Connection Pool Monitoring

The Node.js driver 3.5.0 or higher features Connection Pool Monitoring events, allowing an application or tool to monitor the internal workings of the driver’s connection pool.

NOTE: Connection pool monitoring is only available when the “Unified Topology” is enabled

Overview of CMAP events

Event	Description
connectionPoolCreated	Emitted when a connection pool is created
connectionPoolClosed	Emitted when a connection pool is closed, prior to server instance destruction
connectionCreated	Emitted when a connection is created, but not necessarily when it is used for an operation
connectionReady	Emitted after a connection has successfully completed a handshake, and is ready to be used for operations
connectionClosed	Emitted when a connection is closed
connectionCheckOutStarted	Emitted when an operation attempts to acquire a connection for execution
connectionCheckOutFailed	Emitted when an operation fails to acquire a connection for execution
connectionCheckedOut	Emitted when an operation successfully acquires a connection for execution
connectionCheckedIn	Emitted when a connection is returned to the pool after operation execution
connectionPoolCleared	Emitted when the connection pool’s generation count is increased

Simple Code Example

The following example demonstrates connecting to a replica set and printing out all CMAP related events:

const MongoClient = require('mongodb').MongoClient;
const url = 'mongodb://localhost:31000,localhost:31001/?replicaSet=rs';
const client = new MongoClient(url);

client.on('connectionPoolCreated', event => console.dir(event));
client.on('connectionPoolClosed', event => console.dir(event));
client.on('connectionCreated', event => console.dir(event));
client.on('connectionReady', event => console.dir(event));
client.on('connectionClosed', event => console.dir(event));
client.on('connectionCheckOutStarted', event => console.dir(event));
client.on('connectionCheckOutFailed', event => console.dir(event));
client.on('connectionCheckedOut', event => console.dir(event));
client.on('connectionCheckedIn', event => console.dir(event));
client.on('connectionPoolCleared', event => console.dir(event));

client.connect((err, client) => {
  if (err) throw err;
});

Example Events

connectionPoolCreated

ConnectionPoolCreatedEvent {
  time: 2020-01-14T13:46:15.536Z,
  address: 'localhost:31003',
  options: { ... }
}

connectionPoolClosed

ConnectionPoolClosedEvent {
  time: 2020-01-14T13:54:53.570Z,
  address: '127.0.0.1:34849'
}

connectionCreated

ConnectionCreatedEvent {
  time: 2020-01-14T13:54:53.579Z,
  address: '127.0.0.1:34849',
  connectionId: 1
}

connectionReady

ConnectionReadyEvent {
  time: 2020-01-14T13:54:53.579Z,
  address: '127.0.0.1:34849',
  connectionId: 1
}

connectionClosed

ConnectionClosedEvent {
  time: 2020-01-14T13:54:53.564Z,
  address: '127.0.0.1:34849',
  connectionId: 2,
  reason: ...
}

connectionCheckOutStarted

ConnectionCheckOutStartedEvent {
  time: 2020-01-14T13:49:59.271Z,
  address: 'localhost:31000'
}

connectionCheckOutFailed

ConnectionCheckOutFailedEvent {
  time: 2020-01-14T13:49:59.271Z,
  address: 'localhost:31000'
  reason: ...
}

connectionCheckedOut

ConnectionCheckedOutEvent {
  time: 2020-01-14T13:48:42.541Z,
  address: 'localhost:31000',
  connectionId: 1
}

connectionCheckedIn

ConnectionCheckedInEvent {
  time: 2020-01-14T13:48:42.543Z,
  address: 'localhost:31000',
  connectionId: 1
}

connectionPoolCleared

ConnectionPoolClearedEvent {
  time: 2020-01-14T13:58:11.437Z,
  address: '127.0.0.1:45005'
}