Manage Connections with Connection Pools

maxPoolSize

In addition to the sockets needed to support your application's requests, each MongoClient instance opens up to two connections per server in your MongoDB topology for monitoring the server's state.

For example, a client connected to a three-node replica set opens six monitoring sockets. If the application uses the default setting for maxPoolSize and only queries the primary (default) node, then there can be at most 106 open sockets and 100 connections in the connection pool. If the application uses a read preference to query the secondary nodes, those connection pools grow and there can be 306 total connections including the open monitoring sockets.

To support high numbers of concurrent MongoDB requests within one process, you can increase maxPoolSize.

The following code creates a MongoClient instance with a maximum connection pool size of 200 by specifying the maxPoolSize option in the options object:

const { MongoClient } = require('mongodb');
const uri = '<connection-string>';
const client = new MongoClient(uri, {
  maxPoolSize: 200
});

maxConnecting

Connection pools rate-limit connection establishment. The maxConnecting option determines the number of connections that the pool can create in parallel at any time. For example, if the value of maxConnecting is 2, the third request that attempts to concurrently check out a connection succeeds only when one the following cases occurs:

• The connection pool finishes creating a connection and there are fewer than maxPoolSize connections in the pool.

• An existing connection is checked back into the pool.

The following code creates a MongoClient instance with a maximum number of 2 connections to be established concurrently per pool by specifying the maxConnecting option in the options object:

const { MongoClient } = require('mongodb');
const uri = '<connection-string>';
const client = new MongoClient(uri, {
   maxConnecting: 2
});

minPoolSize

You can set the minimum number of connections to each server with the minPoolSize option. The driver ensures that there are always at least the number of connections set by the minPoolSize option in the connection pool. If sockets are closed, causing the total number of sockets (both in use and idle) to drop below the minimum, more sockets are opened until the minimum is reached.

The following code creates a MongoClient instance with a minimum connnection pool size of 10 by specifying the minPoolSize option in the options object:

const { MongoClient } = require('mongodb');
const uri = '<connection-string>';
const client = new MongoClient(uri, {
  minPoolSize: 10
});

maxIdleTimeMS

You can set the maximum number of milliseconds that a connection can remain idle in the pool by setting the maxIdleTimeMS option. Once a connection has been idle for maxIdleTimeMS, the connection pool removes and replaces it. This option defaults to 0 (no limit).

The following code creates a MongoClient instance with a maximum idle time of 10000 milliseconds (10 seconds) by specifying the maxIdleTimeMS setting in the options object:

const { MongoClient } = require('mongodb');
const uri = '<connection-string>';
const client = new MongoClient(uri, {
  maxIdleTimeMS: 10000
});

waitQueueTimeoutMS

MongoClient supports multiple concurrent requests. For each process, create a client and reuse it for all operations in a process. This practice is more efficient than creating a client for each request.

The driver does not limit the number of requests that can wait for sockets to become available, and it is the application's responsibility to limit the size of its pool to bound queuing during a load spike. Requests wait for the amount of time specified in the waitQueueTimeoutMS option, which defaults to 0 (no limit).

A request that waits more than the length of time defined by waitQueueTimeoutMS for a socket raises a connection error. Use this option if it is more important to bound the duration of operations during a load spike than it is to complete every operation.

The following code creates a MongoClient instance with a maximum wait queue timeout of 10000 milliseconds (10 seconds) by declaring it in the options object:

const { MongoClient } = require('mongodb');
const uri = '<connection-string>';
const client = new MongoClient(uri, {
  waitQueueTimeoutMS: 10000
});