Fundamentals

Monitoring¶

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Overview
Event Subscription Example
Event Descriptions
Example Event Documents

Overview¶

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.

Read this guide if you need to record topology changes in your application or want to explore the information provided in these events.

Event Subscription Example¶

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:

const { MongoClient } = require("mongodb");

// Replace the following with your MongoDB deployment's connection
// string.
const uri =
  "mongodb+srv://<clusterUrl>/?replicaSet=rs&w=majority";

const client = new MongoClient(uri);

// Replace <event name> with the name of the event you are subscribing to.
const eventName = "<event name>";
client.on(eventName, event => {
  console.log(`received ${eventName}: ${JSON.stringify(event, null, 2)}`);
});

async function run() {
  try {
    await client.connect();

    // Establish and verify connection
    await client.db("admin").command({ ping: 1 });
    console.log("Connected successfully");
  } finally {
    // Ensures that the client will close when you finish/error
    await client.close();
  }
}
run().catch(console.dir);

Event Descriptions¶

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 prior to 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. Not applicable to a single instance.
`serverHeartbeatStarted`	Created prior to issuing an `isMaster` command to a MongoDB instance. Not applicable to a single instance.
`serverHeartbeatSucceeded`	Created when the `isMaster` command returns successfully from a MongoDB instance. Not applicable to a single instance.
`serverHeartbeatFailed`	Created when an `isMaster` command issued to a specific MongoDB instance fails to return a successful response Not applicable to a single instance.

Example Event Documents¶

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

`serverDescriptionChanged`¶

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 specification for more details
`RSGhost`	See the RSOther specification for more details

serverHeartbeatStarted¶

ServerHeartbeatStartedEvent {
  connectionId: 'localhost:27017'
}

serverHeartbeatSucceeded¶

ServerHeartbeatSucceededEvent {
  duration: 1.939997,
  reply:{
    hosts: [ 'localhost:27017' ],
    setName: 'rs',
    setVersion: 1,
    ismaster: 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¶

ServerHeartbeatFailed {
  duration: 20,
  failure: MongoError('some error'),
  connectionId: 'localhost:27017'
}

serverOpening¶

ServerOpeningEvent {
  topologyId: 0,
  address: 'localhost:27017'
}

serverClosed¶

ServerClosedEvent {
  topologyId: 0,
  address: 'localhost:27017'
}

topologyOpening¶

TopologyOpeningEvent {
  topologyId: 0
}

topologyClosed¶

TopologyClosedEvent {
  topologyId: 0
}

topologyDescriptionChanged¶

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`	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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