You can connect to MongoDB with the mongoose.connect() method.

mongoose.connect('mongodb://localhost:27017/myapp', {useNewUrlParser: true});

This is the minimum needed to connect the myapp database running locally on the default port (27017). If connecting fails on your machine, try using instead of localhost.

You can also specify several more parameters in the uri:

mongoose.connect('mongodb://username:[email protected]:port/database?options...', {useNewUrlParser: true});

See the mongodb connection string spec for more detail.

Operation Buffering

Mongoose lets you start using your models immediately, without waiting for mongoose to establish a connection to MongoDB.

mongoose.connect('mongodb://localhost:27017/myapp', {useNewUrlParser: true});
var MyModel = mongoose.model('Test', new Schema({ name: String }));
// Works
MyModel.findOne(function(error, result) { /* ... */ });

That's because mongoose buffers model function calls internally. This buffering is convenient, but also a common source of confusion. Mongoose will not throw any errors by default if you use a model without connecting.

var MyModel = mongoose.model('Test', new Schema({ name: String }));
// Will just hang until mongoose successfully connects
MyModel.findOne(function(error, result) { /* ... */ });

setTimeout(function() {
  mongoose.connect('mongodb://localhost:27017/myapp', {useNewUrlParser: true});
}, 60000);

To disable buffering, turn off the bufferCommands option on your schema. If you have bufferCommands on and your connection is hanging, try turning bufferCommands off to see if you haven't opened a connection properly. You can also disable bufferCommands globally:

mongoose.set('bufferCommands', false);

Error Handling

There are two classes of errors that can occur with a Mongoose connection.

  • Error on initial connection. If initial connection fails, Mongoose will not attempt to reconnect, it will emit an 'error' event, and the promise mongoose.connect() returns will reject.
  • Error after initial connection was established. Mongoose will attempt to reconnect, and it will emit an 'error' event.

To handle initial connection errors, you should use .catch() or try/catch with async/await.

mongoose.connect('mongodb://localhost:27017/test', { useNewUrlParser: true }).
  catch(error => handleError(error));

// Or:
try {
  await mongoose.connect('mongodb://localhost:27017/test', { useNewUrlParser: true });
} catch (error) {

To handle errors after initial connection was established, you should listen for error events on the connection. However, you still need to handle initial connection errors as shown above.

mongoose.connection.on('error', err => {


The connect method also accepts an options object which will be passed on to the underlying MongoDB driver.

mongoose.connect(uri, options);

A full list of options can be found on the MongoDB Node.js driver docs for connect(). Mongoose passes options to the driver without modification, modulo a few exceptions that are explained below.

  • bufferCommands - This is a mongoose-specific option (not passed to the MongoDB driver) that disables mongoose's buffering mechanism
  • user/pass - The username and password for authentication. These options are mongoose-specific, they are equivalent to the MongoDB driver's auth.user and auth.password options.
  • autoIndex - By default, mongoose will automatically build indexes defined in your schema when it connects. This is great for development, but not ideal for large production deployments, because index builds can cause performance degradation. If you set autoIndex to false, mongoose will not automatically build indexes for any model associated with this connection.
  • dbName - Specifies which database to connect to and overrides any database specified in the connection string. This is useful if you are unable to specify a default database in the connection string like with some mongodb+srv syntax connections.

Below are some of the options that are important for tuning Mongoose.

  • useNewUrlParser - The underlying MongoDB driver has deprecated their current connection string parser. Because this is a major change, they added the useNewUrlParser flag to allow users to fall back to the old parser if they find a bug in the new parser. You should set useNewUrlParser: true unless that prevents you from connecting. Note that if you specify useNewUrlParser: true, you must specify a port in your connection string, like mongodb://localhost:27017/dbname. The new url parser does not support connection strings that do not have a port, like mongodb://localhost/dbname.
  • useCreateIndex - False by default. Set to true to make Mongoose's default index build use createIndex() instead of ensureIndex() to avoid deprecation warnings from the MongoDB driver.
  • useFindAndModify - True by default. Set to false to make findOneAndUpdate() and findOneAndRemove() use native findOneAndUpdate() rather than findAndModify().
  • useUnifiedTopology- False by default. Set to true to opt in to using the MongoDB driver's new connection management engine. You should set this option to true, except for the unlikely case that it prevents you from maintaining a stable connection.
  • promiseLibrary - Sets the underlying driver's promise library.
  • poolSize - The maximum number of sockets the MongoDB driver will keep open for this connection. By default, poolSize is 5. Keep in mind that, as of MongoDB 3.4, MongoDB only allows one operation per socket at a time, so you may want to increase this if you find you have a few slow queries that are blocking faster queries from proceeding. See Slow Trains in MongoDB and Node.js.
  • connectTimeoutMS - How long the MongoDB driver will wait before killing a socket due to inactivity during initial connection. Defaults to 30000. This option is passed transparently to Node.js' socket#setTimeout() function.
  • socketTimeoutMS - How long the MongoDB driver will wait before killing a socket due to inactivity after initial connection. A socket may be inactive because of either no activity or a long-running operation. This is set to 30000 by default, you should set this to 2-3x your longest running operation if you expect some of your database operations to run longer than 20 seconds. This option is passed to Node.js socket#setTimeout() function after the MongoDB driver successfully completes.
  • family - Whether to connect using IPv4 or IPv6. This option passed to Node.js' dns.lookup() function. If you don't specify this option, the MongoDB driver will try IPv6 first and then IPv4 if IPv6 fails. If your mongoose.connect(uri) call takes a long time, try mongoose.connect(uri, { family: 4 })

The following options are important for tuning Mongoose only if you are running without the useUnifiedTopology option:

  • autoReconnect - The underlying MongoDB driver will automatically try to reconnect when it loses connection to MongoDB. Unless you are an extremely advanced user that wants to manage their own connection pool, do not set this option to false.
  • reconnectTries - If you're connected to a single server or mongos proxy (as opposed to a replica set), the MongoDB driver will try to reconnect every reconnectInterval milliseconds for reconnectTries times, and give up afterward. When the driver gives up, the mongoose connection emits a reconnectFailed event. This option does nothing for replica set connections.
  • reconnectInterval - See reconnectTries
  • bufferMaxEntries - The MongoDB driver also has its own buffering mechanism that kicks in when the driver is disconnected. Set this option to 0 and set bufferCommands to false on your schemas if you want your database operations to fail immediately when the driver is not connected, as opposed to waiting for reconnection.


const options = {
  useNewUrlParser: true,
  useCreateIndex: true,
  useFindAndModify: false,
  autoIndex: false, // Don't build indexes
  reconnectTries: Number.MAX_VALUE, // Never stop trying to reconnect
  reconnectInterval: 500, // Reconnect every 500ms
  poolSize: 10, // Maintain up to 10 socket connections
  // If not connected, return errors immediately rather than waiting for reconnect
  bufferMaxEntries: 0,
  connectTimeoutMS: 10000, // Give up initial connection after 10 seconds
  socketTimeoutMS: 45000, // Close sockets after 45 seconds of inactivity
  family: 4 // Use IPv4, skip trying IPv6
mongoose.connect(uri, options);

See this page for more information about connectTimeoutMS and socketTimeoutMS


The connect() function also accepts a callback parameter and returns a promise.

mongoose.connect(uri, options, function(error) {
  // Check error in initial connection. There is no 2nd param to the callback.

// Or using promises
mongoose.connect(uri, options).then(
  () => { /** ready to use. The `mongoose.connect()` promise resolves to mongoose instance. */ },
  err => { /** handle initial connection error */ }

Connection String Options

You can also specify driver options in your connection string as parameters in the query string portion of the URI. This only applies to options passed to the MongoDB driver. You can't set Mongoose-specific options like bufferCommands in the query string.

// The above is equivalent to:
mongoose.connect('mongodb://localhost:27017/test', {
  connectTimeoutMS: 1000
  // Note that mongoose will **not** pull `bufferCommands` from the query string

The disadvantage of putting options in the query string is that query string options are harder to read. The advantage is that you only need a single configuration option, the URI, rather than separate options for socketTimeoutMS, connectTimeoutMS, etc. Best practice is to put options that likely differ between development and production, like replicaSet or ssl, in the connection string, and options that should remain constant, like connectTimeoutMS or poolSize, in the options object.

The MongoDB docs have a full list of supported connection string options

Connection Events

Connections inherit from Node.js' EventEmitter class, and emit events when something happens to the connection, like losing connectivity to the MongoDB server. Below is a list of events that a connection may emit.

  • connecting: Emitted when Mongoose starts making its initial connection to the MongoDB server
  • connected: Emitted when Mongoose successfully makes its initial connection to the MongoDB server
  • open: Equivalent to connected
  • disconnecting: Your app called Connection#close() to disconnect from MongoDB
  • disconnected: Emitted when Mongoose lost connection to the MongoDB server. This event may be due to your code explicitly closing the connection, the database server crashing, or network connectivity issues.
  • close: Emitted after Connection#close() successfully closes the connection. If you call conn.close(), you'll get both a 'disconnected' event and a 'close' event.
  • reconnected: Emitted if Mongoose lost connectivity to MongoDB and successfully reconnected. Mongoose attempts to automatically reconnect when it loses connection to the database.
  • error: Emitted if an error occurs on a connection, like a parseError due to malformed data or a payload larger than 16MB.
  • fullsetup: Emitted when you're connecting to a replica set and Mongoose has successfully connected to the primary and at least one secondary.
  • all: Emitted when you're connecting to a replica set and Mongoose has successfully connected to all servers specified in your connection string.
  • reconnectFailed: Emitted when you're connected to a standalone server and Mongoose has run out of reconnectTries. The MongoDB driver will no longer attempt to reconnect after this event is emitted. This event will never be emitted if you're connected to a replica set.

A note about keepAlive

For long running applications, it is often prudent to enable keepAlive with a number of milliseconds. Without it, after some period of time you may start to see "connection closed" errors for what seems like no reason. If so, after reading this, you may decide to enable keepAlive:

mongoose.connect(uri, { keepAlive: true, keepAliveInitialDelay: 300000 });

keepAliveInitialDelay is the number of milliseconds to wait before initiating keepAlive on the socket. keepAlive is true by default since mongoose 5.2.0.

Replica Set Connections

To connect to a replica set you pass a comma delimited list of hosts to connect to rather than a single host.

mongoose.connect('mongodb://[username:[email protected]]host1[:port1][,host2[:port2],...[,hostN[:portN]]][/[database][?options]]' [, options]);

For example:

mongoose.connect('mongodb://user:[email protected]:27017,host2.com:27017,host3.com:27017/testdb');

To connect to a single node replica set, specify the replicaSet option.


Server Selection

If you enable the useUnifiedTopology option, the underlying MongoDB driver will use server selection to connect to MongoDB and send operations to MongoDB. If the MongoDB driver can't find a server to send an operation to after serverSelectionTimeoutMS, you'll get the below error:

MongoTimeoutError: Server selection timed out after 30000 ms

You can configure the timeout using the serverSelectionTimeoutMS option to mongoose.connect():

mongoose.connect(uri, {
  useNewUrlParser: true,
  useUnifiedTopology: true,
  serverSelectionTimeoutMS: 5000 // Timeout after 5s instead of 30s

A MongoTimeoutError has a reason property that explains why server selection timed out. For example, if you're connecting to a standalone server with an incorrect password, reason will contain an "Authentication failed" error.

const mongoose = require('mongoose');

const uri = 'mongodb+srv://username:[email protected]/' +
// Prints "MongoError: bad auth Authentication failed."
mongoose.connect(uri, {
  useNewUrlParser: true,
  useUnifiedTopology: true,
  serverSelectionTimeoutMS: 5000
}).catch(err => console.log(err.reason));

Replica Set Host Names

MongoDB replica sets rely on being able to reliably figure out the domain name for each member. On Linux and OSX, the MongoDB server uses the output of the hostname command to figure out the domain name to report to the replica set. This can cause confusing errors if you're connecting to a remote MongoDB replica set running on a machine that reports its hostname as localhost:

// Can get this error even if your connection string doesn't include
// `localhost` if `rs.conf()` reports that one replica set member has
// `localhost` as its host name.
failed to connect to server [localhost:27017] on first connect

If you're experiencing a similar error, connect to the replica set using the mongo shell and run the rs.conf() command to check the host names of each replica set member. Follow this page's instructions to change a replica set member's host name.

Multi-mongos support

You can also connect to multiple mongos instances for high availability in a sharded cluster. You do not need to pass any special options to connect to multiple mongos in mongoose 5.x.

// Connect to 2 mongos servers
mongoose.connect('mongodb://mongosA:27501,mongosB:27501', cb);

Multiple connections

So far we've seen how to connect to MongoDB using Mongoose's default connection. At times we may need multiple connections open to Mongo, each with different read/write settings, or maybe just to different databases for example. In these cases we can utilize mongoose.createConnection() which accepts all the arguments already discussed and returns a fresh connection for you.

const conn = mongoose.createConnection('mongodb://[username:[email protected]]host1[:port1][,host2[:port2],...[,hostN[:portN]]][/[database][?options]]', options);

This connection object is then used to create and retrieve models. Models are always scoped to a single connection.

const UserModel = conn.model('User', userSchema);

If you use multiple connections, you should make sure you export schemas, not models.

const userSchema = new Schema({ name: String, email: String });

// The correct pattern is to export a schema
module.exports = userSchema;

// Because if you export a model as shown below, the model will be scoped
// to Mongoose's default connection.
// module.exports = mongoose.model('User', userSchema);

In addition, you should define a factory function that registers models on a connection to make it easy to register all your models on a given connection.

const userSchema = require('./userSchema');

module.exports = conn => {
  conn.model('User', userSchema);

Mongoose creates a default connection when you call mongoose.connect(). You can access the default connection using mongoose.connection.

Connection Pools

Each connection, whether created with mongoose.connect or mongoose.createConnection are all backed by an internal configurable connection pool defaulting to a maximum size of 5. Adjust the pool size using your connection options:

// With object options
mongoose.createConnection(uri, { poolSize: 4 });

const uri = 'mongodb://localhost:27017/test?poolSize=4';

Option Changes in v5.x

You may see the following deprecation warning if upgrading from 4.x to 5.x and you didn't use the useMongoClient option in 4.x:

the server/replset/mongos options are deprecated, all their options are supported at the top level of the options object

In older version of the MongoDB driver you had to specify distinct options for server connections, replica set connections, and mongos connections:

mongoose.connect(myUri, {
  server: {
    socketOptions: {
      socketTimeoutMS: 0,
      keepAlive: true
    reconnectTries: 30
  replset: {
    socketOptions: {
      socketTimeoutMS: 0,
      keepAlive: true
    reconnectTries: 30
  mongos: {
    socketOptions: {
      socketTimeoutMS: 0,
      keepAlive: true
    reconnectTries: 30

In mongoose v5.x you can instead declare these options at the top level, without all that extra nesting. Here's the list of all supported options.

// Equivalent to the above code
mongoose.connect(myUri, {
  socketTimeoutMS: 0,
  keepAlive: true,
  reconnectTries: 30

Next Up

Now that we've covered connections, let's take a look at models.

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