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Cluster Monitoring

On this page

  • Overview
  • Event Subscription Example
  • Event Descriptions
  • Example Event Documents
  • serverDescriptionChanged
  • serverHeartbeatStarted
  • serverHeartbeatSucceeded
  • serverHeartbeatFailed
  • serverOpening
  • serverClosed
  • topologyOpening
  • topologyClosed
  • topologyDescriptionChanged

This guide shows you how to monitor topology events in a MongoDB instance, replica set, or sharded cluster. The driver creates topology events, also known as Server Discovery and Monitoring (SDAM) events, when there is a change in the state of the instance or cluster that you connected to. For example, the driver creates an event when you establish a new connection or if the cluster elects a new primary.

The following sections demonstrate how to record topology changes in your application and explore the information provided in these events.

You can access one or more SDAM events using the driver by subscribing to them in your application. The following example demonstrates connecting to a replica set and subscribing to one of the SDAM events created by the MongoDB deployment:

/* Subscribe to SDAM event */
const { MongoClient } = require("mongodb");
// Replace the following with your MongoDB deployment's connection string
const uri = "mongodb+srv://<clusterUrl>/?replicaSet=rs&writeConcern=majority";
const client = new MongoClient(uri);
// Replace <event name> with the name of the event you are subscribing to
const eventName = "<event name>";
// Subscribe to a specified event and print a message when the event is received
client.on(eventName, event => {
console.log(`received ${eventName}: ${JSON.stringify(event, null, 2)}`);
});
async function run() {
try {
// Establish and verify connection to the database
await client.db("admin").command({ ping: 1 });
console.log("Connected successfully");
} finally {
// Close the database connection on completion or error
await client.close();
}
}
run().catch(console.dir);

You can subscribe to any of the following SDAM events:

Event Name
Description
serverOpening
Created when a connection to an instance opens.
serverClosed
Created when a connection to an instance closes.
serverDescriptionChanged
Created when an instance state changes (such as from secondary to primary).
topologyOpening
Created before attempting a connection to an instance.
topologyClosed
Created after all instance connections in the topology close.
topologyDescriptionChanged
Created when the topology changes, such as an election of a new primary or a mongos proxy disconnecting.
serverHeartbeatStarted
Created before issuing a hello command to a MongoDB instance.
serverHeartbeatSucceeded
Created when the hello command returns successfully from a MongoDB instance.
serverHeartbeatFailed
Created when a hello command issued to a specific MongoDB instance fails to return a successful response.

The following sections show sample output for each type of SDAM event.

ServerDescriptionChangedEvent {
topologyId: 0,
address: 'localhost:27017',
previousDescription: ServerDescription {
address: 'localhost:27017',
error: null,
roundTripTime: 0,
lastUpdateTime: 1571251089030,
lastWriteDate: null,
opTime: null,
type: 'Unknown',
minWireVersion: 0,
maxWireVersion: 0,
hosts: [],
passives: [],
arbiters: [],
tags: []
},
newDescription: ServerDescription {
address: 'localhost:27017',
error: null,
roundTripTime: 0,
lastUpdateTime: 1571251089051,
lastWriteDate: 2019-10-16T18:38:07.000Z,
opTime: { ts: Timestamp, t: 18 },
type: 'RSPrimary',
minWireVersion: 0,
maxWireVersion: 7,
maxBsonObjectSize: 16777216,
maxMessageSizeBytes: 48000000,
maxWriteBatchSize: 100000,
me: 'localhost:27017',
hosts: [ 'localhost:27017' ],
passives: [],
arbiters: [],
tags: [],
setName: 'rs',
setVersion: 1,
electionId: ObjectID,
primary: 'localhost:27017',
logicalSessionTimeoutMinutes: 30,
'$clusterTime': ClusterTime
}
}

The type field of the ServerDescription object in this event contains one of the following possible values:

Type
Description
Unknown
Unknown instance
Standalone
Standalone instance
Mongos
Mongos proxy instance
PossiblePrimary
At least one server recognizes this as the primary, but is not yet verified by all instances.
RSPrimary
Primary instance
RSSecondary
Secondary instance
RSArbiter
Arbiter instance
RSOther
See the RSGhost and RSOther specification for more details
RSGhost
See the RSGhost and RSOther specification for more details
ServerHeartbeatStartedEvent {
connectionId: 'localhost:27017'
}
ServerHeartbeatSucceededEvent {
duration: 1.939997,
reply:{
hosts: [ 'localhost:27017' ],
setName: 'rs',
setVersion: 1,
isWritablePrimary: true,
secondary: false,
primary: 'localhost:27017',
me: 'localhost:27017',
electionId: ObjectID,
lastWrite: {
opTime: { ts: [Timestamp], t: 18 },
lastWriteDate: 2019-10-16T18:38:17.000Z,
majorityOpTime: { ts: [Timestamp], t: 18 },
majorityWriteDate: 2019-10-16T18:38:17.000Z
},
maxBsonObjectSize: 16777216,
maxMessageSizeBytes: 48000000,
maxWriteBatchSize: 100000,
localTime: 2019-10-16T18:38:19.589Z,
logicalSessionTimeoutMinutes: 30,
minWireVersion: 0,
maxWireVersion: 7,
readOnly: false,
ok: 1,
operationTime: Timestamp,
'$clusterTime': ClusterTime
},
connectionId: 'localhost:27017'
}
ServerHeartbeatFailed {
duration: 20,
failure: MongoError('some error'),
connectionId: 'localhost:27017'
}
ServerOpeningEvent {
topologyId: 0,
address: 'localhost:27017'
}
ServerClosedEvent {
topologyId: 0,
address: 'localhost:27017'
}
TopologyOpeningEvent {
topologyId: 0
}
TopologyClosedEvent {
topologyId: 0
}
TopologyDescriptionChangedEvent {
topologyId: 0,
previousDescription: TopologyDescription {
type: 'ReplicaSetNoPrimary',
setName: null,
maxSetVersion: null,
maxElectionId: null,
servers: Map {
'localhost:27017' => ServerDescription
},
stale: false,
compatible: true,
compatibilityError: null,
logicalSessionTimeoutMinutes: null,
heartbeatFrequencyMS: 10000,
localThresholdMS: 15,
options: Object,
error: undefined,
commonWireVersion: null
},
newDescription: TopologyDescription {
type: 'ReplicaSetWithPrimary',
setName: 'rs',
maxSetVersion: 1,
maxElectionId: null,
servers: Map {
'localhost:27017' => ServerDescription
},
stale: false,
compatible: true,
compatibilityError: null,
logicalSessionTimeoutMinutes: 30,
heartbeatFrequencyMS: 10000,
localThresholdMS: 15,
options: Object,
error: undefined,
commonWireVersion: 7
}
}

The type field of the TopologyDescription object in this event contains one of the following possible values:

Type
Description
Single
Standalone instance
ReplicaSetWithPrimary
Replica set with a primary
ReplicaSetNoPrimary
Replica set with no primary
Sharded
Sharded cluster
Unknown
Unknown topology
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