if (!pendingOps.length) return;
      err = new ShareDBError(
        ERROR_CODE.ERR_PENDING_OP_REMOVED_BY_OP_SUBMIT_REJECTED,
        'Discarding pending op because of hard rollback during ERR_OP_SUBMIT_REJECTED'
      );
    }

    if (inflightOp) pendingOps.unshift(inflightOp);
    var allOpsHadCallbacks = !!pendingOps.length;
    for (var i = 0; i < pendingOps.length; i++) {
      allOpsHadCallbacks = util.callEach(pendingOps[i].callbacks, err) && allOpsHadCallbacks;
    }
    if (err && !allOpsHadCallbacks) doc.emit('error', err);
  });
};

Doc.prototype._clearInflightOp = function(err) {
  var inflightOp = this.inflightOp;

  this.inflightOp = null;

  var called = util.callEach(inflightOp.callbacks, err);

  this.flush();
  this._emitNothingPending();

  if (err && !called) return this.emit('error', err);
};