Initial commit

node_modules/mongodb/lib/core/topologies/read_preference.js

@@ -0,0 +1,266 @@
+'use strict';
+
+/**
+ * The **ReadPreference** class is a class that represents a MongoDB ReadPreference and is
+ * used to construct connections.
+ * @class
+ * @param {string} mode A string describing the read preference mode (primary|primaryPreferred|secondary|secondaryPreferred|nearest)
+ * @param {array} tags The tags object
+ * @param {object} [options] Additional read preference options
+ * @param {number} [options.maxStalenessSeconds] Max secondary read staleness in seconds, Minimum value is 90 seconds.
+ * @param {object} [options.hedge] Server mode in which the same query is dispatched in parallel to multiple replica set members.
+ * @param {boolean} [options.hedge.enabled] Explicitly enable or disable hedged reads.
+ * @see https://docs.mongodb.com/manual/core/read-preference/
+ * @return {ReadPreference}
+ */
+const ReadPreference = function(mode, tags, options) {
+  if (!ReadPreference.isValid(mode)) {
+    throw new TypeError(`Invalid read preference mode ${mode}`);
+  }
+
+  // TODO(major): tags MUST be an array of tagsets
+  if (tags && !Array.isArray(tags)) {
+    console.warn(
+      'ReadPreference tags must be an array, this will change in the next major version'
+    );
+
+    const tagsHasMaxStalenessSeconds = typeof tags.maxStalenessSeconds !== 'undefined';
+    const tagsHasHedge = typeof tags.hedge !== 'undefined';
+    const tagsHasOptions = tagsHasMaxStalenessSeconds || tagsHasHedge;
+    if (tagsHasOptions) {
+      // this is likely an options object
+      options = tags;
+      tags = undefined;
+    } else {
+      tags = [tags];
+    }
+  }
+
+  this.mode = mode;
+  this.tags = tags;
+  this.hedge = options && options.hedge;
+
+  options = options || {};
+  if (options.maxStalenessSeconds != null) {
+    if (options.maxStalenessSeconds <= 0) {
+      throw new TypeError('maxStalenessSeconds must be a positive integer');
+    }
+
+    this.maxStalenessSeconds = options.maxStalenessSeconds;
+
+    // NOTE: The minimum required wire version is 5 for this read preference. If the existing
+    //       topology has a lower value then a MongoError will be thrown during server selection.
+    this.minWireVersion = 5;
+  }
+
+  if (this.mode === ReadPreference.PRIMARY) {
+    if (this.tags && Array.isArray(this.tags) && this.tags.length > 0) {
+      throw new TypeError('Primary read preference cannot be combined with tags');
+    }
+
+    if (this.maxStalenessSeconds) {
+      throw new TypeError('Primary read preference cannot be combined with maxStalenessSeconds');
+    }
+
+    if (this.hedge) {
+      throw new TypeError('Primary read preference cannot be combined with hedge');
+    }
+  }
+};
+
+// Support the deprecated `preference` property introduced in the porcelain layer
+Object.defineProperty(ReadPreference.prototype, 'preference', {
+  enumerable: true,
+  get: function() {
+    return this.mode;
+  }
+});
+
+/*
+ * Read preference mode constants
+ */
+ReadPreference.PRIMARY = 'primary';
+ReadPreference.PRIMARY_PREFERRED = 'primaryPreferred';
+ReadPreference.SECONDARY = 'secondary';
+ReadPreference.SECONDARY_PREFERRED = 'secondaryPreferred';
+ReadPreference.NEAREST = 'nearest';
+
+const VALID_MODES = [
+  ReadPreference.PRIMARY,
+  ReadPreference.PRIMARY_PREFERRED,
+  ReadPreference.SECONDARY,
+  ReadPreference.SECONDARY_PREFERRED,
+  ReadPreference.NEAREST,
+  null
+];
+
+/**
+ * Construct a ReadPreference given an options object.
+ *
+ * @param {object} options The options object from which to extract the read preference.
+ * @return {ReadPreference}
+ */
+ReadPreference.fromOptions = function(options) {
+  if (!options) return null;
+  const readPreference = options.readPreference;
+  if (!readPreference) return null;
+  const readPreferenceTags = options.readPreferenceTags;
+  const maxStalenessSeconds = options.maxStalenessSeconds;
+  if (typeof readPreference === 'string') {
+    return new ReadPreference(readPreference, readPreferenceTags);
+  } else if (!(readPreference instanceof ReadPreference) && typeof readPreference === 'object') {
+    const mode = readPreference.mode || readPreference.preference;
+    if (mode && typeof mode === 'string') {
+      return new ReadPreference(mode, readPreference.tags, {
+        maxStalenessSeconds: readPreference.maxStalenessSeconds || maxStalenessSeconds,
+        hedge: readPreference.hedge
+      });
+    }
+  }
+
+  return readPreference;
+};
+
+/**
+ * Resolves a read preference based on well-defined inheritance rules. This method will not only
+ * determine the read preference (if there is one), but will also ensure the returned value is a
+ * properly constructed instance of `ReadPreference`.
+ *
+ * @param {Collection|Db|MongoClient} parent The parent of the operation on which to determine the read
+ * preference, used for determining the inherited read preference.
+ * @param {object} options The options passed into the method, potentially containing a read preference
+ * @returns {(ReadPreference|null)} The resolved read preference
+ */
+ReadPreference.resolve = function(parent, options) {
+  options = options || {};
+  const session = options.session;
+
+  const inheritedReadPreference = parent && parent.readPreference;
+
+  let readPreference;
+  if (options.readPreference) {
+    readPreference = ReadPreference.fromOptions(options);
+  } else if (session && session.inTransaction() && session.transaction.options.readPreference) {
+    // The transaction’s read preference MUST override all other user configurable read preferences.
+    readPreference = session.transaction.options.readPreference;
+  } else if (inheritedReadPreference != null) {
+    readPreference = inheritedReadPreference;
+  } else {
+    readPreference = ReadPreference.primary;
+  }
+
+  return typeof readPreference === 'string' ? new ReadPreference(readPreference) : readPreference;
+};
+
+/**
+ * Replaces options.readPreference with a ReadPreference instance
+ */
+ReadPreference.translate = function(options) {
+  if (options.readPreference == null) return options;
+  const r = options.readPreference;
+
+  if (typeof r === 'string') {
+    options.readPreference = new ReadPreference(r);
+  } else if (r && !(r instanceof ReadPreference) && typeof r === 'object') {
+    const mode = r.mode || r.preference;
+    if (mode && typeof mode === 'string') {
+      options.readPreference = new ReadPreference(mode, r.tags, {
+        maxStalenessSeconds: r.maxStalenessSeconds
+      });
+    }
+  } else if (!(r instanceof ReadPreference)) {
+    throw new TypeError('Invalid read preference: ' + r);
+  }
+
+  return options;
+};
+
+/**
+ * Validate if a mode is legal
+ *
+ * @method
+ * @param {string} mode The string representing the read preference mode.
+ * @return {boolean} True if a mode is valid
+ */
+ReadPreference.isValid = function(mode) {
+  return VALID_MODES.indexOf(mode) !== -1;
+};
+
+/**
+ * Validate if a mode is legal
+ *
+ * @method
+ * @param {string} mode The string representing the read preference mode.
+ * @return {boolean} True if a mode is valid
+ */
+ReadPreference.prototype.isValid = function(mode) {
+  return ReadPreference.isValid(typeof mode === 'string' ? mode : this.mode);
+};
+
+const needSlaveOk = ['primaryPreferred', 'secondary', 'secondaryPreferred', 'nearest'];
+
+/**
+ * Indicates that this readPreference needs the "slaveOk" bit when sent over the wire
+ * @method
+ * @return {boolean}
+ * @see https://docs.mongodb.com/manual/reference/mongodb-wire-protocol/#op-query
+ */
+ReadPreference.prototype.slaveOk = function() {
+  return needSlaveOk.indexOf(this.mode) !== -1;
+};
+
+/**
+ * Are the two read preference equal
+ * @method
+ * @param {ReadPreference} readPreference The read preference with which to check equality
+ * @return {boolean} True if the two ReadPreferences are equivalent
+ */
+ReadPreference.prototype.equals = function(readPreference) {
+  return readPreference.mode === this.mode;
+};
+
+/**
+ * Return JSON representation
+ * @method
+ * @return {Object} A JSON representation of the ReadPreference
+ */
+ReadPreference.prototype.toJSON = function() {
+  const readPreference = { mode: this.mode };
+  if (Array.isArray(this.tags)) readPreference.tags = this.tags;
+  if (this.maxStalenessSeconds) readPreference.maxStalenessSeconds = this.maxStalenessSeconds;
+  if (this.hedge) readPreference.hedge = this.hedge;
+  return readPreference;
+};
+
+/**
+ * Primary read preference
+ * @member
+ * @type {ReadPreference}
+ */
+ReadPreference.primary = new ReadPreference('primary');
+/**
+ * Primary Preferred read preference
+ * @member
+ * @type {ReadPreference}
+ */
+ReadPreference.primaryPreferred = new ReadPreference('primaryPreferred');
+/**
+ * Secondary read preference
+ * @member
+ * @type {ReadPreference}
+ */
+ReadPreference.secondary = new ReadPreference('secondary');
+/**
+ * Secondary Preferred read preference
+ * @member
+ * @type {ReadPreference}
+ */
+ReadPreference.secondaryPreferred = new ReadPreference('secondaryPreferred');
+/**
+ * Nearest read preference
+ * @member
+ * @type {ReadPreference}
+ */
+ReadPreference.nearest = new ReadPreference('nearest');
+
+module.exports = ReadPreference;

node_modules/mongodb/lib/core/topologies/server.js

@@ -0,0 +1,990 @@
+'use strict';
+
+var inherits = require('util').inherits,
+  f = require('util').format,
+  EventEmitter = require('events').EventEmitter,
+  ReadPreference = require('./read_preference'),
+  Logger = require('../connection/logger'),
+  debugOptions = require('../connection/utils').debugOptions,
+  retrieveBSON = require('../connection/utils').retrieveBSON,
+  Pool = require('../connection/pool'),
+  MongoError = require('../error').MongoError,
+  MongoNetworkError = require('../error').MongoNetworkError,
+  wireProtocol = require('../wireprotocol'),
+  CoreCursor = require('../cursor').CoreCursor,
+  sdam = require('./shared'),
+  createCompressionInfo = require('./shared').createCompressionInfo,
+  resolveClusterTime = require('./shared').resolveClusterTime,
+  SessionMixins = require('./shared').SessionMixins,
+  relayEvents = require('../utils').relayEvents;
+
+const collationNotSupported = require('../utils').collationNotSupported;
+const makeClientMetadata = require('../utils').makeClientMetadata;
+
+// Used for filtering out fields for loggin
+var debugFields = [
+  'reconnect',
+  'reconnectTries',
+  'reconnectInterval',
+  'emitError',
+  'cursorFactory',
+  'host',
+  'port',
+  'size',
+  'keepAlive',
+  'keepAliveInitialDelay',
+  'noDelay',
+  'connectionTimeout',
+  'checkServerIdentity',
+  'socketTimeout',
+  'ssl',
+  'ca',
+  'crl',
+  'cert',
+  'key',
+  'rejectUnauthorized',
+  'promoteLongs',
+  'promoteValues',
+  'promoteBuffers',
+  'servername'
+];
+
+// Server instance id
+var id = 0;
+var serverAccounting = false;
+var servers = {};
+var BSON = retrieveBSON();
+
+function topologyId(server) {
+  return server.s.parent == null ? server.id : server.s.parent.id;
+}
+
+/**
+ * Creates a new Server instance
+ * @class
+ * @param {boolean} [options.reconnect=true] Server will attempt to reconnect on loss of connection
+ * @param {number} [options.reconnectTries=30] Server attempt to reconnect #times
+ * @param {number} [options.reconnectInterval=1000] Server will wait # milliseconds between retries
+ * @param {number} [options.monitoring=true] Enable the server state monitoring (calling ismaster at monitoringInterval)
+ * @param {number} [options.monitoringInterval=5000] The interval of calling ismaster when monitoring is enabled.
+ * @param {Cursor} [options.cursorFactory=Cursor] The cursor factory class used for all query cursors
+ * @param {string} options.host The server host
+ * @param {number} options.port The server port
+ * @param {number} [options.size=5] Server connection pool size
+ * @param {boolean} [options.keepAlive=true] TCP Connection keep alive enabled
+ * @param {number} [options.keepAliveInitialDelay=120000] Initial delay before TCP keep alive enabled
+ * @param {boolean} [options.noDelay=true] TCP Connection no delay
+ * @param {number} [options.connectionTimeout=30000] TCP Connection timeout setting
+ * @param {number} [options.socketTimeout=360000] TCP Socket timeout setting
+ * @param {boolean} [options.ssl=false] Use SSL for connection
+ * @param {boolean|function} [options.checkServerIdentity=true] Ensure we check server identify during SSL, set to false to disable checking. Only works for Node 0.12.x or higher. You can pass in a boolean or your own checkServerIdentity override function.
+ * @param {Buffer} [options.ca] SSL Certificate store binary buffer
+ * @param {Buffer} [options.crl] SSL Certificate revocation store binary buffer
+ * @param {Buffer} [options.cert] SSL Certificate binary buffer
+ * @param {Buffer} [options.key] SSL Key file binary buffer
+ * @param {string} [options.passphrase] SSL Certificate pass phrase
+ * @param {boolean} [options.rejectUnauthorized=true] Reject unauthorized server certificates
+ * @param {string} [options.servername=null] String containing the server name requested via TLS SNI.
+ * @param {boolean} [options.promoteLongs=true] Convert Long values from the db into Numbers if they fit into 53 bits
+ * @param {boolean} [options.promoteValues=true] Promotes BSON values to native types where possible, set to false to only receive wrapper types.
+ * @param {boolean} [options.promoteBuffers=false] Promotes Binary BSON values to native Node Buffers.
+ * @param {string} [options.appname=null] Application name, passed in on ismaster call and logged in mongod server logs. Maximum size 128 bytes.
+ * @param {boolean} [options.domainsEnabled=false] Enable the wrapping of the callback in the current domain, disabled by default to avoid perf hit.
+ * @param {boolean} [options.monitorCommands=false] Enable command monitoring for this topology
+ * @return {Server} A cursor instance
+ * @fires Server#connect
+ * @fires Server#close
+ * @fires Server#error
+ * @fires Server#timeout
+ * @fires Server#parseError
+ * @fires Server#reconnect
+ * @fires Server#reconnectFailed
+ * @fires Server#serverHeartbeatStarted
+ * @fires Server#serverHeartbeatSucceeded
+ * @fires Server#serverHeartbeatFailed
+ * @fires Server#topologyOpening
+ * @fires Server#topologyClosed
+ * @fires Server#topologyDescriptionChanged
+ * @property {string} type the topology type.
+ * @property {string} parserType the parser type used (c++ or js).
+ */
+var Server = function(options) {
+  options = options || {};
+
+  // Add event listener
+  EventEmitter.call(this);
+
+  // Server instance id
+  this.id = id++;
+
+  // Internal state
+  this.s = {
+    // Options
+    options: Object.assign({ metadata: makeClientMetadata(options) }, options),
+    // Logger
+    logger: Logger('Server', options),
+    // Factory overrides
+    Cursor: options.cursorFactory || CoreCursor,
+    // BSON instance
+    bson:
+      options.bson ||
+      new BSON([
+        BSON.Binary,
+        BSON.Code,
+        BSON.DBRef,
+        BSON.Decimal128,
+        BSON.Double,
+        BSON.Int32,
+        BSON.Long,
+        BSON.Map,
+        BSON.MaxKey,
+        BSON.MinKey,
+        BSON.ObjectId,
+        BSON.BSONRegExp,
+        BSON.Symbol,
+        BSON.Timestamp
+      ]),
+    // Pool
+    pool: null,
+    // Disconnect handler
+    disconnectHandler: options.disconnectHandler,
+    // Monitor thread (keeps the connection alive)
+    monitoring: typeof options.monitoring === 'boolean' ? options.monitoring : true,
+    // Is the server in a topology
+    inTopology: !!options.parent,
+    // Monitoring timeout
+    monitoringInterval:
+      typeof options.monitoringInterval === 'number' ? options.monitoringInterval : 5000,
+    compression: { compressors: createCompressionInfo(options) },
+    // Optional parent topology
+    parent: options.parent
+  };
+
+  // If this is a single deployment we need to track the clusterTime here
+  if (!this.s.parent) {
+    this.s.clusterTime = null;
+  }
+
+  // Curent ismaster
+  this.ismaster = null;
+  // Current ping time
+  this.lastIsMasterMS = -1;
+  // The monitoringProcessId
+  this.monitoringProcessId = null;
+  // Initial connection
+  this.initialConnect = true;
+  // Default type
+  this._type = 'server';
+
+  // Max Stalleness values
+  // last time we updated the ismaster state
+  this.lastUpdateTime = 0;
+  // Last write time
+  this.lastWriteDate = 0;
+  // Stalleness
+  this.staleness = 0;
+};
+
+inherits(Server, EventEmitter);
+Object.assign(Server.prototype, SessionMixins);
+
+Object.defineProperty(Server.prototype, 'type', {
+  enumerable: true,
+  get: function() {
+    return this._type;
+  }
+});
+
+Object.defineProperty(Server.prototype, 'parserType', {
+  enumerable: true,
+  get: function() {
+    return BSON.native ? 'c++' : 'js';
+  }
+});
+
+Object.defineProperty(Server.prototype, 'logicalSessionTimeoutMinutes', {
+  enumerable: true,
+  get: function() {
+    if (!this.ismaster) return null;
+    return this.ismaster.logicalSessionTimeoutMinutes || null;
+  }
+});
+
+Object.defineProperty(Server.prototype, 'clientMetadata', {
+  enumerable: true,
+  get: function() {
+    return this.s.options.metadata;
+  }
+});
+
+// In single server deployments we track the clusterTime directly on the topology, however
+// in Mongos and ReplSet deployments we instead need to delegate the clusterTime up to the
+// tracking objects so we can ensure we are gossiping the maximum time received from the
+// server.
+Object.defineProperty(Server.prototype, 'clusterTime', {
+  enumerable: true,
+  set: function(clusterTime) {
+    const settings = this.s.parent ? this.s.parent : this.s;
+    resolveClusterTime(settings, clusterTime);
+  },
+  get: function() {
+    const settings = this.s.parent ? this.s.parent : this.s;
+    return settings.clusterTime || null;
+  }
+});
+
+Server.enableServerAccounting = function() {
+  serverAccounting = true;
+  servers = {};
+};
+
+Server.disableServerAccounting = function() {
+  serverAccounting = false;
+};
+
+Server.servers = function() {
+  return servers;
+};
+
+Object.defineProperty(Server.prototype, 'name', {
+  enumerable: true,
+  get: function() {
+    return this.s.options.host + ':' + this.s.options.port;
+  }
+});
+
+function disconnectHandler(self, type, ns, cmd, options, callback) {
+  // Topology is not connected, save the call in the provided store to be
+  // Executed at some point when the handler deems it's reconnected
+  if (
+    !self.s.pool.isConnected() &&
+    self.s.options.reconnect &&
+    self.s.disconnectHandler != null &&
+    !options.monitoring
+  ) {
+    self.s.disconnectHandler.add(type, ns, cmd, options, callback);
+    return true;
+  }
+
+  // If we have no connection error
+  if (!self.s.pool.isConnected()) {
+    callback(new MongoError(f('no connection available to server %s', self.name)));
+    return true;
+  }
+}
+
+function monitoringProcess(self) {
+  return function() {
+    // Pool was destroyed do not continue process
+    if (self.s.pool.isDestroyed()) return;
+    // Emit monitoring Process event
+    self.emit('monitoring', self);
+    // Perform ismaster call
+    // Get start time
+    var start = new Date().getTime();
+
+    // Execute the ismaster query
+    self.command(
+      'admin.$cmd',
+      { ismaster: true },
+      {
+        socketTimeout:
+          typeof self.s.options.connectionTimeout !== 'number'
+            ? 2000
+            : self.s.options.connectionTimeout,
+        monitoring: true
+      },
+      (err, result) => {
+        // Set initial lastIsMasterMS
+        self.lastIsMasterMS = new Date().getTime() - start;
+        if (self.s.pool.isDestroyed()) return;
+        // Update the ismaster view if we have a result
+        if (result) {
+          self.ismaster = result.result;
+        }
+        // Re-schedule the monitoring process
+        self.monitoringProcessId = setTimeout(monitoringProcess(self), self.s.monitoringInterval);
+      }
+    );
+  };
+}
+
+var eventHandler = function(self, event) {
+  return function(err, conn) {
+    // Log information of received information if in info mode
+    if (self.s.logger.isInfo()) {
+      var object = err instanceof MongoError ? JSON.stringify(err) : {};
+      self.s.logger.info(
+        f('server %s fired event %s out with message %s', self.name, event, object)
+      );
+    }
+
+    // Handle connect event
+    if (event === 'connect') {
+      self.initialConnect = false;
+      self.ismaster = conn.ismaster;
+      self.lastIsMasterMS = conn.lastIsMasterMS;
+      if (conn.agreedCompressor) {
+        self.s.pool.options.agreedCompressor = conn.agreedCompressor;
+      }
+
+      if (conn.zlibCompressionLevel) {
+        self.s.pool.options.zlibCompressionLevel = conn.zlibCompressionLevel;
+      }
+
+      if (conn.ismaster.$clusterTime) {
+        const $clusterTime = conn.ismaster.$clusterTime;
+        self.clusterTime = $clusterTime;
+      }
+
+      // It's a proxy change the type so
+      // the wireprotocol will send $readPreference
+      if (self.ismaster.msg === 'isdbgrid') {
+        self._type = 'mongos';
+      }
+
+      // Have we defined self monitoring
+      if (self.s.monitoring) {
+        self.monitoringProcessId = setTimeout(monitoringProcess(self), self.s.monitoringInterval);
+      }
+
+      // Emit server description changed if something listening
+      sdam.emitServerDescriptionChanged(self, {
+        address: self.name,
+        arbiters: [],
+        hosts: [],
+        passives: [],
+        type: sdam.getTopologyType(self)
+      });
+
+      if (!self.s.inTopology) {
+        // Emit topology description changed if something listening
+        sdam.emitTopologyDescriptionChanged(self, {
+          topologyType: 'Single',
+          servers: [
+            {
+              address: self.name,
+              arbiters: [],
+              hosts: [],
+              passives: [],
+              type: sdam.getTopologyType(self)
+            }
+          ]
+        });
+      }
+
+      // Log the ismaster if available
+      if (self.s.logger.isInfo()) {
+        self.s.logger.info(
+          f('server %s connected with ismaster [%s]', self.name, JSON.stringify(self.ismaster))
+        );
+      }
+
+      // Emit connect
+      self.emit('connect', self);
+    } else if (
+      event === 'error' ||
+      event === 'parseError' ||
+      event === 'close' ||
+      event === 'timeout' ||
+      event === 'reconnect' ||
+      event === 'attemptReconnect' ||
+      'reconnectFailed'
+    ) {
+      // Remove server instance from accounting
+      if (
+        serverAccounting &&
+        ['close', 'timeout', 'error', 'parseError', 'reconnectFailed'].indexOf(event) !== -1
+      ) {
+        // Emit toplogy opening event if not in topology
+        if (!self.s.inTopology) {
+          self.emit('topologyOpening', { topologyId: self.id });
+        }
+
+        delete servers[self.id];
+      }
+
+      if (event === 'close') {
+        // Closing emits a server description changed event going to unknown.
+        sdam.emitServerDescriptionChanged(self, {
+          address: self.name,
+          arbiters: [],
+          hosts: [],
+          passives: [],
+          type: 'Unknown'
+        });
+      }
+
+      // Reconnect failed return error
+      if (event === 'reconnectFailed') {
+        self.emit('reconnectFailed', err);
+        // Emit error if any listeners
+        if (self.listeners('error').length > 0) {
+          self.emit('error', err);
+        }
+        // Terminate
+        return;
+      }
+
+      // On first connect fail
+      if (
+        ['disconnected', 'connecting'].indexOf(self.s.pool.state) !== -1 &&
+        self.initialConnect &&
+        ['close', 'timeout', 'error', 'parseError'].indexOf(event) !== -1
+      ) {
+        self.initialConnect = false;
+        return self.emit(
+          'error',
+          new MongoNetworkError(
+            f('failed to connect to server [%s] on first connect [%s]', self.name, err)
+          )
+        );
+      }
+
+      // Reconnect event, emit the server
+      if (event === 'reconnect') {
+        // Reconnecting emits a server description changed event going from unknown to the
+        // current server type.
+        sdam.emitServerDescriptionChanged(self, {
+          address: self.name,
+          arbiters: [],
+          hosts: [],
+          passives: [],
+          type: sdam.getTopologyType(self)
+        });
+        return self.emit(event, self);
+      }
+
+      // Emit the event
+      self.emit(event, err);
+    }
+  };
+};
+
+/**
+ * Initiate server connect
+ */
+Server.prototype.connect = function(options) {
+  var self = this;
+  options = options || {};
+
+  // Set the connections
+  if (serverAccounting) servers[this.id] = this;
+
+  // Do not allow connect to be called on anything that's not disconnected
+  if (self.s.pool && !self.s.pool.isDisconnected() && !self.s.pool.isDestroyed()) {
+    throw new MongoError(f('server instance in invalid state %s', self.s.pool.state));
+  }
+
+  // Create a pool
+  self.s.pool = new Pool(this, Object.assign(self.s.options, options, { bson: this.s.bson }));
+
+  // Set up listeners
+  self.s.pool.on('close', eventHandler(self, 'close'));
+  self.s.pool.on('error', eventHandler(self, 'error'));
+  self.s.pool.on('timeout', eventHandler(self, 'timeout'));
+  self.s.pool.on('parseError', eventHandler(self, 'parseError'));
+  self.s.pool.on('connect', eventHandler(self, 'connect'));
+  self.s.pool.on('reconnect', eventHandler(self, 'reconnect'));
+  self.s.pool.on('reconnectFailed', eventHandler(self, 'reconnectFailed'));
+
+  // Set up listeners for command monitoring
+  relayEvents(self.s.pool, self, ['commandStarted', 'commandSucceeded', 'commandFailed']);
+
+  // Emit toplogy opening event if not in topology
+  if (!self.s.inTopology) {
+    this.emit('topologyOpening', { topologyId: topologyId(self) });
+  }
+
+  // Emit opening server event
+  self.emit('serverOpening', { topologyId: topologyId(self), address: self.name });
+
+  self.s.pool.connect();
+};
+
+/**
+ * Authenticate the topology.
+ * @method
+ * @param {MongoCredentials} credentials The credentials for authentication we are using
+ * @param {authResultCallback} callback A callback function
+ */
+Server.prototype.auth = function(credentials, callback) {
+  if (typeof callback === 'function') callback(null, null);
+};
+
+/**
+ * Get the server description
+ * @method
+ * @return {object}
+ */
+Server.prototype.getDescription = function() {
+  var ismaster = this.ismaster || {};
+  var description = {
+    type: sdam.getTopologyType(this),
+    address: this.name
+  };
+
+  // Add fields if available
+  if (ismaster.hosts) description.hosts = ismaster.hosts;
+  if (ismaster.arbiters) description.arbiters = ismaster.arbiters;
+  if (ismaster.passives) description.passives = ismaster.passives;
+  if (ismaster.setName) description.setName = ismaster.setName;
+  return description;
+};
+
+/**
+ * Returns the last known ismaster document for this server
+ * @method
+ * @return {object}
+ */
+Server.prototype.lastIsMaster = function() {
+  return this.ismaster;
+};
+
+/**
+ * Unref all connections belong to this server
+ * @method
+ */
+Server.prototype.unref = function() {
+  this.s.pool.unref();
+};
+
+/**
+ * Figure out if the server is connected
+ * @method
+ * @return {boolean}
+ */
+Server.prototype.isConnected = function() {
+  if (!this.s.pool) return false;
+  return this.s.pool.isConnected();
+};
+
+/**
+ * Figure out if the server instance was destroyed by calling destroy
+ * @method
+ * @return {boolean}
+ */
+Server.prototype.isDestroyed = function() {
+  if (!this.s.pool) return false;
+  return this.s.pool.isDestroyed();
+};
+
+function basicWriteValidations(self) {
+  if (!self.s.pool) return new MongoError('server instance is not connected');
+  if (self.s.pool.isDestroyed()) return new MongoError('server instance pool was destroyed');
+}
+
+function basicReadValidations(self, options) {
+  basicWriteValidations(self, options);
+
+  if (options.readPreference && !(options.readPreference instanceof ReadPreference)) {
+    throw new Error('readPreference must be an instance of ReadPreference');
+  }
+}
+
+/**
+ * Execute a command
+ * @method
+ * @param {string} ns The MongoDB fully qualified namespace (ex: db1.collection1)
+ * @param {object} cmd The command hash
+ * @param {ReadPreference} [options.readPreference] Specify read preference if command supports it
+ * @param {Boolean} [options.serializeFunctions=false] Specify if functions on an object should be serialized.
+ * @param {Boolean} [options.checkKeys=false] Specify if the bson parser should validate keys.
+ * @param {Boolean} [options.ignoreUndefined=false] Specify if the BSON serializer should ignore undefined fields.
+ * @param {Boolean} [options.fullResult=false] Return the full envelope instead of just the result document.
+ * @param {ClientSession} [options.session=null] Session to use for the operation
+ * @param {opResultCallback} callback A callback function
+ */
+Server.prototype.command = function(ns, cmd, options, callback) {
+  var self = this;
+  if (typeof options === 'function') {
+    (callback = options), (options = {}), (options = options || {});
+  }
+
+  var result = basicReadValidations(self, options);
+  if (result) return callback(result);
+
+  // Clone the options
+  options = Object.assign({}, options, { wireProtocolCommand: false });
+
+  // Debug log
+  if (self.s.logger.isDebug())
+    self.s.logger.debug(
+      f(
+        'executing command [%s] against %s',
+        JSON.stringify({
+          ns: ns,
+          cmd: cmd,
+          options: debugOptions(debugFields, options)
+        }),
+        self.name
+      )
+    );
+
+  // If we are not connected or have a disconnectHandler specified
+  if (disconnectHandler(self, 'command', ns, cmd, options, callback)) return;
+
+  // error if collation not supported
+  if (collationNotSupported(this, cmd)) {
+    return callback(new MongoError(`server ${this.name} does not support collation`));
+  }
+
+  wireProtocol.command(self, ns, cmd, options, callback);
+};
+
+/**
+ * Execute a query against the server
+ *
+ * @param {string} ns The MongoDB fully qualified namespace (ex: db1.collection1)
+ * @param {object} cmd The command document for the query
+ * @param {object} options Optional settings
+ * @param {function} callback
+ */
+Server.prototype.query = function(ns, cmd, cursorState, options, callback) {
+  wireProtocol.query(this, ns, cmd, cursorState, options, callback);
+};
+
+/**
+ * Execute a `getMore` against the server
+ *
+ * @param {string} ns The MongoDB fully qualified namespace (ex: db1.collection1)
+ * @param {object} cursorState State data associated with the cursor calling this method
+ * @param {object} options Optional settings
+ * @param {function} callback
+ */
+Server.prototype.getMore = function(ns, cursorState, batchSize, options, callback) {
+  wireProtocol.getMore(this, ns, cursorState, batchSize, options, callback);
+};
+
+/**
+ * Execute a `killCursors` command against the server
+ *
+ * @param {string} ns The MongoDB fully qualified namespace (ex: db1.collection1)
+ * @param {object} cursorState State data associated with the cursor calling this method
+ * @param {function} callback
+ */
+Server.prototype.killCursors = function(ns, cursorState, callback) {
+  wireProtocol.killCursors(this, ns, cursorState, callback);
+};
+
+/**
+ * Insert one or more documents
+ * @method
+ * @param {string} ns The MongoDB fully qualified namespace (ex: db1.collection1)
+ * @param {array} ops An array of documents to insert
+ * @param {boolean} [options.ordered=true] Execute in order or out of order
+ * @param {object} [options.writeConcern={}] Write concern for the operation
+ * @param {Boolean} [options.serializeFunctions=false] Specify if functions on an object should be serialized.
+ * @param {Boolean} [options.ignoreUndefined=false] Specify if the BSON serializer should ignore undefined fields.
+ * @param {ClientSession} [options.session=null] Session to use for the operation
+ * @param {opResultCallback} callback A callback function
+ */
+Server.prototype.insert = function(ns, ops, options, callback) {
+  var self = this;
+  if (typeof options === 'function') {
+    (callback = options), (options = {}), (options = options || {});
+  }
+
+  var result = basicWriteValidations(self, options);
+  if (result) return callback(result);
+
+  // If we are not connected or have a disconnectHandler specified
+  if (disconnectHandler(self, 'insert', ns, ops, options, callback)) return;
+
+  // Setup the docs as an array
+  ops = Array.isArray(ops) ? ops : [ops];
+
+  // Execute write
+  return wireProtocol.insert(self, ns, ops, options, callback);
+};
+
+/**
+ * Perform one or more update operations
+ * @method
+ * @param {string} ns The MongoDB fully qualified namespace (ex: db1.collection1)
+ * @param {array} ops An array of updates
+ * @param {boolean} [options.ordered=true] Execute in order or out of order
+ * @param {object} [options.writeConcern={}] Write concern for the operation
+ * @param {Boolean} [options.serializeFunctions=false] Specify if functions on an object should be serialized.
+ * @param {Boolean} [options.ignoreUndefined=false] Specify if the BSON serializer should ignore undefined fields.
+ * @param {ClientSession} [options.session=null] Session to use for the operation
+ * @param {opResultCallback} callback A callback function
+ */
+Server.prototype.update = function(ns, ops, options, callback) {
+  var self = this;
+  if (typeof options === 'function') {
+    (callback = options), (options = {}), (options = options || {});
+  }
+
+  var result = basicWriteValidations(self, options);
+  if (result) return callback(result);
+
+  // If we are not connected or have a disconnectHandler specified
+  if (disconnectHandler(self, 'update', ns, ops, options, callback)) return;
+
+  // error if collation not supported
+  if (collationNotSupported(this, options)) {
+    return callback(new MongoError(`server ${this.name} does not support collation`));
+  }
+
+  // Setup the docs as an array
+  ops = Array.isArray(ops) ? ops : [ops];
+  // Execute write
+  return wireProtocol.update(self, ns, ops, options, callback);
+};
+
+/**
+ * Perform one or more remove operations
+ * @method
+ * @param {string} ns The MongoDB fully qualified namespace (ex: db1.collection1)
+ * @param {array} ops An array of removes
+ * @param {boolean} [options.ordered=true] Execute in order or out of order
+ * @param {object} [options.writeConcern={}] Write concern for the operation
+ * @param {Boolean} [options.serializeFunctions=false] Specify if functions on an object should be serialized.
+ * @param {Boolean} [options.ignoreUndefined=false] Specify if the BSON serializer should ignore undefined fields.
+ * @param {ClientSession} [options.session=null] Session to use for the operation
+ * @param {opResultCallback} callback A callback function
+ */
+Server.prototype.remove = function(ns, ops, options, callback) {
+  var self = this;
+  if (typeof options === 'function') {
+    (callback = options), (options = {}), (options = options || {});
+  }
+
+  var result = basicWriteValidations(self, options);
+  if (result) return callback(result);
+
+  // If we are not connected or have a disconnectHandler specified
+  if (disconnectHandler(self, 'remove', ns, ops, options, callback)) return;
+
+  // error if collation not supported
+  if (collationNotSupported(this, options)) {
+    return callback(new MongoError(`server ${this.name} does not support collation`));
+  }
+
+  // Setup the docs as an array
+  ops = Array.isArray(ops) ? ops : [ops];
+  // Execute write
+  return wireProtocol.remove(self, ns, ops, options, callback);
+};
+
+/**
+ * Get a new cursor
+ * @method
+ * @param {string} ns The MongoDB fully qualified namespace (ex: db1.collection1)
+ * @param {object|Long} cmd Can be either a command returning a cursor or a cursorId
+ * @param {object} [options] Options for the cursor
+ * @param {object} [options.batchSize=0] Batchsize for the operation
+ * @param {array} [options.documents=[]] Initial documents list for cursor
+ * @param {ReadPreference} [options.readPreference] Specify read preference if command supports it
+ * @param {Boolean} [options.serializeFunctions=false] Specify if functions on an object should be serialized.
+ * @param {Boolean} [options.ignoreUndefined=false] Specify if the BSON serializer should ignore undefined fields.
+ * @param {ClientSession} [options.session=null] Session to use for the operation
+ * @param {object} [options.topology] The internal topology of the created cursor
+ * @returns {Cursor}
+ */
+Server.prototype.cursor = function(ns, cmd, options) {
+  options = options || {};
+  const topology = options.topology || this;
+
+  // Set up final cursor type
+  var FinalCursor = options.cursorFactory || this.s.Cursor;
+
+  // Return the cursor
+  return new FinalCursor(topology, ns, cmd, options);
+};
+
+/**
+ * Compare two server instances
+ * @method
+ * @param {Server} server Server to compare equality against
+ * @return {boolean}
+ */
+Server.prototype.equals = function(server) {
+  if (typeof server === 'string') return this.name.toLowerCase() === server.toLowerCase();
+  if (server.name) return this.name.toLowerCase() === server.name.toLowerCase();
+  return false;
+};
+
+/**
+ * All raw connections
+ * @method
+ * @return {Connection[]}
+ */
+Server.prototype.connections = function() {
+  return this.s.pool.allConnections();
+};
+
+/**
+ * Selects a server
+ * @method
+ * @param {function} selector Unused
+ * @param {ReadPreference} [options.readPreference] Unused
+ * @param {ClientSession} [options.session] Unused
+ * @return {Server}
+ */
+Server.prototype.selectServer = function(selector, options, callback) {
+  if (typeof selector === 'function' && typeof callback === 'undefined')
+    (callback = selector), (selector = undefined), (options = {});
+  if (typeof options === 'function')
+    (callback = options), (options = selector), (selector = undefined);
+
+  callback(null, this);
+};
+
+var listeners = ['close', 'error', 'timeout', 'parseError', 'connect'];
+
+/**
+ * Destroy the server connection
+ * @method
+ * @param {boolean} [options.emitClose=false] Emit close event on destroy
+ * @param {boolean} [options.emitDestroy=false] Emit destroy event on destroy
+ * @param {boolean} [options.force=false] Force destroy the pool
+ */
+Server.prototype.destroy = function(options, callback) {
+  if (this._destroyed) {
+    if (typeof callback === 'function') callback(null, null);
+    return;
+  }
+
+  if (typeof options === 'function') {
+    callback = options;
+    options = {};
+  }
+
+  options = options || {};
+  var self = this;
+
+  // Set the connections
+  if (serverAccounting) delete servers[this.id];
+
+  // Destroy the monitoring process if any
+  if (this.monitoringProcessId) {
+    clearTimeout(this.monitoringProcessId);
+  }
+
+  // No pool, return
+  if (!self.s.pool) {
+    this._destroyed = true;
+    if (typeof callback === 'function') callback(null, null);
+    return;
+  }
+
+  // Emit close event
+  if (options.emitClose) {
+    self.emit('close', self);
+  }
+
+  // Emit destroy event
+  if (options.emitDestroy) {
+    self.emit('destroy', self);
+  }
+
+  // Remove all listeners
+  listeners.forEach(function(event) {
+    self.s.pool.removeAllListeners(event);
+  });
+
+  // Emit opening server event
+  if (self.listeners('serverClosed').length > 0)
+    self.emit('serverClosed', { topologyId: topologyId(self), address: self.name });
+
+  // Emit toplogy opening event if not in topology
+  if (self.listeners('topologyClosed').length > 0 && !self.s.inTopology) {
+    self.emit('topologyClosed', { topologyId: topologyId(self) });
+  }
+
+  if (self.s.logger.isDebug()) {
+    self.s.logger.debug(f('destroy called on server %s', self.name));
+  }
+
+  // Destroy the pool
+  this.s.pool.destroy(options.force, callback);
+  this._destroyed = true;
+};
+
+/**
+ * A server connect event, used to verify that the connection is up and running
+ *
+ * @event Server#connect
+ * @type {Server}
+ */
+
+/**
+ * A server reconnect event, used to verify that the server topology has reconnected
+ *
+ * @event Server#reconnect
+ * @type {Server}
+ */
+
+/**
+ * A server opening SDAM monitoring event
+ *
+ * @event Server#serverOpening
+ * @type {object}
+ */
+
+/**
+ * A server closed SDAM monitoring event
+ *
+ * @event Server#serverClosed
+ * @type {object}
+ */
+
+/**
+ * A server description SDAM change monitoring event
+ *
+ * @event Server#serverDescriptionChanged
+ * @type {object}
+ */
+
+/**
+ * A topology open SDAM event
+ *
+ * @event Server#topologyOpening
+ * @type {object}
+ */
+
+/**
+ * A topology closed SDAM event
+ *
+ * @event Server#topologyClosed
+ * @type {object}
+ */
+
+/**
+ * A topology structure SDAM change event
+ *
+ * @event Server#topologyDescriptionChanged
+ * @type {object}
+ */
+
+/**
+ * Server reconnect failed
+ *
+ * @event Server#reconnectFailed
+ * @type {Error}
+ */
+
+/**
+ * Server connection pool closed
+ *
+ * @event Server#close
+ * @type {object}
+ */
+
+/**
+ * Server connection pool caused an error
+ *
+ * @event Server#error
+ * @type {Error}
+ */
+
+/**
+ * Server destroyed was called
+ *
+ * @event Server#destroy
+ * @type {Server}
+ */
+
+module.exports = Server;

node_modules/mongodb/lib/core/topologies/shared.js

@@ -0,0 +1,456 @@
+'use strict';
+const ReadPreference = require('./read_preference');
+const TopologyType = require('../sdam/common').TopologyType;
+const MongoError = require('../error').MongoError;
+const isRetryableWriteError = require('../error').isRetryableWriteError;
+const maxWireVersion = require('../utils').maxWireVersion;
+const MongoNetworkError = require('../error').MongoNetworkError;
+const MMAPv1_RETRY_WRITES_ERROR_CODE = 20;
+
+/**
+ * Emit event if it exists
+ * @method
+ */
+function emitSDAMEvent(self, event, description) {
+  if (self.listeners(event).length > 0) {
+    self.emit(event, description);
+  }
+}
+
+function createCompressionInfo(options) {
+  if (!options.compression || !options.compression.compressors) {
+    return [];
+  }
+
+  // Check that all supplied compressors are valid
+  options.compression.compressors.forEach(function(compressor) {
+    if (compressor !== 'snappy' && compressor !== 'zlib') {
+      throw new Error('compressors must be at least one of snappy or zlib');
+    }
+  });
+
+  return options.compression.compressors;
+}
+
+function clone(object) {
+  return JSON.parse(JSON.stringify(object));
+}
+
+var getPreviousDescription = function(self) {
+  if (!self.s.serverDescription) {
+    self.s.serverDescription = {
+      address: self.name,
+      arbiters: [],
+      hosts: [],
+      passives: [],
+      type: 'Unknown'
+    };
+  }
+
+  return self.s.serverDescription;
+};
+
+var emitServerDescriptionChanged = function(self, description) {
+  if (self.listeners('serverDescriptionChanged').length > 0) {
+    // Emit the server description changed events
+    self.emit('serverDescriptionChanged', {
+      topologyId: self.s.topologyId !== -1 ? self.s.topologyId : self.id,
+      address: self.name,
+      previousDescription: getPreviousDescription(self),
+      newDescription: description
+    });
+
+    self.s.serverDescription = description;
+  }
+};
+
+var getPreviousTopologyDescription = function(self) {
+  if (!self.s.topologyDescription) {
+    self.s.topologyDescription = {
+      topologyType: 'Unknown',
+      servers: [
+        {
+          address: self.name,
+          arbiters: [],
+          hosts: [],
+          passives: [],
+          type: 'Unknown'
+        }
+      ]
+    };
+  }
+
+  return self.s.topologyDescription;
+};
+
+var emitTopologyDescriptionChanged = function(self, description) {
+  if (self.listeners('topologyDescriptionChanged').length > 0) {
+    // Emit the server description changed events
+    self.emit('topologyDescriptionChanged', {
+      topologyId: self.s.topologyId !== -1 ? self.s.topologyId : self.id,
+      address: self.name,
+      previousDescription: getPreviousTopologyDescription(self),
+      newDescription: description
+    });
+
+    self.s.serverDescription = description;
+  }
+};
+
+var changedIsMaster = function(self, currentIsmaster, ismaster) {
+  var currentType = getTopologyType(self, currentIsmaster);
+  var newType = getTopologyType(self, ismaster);
+  if (newType !== currentType) return true;
+  return false;
+};
+
+var getTopologyType = function(self, ismaster) {
+  if (!ismaster) {
+    ismaster = self.ismaster;
+  }
+
+  if (!ismaster) return 'Unknown';
+  if (ismaster.ismaster && ismaster.msg === 'isdbgrid') return 'Mongos';
+  if (ismaster.ismaster && !ismaster.hosts) return 'Standalone';
+  if (ismaster.ismaster) return 'RSPrimary';
+  if (ismaster.secondary) return 'RSSecondary';
+  if (ismaster.arbiterOnly) return 'RSArbiter';
+  return 'Unknown';
+};
+
+var inquireServerState = function(self) {
+  return function(callback) {
+    if (self.s.state === 'destroyed') return;
+    // Record response time
+    var start = new Date().getTime();
+
+    // emitSDAMEvent
+    emitSDAMEvent(self, 'serverHeartbeatStarted', { connectionId: self.name });
+
+    // Attempt to execute ismaster command
+    self.command('admin.$cmd', { ismaster: true }, { monitoring: true }, function(err, r) {
+      if (!err) {
+        // Legacy event sender
+        self.emit('ismaster', r, self);
+
+        // Calculate latencyMS
+        var latencyMS = new Date().getTime() - start;
+
+        // Server heart beat event
+        emitSDAMEvent(self, 'serverHeartbeatSucceeded', {
+          durationMS: latencyMS,
+          reply: r.result,
+          connectionId: self.name
+        });
+
+        // Did the server change
+        if (changedIsMaster(self, self.s.ismaster, r.result)) {
+          // Emit server description changed if something listening
+          emitServerDescriptionChanged(self, {
+            address: self.name,
+            arbiters: [],
+            hosts: [],
+            passives: [],
+            type: !self.s.inTopology ? 'Standalone' : getTopologyType(self)
+          });
+        }
+
+        // Updat ismaster view
+        self.s.ismaster = r.result;
+
+        // Set server response time
+        self.s.isMasterLatencyMS = latencyMS;
+      } else {
+        emitSDAMEvent(self, 'serverHeartbeatFailed', {
+          durationMS: latencyMS,
+          failure: err,
+          connectionId: self.name
+        });
+      }
+
+      // Peforming an ismaster monitoring callback operation
+      if (typeof callback === 'function') {
+        return callback(err, r);
+      }
+
+      // Perform another sweep
+      self.s.inquireServerStateTimeout = setTimeout(inquireServerState(self), self.s.haInterval);
+    });
+  };
+};
+
+//
+// Clone the options
+var cloneOptions = function(options) {
+  var opts = {};
+  for (var name in options) {
+    opts[name] = options[name];
+  }
+  return opts;
+};
+
+function Interval(fn, time) {
+  var timer = false;
+
+  this.start = function() {
+    if (!this.isRunning()) {
+      timer = setInterval(fn, time);
+    }
+
+    return this;
+  };
+
+  this.stop = function() {
+    clearInterval(timer);
+    timer = false;
+    return this;
+  };
+
+  this.isRunning = function() {
+    return timer !== false;
+  };
+}
+
+function Timeout(fn, time) {
+  var timer = false;
+  var func = () => {
+    if (timer) {
+      clearTimeout(timer);
+      timer = false;
+
+      fn();
+    }
+  };
+
+  this.start = function() {
+    if (!this.isRunning()) {
+      timer = setTimeout(func, time);
+    }
+    return this;
+  };
+
+  this.stop = function() {
+    clearTimeout(timer);
+    timer = false;
+    return this;
+  };
+
+  this.isRunning = function() {
+    return timer !== false;
+  };
+}
+
+function diff(previous, current) {
+  // Difference document
+  var diff = {
+    servers: []
+  };
+
+  // Previous entry
+  if (!previous) {
+    previous = { servers: [] };
+  }
+
+  // Check if we have any previous servers missing in the current ones
+  for (var i = 0; i < previous.servers.length; i++) {
+    var found = false;
+
+    for (var j = 0; j < current.servers.length; j++) {
+      if (current.servers[j].address.toLowerCase() === previous.servers[i].address.toLowerCase()) {
+        found = true;
+        break;
+      }
+    }
+
+    if (!found) {
+      // Add to the diff
+      diff.servers.push({
+        address: previous.servers[i].address,
+        from: previous.servers[i].type,
+        to: 'Unknown'
+      });
+    }
+  }
+
+  // Check if there are any severs that don't exist
+  for (j = 0; j < current.servers.length; j++) {
+    found = false;
+
+    // Go over all the previous servers
+    for (i = 0; i < previous.servers.length; i++) {
+      if (previous.servers[i].address.toLowerCase() === current.servers[j].address.toLowerCase()) {
+        found = true;
+        break;
+      }
+    }
+
+    // Add the server to the diff
+    if (!found) {
+      diff.servers.push({
+        address: current.servers[j].address,
+        from: 'Unknown',
+        to: current.servers[j].type
+      });
+    }
+  }
+
+  // Got through all the servers
+  for (i = 0; i < previous.servers.length; i++) {
+    var prevServer = previous.servers[i];
+
+    // Go through all current servers
+    for (j = 0; j < current.servers.length; j++) {
+      var currServer = current.servers[j];
+
+      // Matching server
+      if (prevServer.address.toLowerCase() === currServer.address.toLowerCase()) {
+        // We had a change in state
+        if (prevServer.type !== currServer.type) {
+          diff.servers.push({
+            address: prevServer.address,
+            from: prevServer.type,
+            to: currServer.type
+          });
+        }
+      }
+    }
+  }
+
+  // Return difference
+  return diff;
+}
+
+/**
+ * Shared function to determine clusterTime for a given topology
+ *
+ * @param {*} topology
+ * @param {*} clusterTime
+ */
+function resolveClusterTime(topology, $clusterTime) {
+  if (topology.clusterTime == null) {
+    topology.clusterTime = $clusterTime;
+  } else {
+    if ($clusterTime.clusterTime.greaterThan(topology.clusterTime.clusterTime)) {
+      topology.clusterTime = $clusterTime;
+    }
+  }
+}
+
+// NOTE: this is a temporary move until the topologies can be more formally refactored
+//       to share code.
+const SessionMixins = {
+  endSessions: function(sessions, callback) {
+    if (!Array.isArray(sessions)) {
+      sessions = [sessions];
+    }
+
+    // TODO:
+    //   When connected to a sharded cluster the endSessions command
+    //   can be sent to any mongos. When connected to a replica set the
+    //   endSessions command MUST be sent to the primary if the primary
+    //   is available, otherwise it MUST be sent to any available secondary.
+    //   Is it enough to use: ReadPreference.primaryPreferred ?
+    this.command(
+      'admin.$cmd',
+      { endSessions: sessions },
+      { readPreference: ReadPreference.primaryPreferred },
+      () => {
+        // intentionally ignored, per spec
+        if (typeof callback === 'function') callback();
+      }
+    );
+  }
+};
+
+function topologyType(topology) {
+  if (topology.description) {
+    return topology.description.type;
+  }
+
+  if (topology.type === 'mongos') {
+    return TopologyType.Sharded;
+  } else if (topology.type === 'replset') {
+    return TopologyType.ReplicaSetWithPrimary;
+  }
+
+  return TopologyType.Single;
+}
+
+const RETRYABLE_WIRE_VERSION = 6;
+
+/**
+ * Determines whether the provided topology supports retryable writes
+ *
+ * @param {Mongos|Replset} topology
+ */
+const isRetryableWritesSupported = function(topology) {
+  const maxWireVersion = topology.lastIsMaster().maxWireVersion;
+  if (maxWireVersion < RETRYABLE_WIRE_VERSION) {
+    return false;
+  }
+
+  if (!topology.logicalSessionTimeoutMinutes) {
+    return false;
+  }
+
+  if (topologyType(topology) === TopologyType.Single) {
+    return false;
+  }
+
+  return true;
+};
+
+const MMAPv1_RETRY_WRITES_ERROR_MESSAGE =
+  'This MongoDB deployment does not support retryable writes. Please add retryWrites=false to your connection string.';
+
+function getMMAPError(err) {
+  if (err.code !== MMAPv1_RETRY_WRITES_ERROR_CODE || !err.errmsg.includes('Transaction numbers')) {
+    return err;
+  }
+
+  // According to the retryable writes spec, we must replace the error message in this case.
+  // We need to replace err.message so the thrown message is correct and we need to replace err.errmsg to meet the spec requirement.
+  const newErr = new MongoError({
+    message: MMAPv1_RETRY_WRITES_ERROR_MESSAGE,
+    errmsg: MMAPv1_RETRY_WRITES_ERROR_MESSAGE,
+    originalError: err
+  });
+  return newErr;
+}
+
+// NOTE: only used for legacy topology types
+function legacyIsRetryableWriteError(err, topology) {
+  if (!(err instanceof MongoError)) {
+    return false;
+  }
+
+  // if pre-4.4 server, then add error label if its a retryable write error
+  if (
+    isRetryableWritesSupported(topology) &&
+    (err instanceof MongoNetworkError ||
+      (maxWireVersion(topology) < 9 && isRetryableWriteError(err)))
+  ) {
+    err.addErrorLabel('RetryableWriteError');
+  }
+
+  return err.hasErrorLabel('RetryableWriteError');
+}
+
+module.exports = {
+  SessionMixins,
+  resolveClusterTime,
+  inquireServerState,
+  getTopologyType,
+  emitServerDescriptionChanged,
+  emitTopologyDescriptionChanged,
+  cloneOptions,
+  createCompressionInfo,
+  clone,
+  diff,
+  Interval,
+  Timeout,
+  isRetryableWritesSupported,
+  getMMAPError,
+  topologyType,
+  legacyIsRetryableWriteError
+};