@@ -55,6 +55,10 @@ 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
'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'),
  createClientInfo = require('./shared').createClientInfo,
  createCompressionInfo = require('./shared').createCompressionInfo,
  resolveClusterTime = require('./shared').resolveClusterTime,
  SessionMixins = require('./shared').SessionMixins,
  relayEvents = require('../utils').relayEvents;

const collationNotSupported = require('../utils').collationNotSupported;

// 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();

/**
 * 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=300000] 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: 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,
    // Topology id
    topologyId: -1,
    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';
  // Set the client info
  this.clientInfo = createClientInfo(options);

  // 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;
  }
});

// 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: self.id });
  }

  // Emit opening server event
  self.emit('serverOpening', {
    topologyId: self.s.topologyId !== -1 ? self.s.topologyId : self.id,
    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: self.s.topologyId !== -1 ? self.s.topologyId : self.id,
      address: self.name
    });

  // Emit toplogy opening event if not in topology
  if (self.listeners('topologyClosed').length > 0 && !self.s.inTopology) {
    self.emit('topologyClosed', { topologyId: self.id });
  }

  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;

is there ever a chance that `server.s` is null? Mu guess is no, but idk.