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Topology Management

The Node.js driver 2.1.10 or higher features SDAM Monitoring events, allowing an application or tool to monitor changes in the drivers view of a single server, replica set or mongos. This allows an application to react to changes of topology, such as a secondary joining or leaving a replica set.

Overview of SDAM events

Event Applies To Description
serverOpening Server, Replicaset, Mongos Emitted when server connection is established.
serverClosed Server, Replicaset, Mongos Emitted when server connection gets closed.
serverDescriptionChanged Server, Replicaset, Mongos Emitted when server state changes (such as from secondary to primary).
topologyOpening Server, Replicaset, Mongos Emitted before any server connections are performed.
topologyClosed Server, Replicaset, Mongos Emitted when topology connections have all closed.
topologyDescriptionChanged Replicaset, Mongos Emitted when the topology shape changes, such as a new primary being elected or a mongos proxy disconnecting.
serverHeartbeatStarted Replicaset, Mongos Emitted before the ismaster command is issued to a MongoDB server.
serverHeartbeatSucceeded Replicaset, Mongos Emitted after a successful ismaster command was issued to a MongoDB server.
serverHeartbeatFailed Replicaset, Mongos Emitted if a ismaster command failed against a specific MongoDB server.

Simple Code Example

The following example demonstrates how to connect to a replica set and monitor all the events that are emitted by the replica set topology.

const MongoClient = require('mongodb').MongoClient;

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

client.on('serverDescriptionChanged', function(event) {
  console.log('received serverDescriptionChanged');
  console.log(JSON.stringify(event, null, 2));
});

client.on('serverHeartbeatStarted', function(event) {
  console.log('received serverHeartbeatStarted');
  console.log(JSON.stringify(event, null, 2));
});

client.on('serverHeartbeatSucceeded', function(event) {
  console.log('received serverHeartbeatSucceeded');
  console.log(JSON.stringify(event, null, 2));
});

client.on('serverHeartbeatFailed', function(event) {
  console.log('received serverHeartbeatFailed');
  console.log(JSON.stringify(event, null, 2));
});

client.on('serverOpening', function(event) {
  console.log('received serverOpening');
  console.log(JSON.stringify(event, null, 2));
});

client.on('serverClosed', function(event) {
  console.log('received serverClosed');
  console.log(JSON.stringify(event, null, 2));
});

client.on('topologyOpening', function(event) {
  console.log('received topologyOpening');
  console.log(JSON.stringify(event, null, 2));
});

client.on('topologyClosed', function(event) {
  console.log('received topologyClosed');
  console.log(JSON.stringify(event, null, 2));
});

client.on('topologyDescriptionChanged', function(event) {
  console.log('received topologyDescriptionChanged');
  console.log(JSON.stringify(event, null, 2));
});

client.connect(url, function(err, client) {
  if(err) throw err;
});

Example Documents Returned For Each Event Type

The following examples serve as a guide to the format of the returned documents.

serverDescriptionChanged

{ address: 'localhost:27017',
  arbiters: [],
  hosts: [],
  passives: [],
  type: 'RSPrimary' }

The type can be one of the following values.

Type Description
RSPrimary Primary server
RSSecondary Secondary server
RSArbiter Arbiter
Standalone Standalone server
Unknown Unknown server
Mongos Mongos proxy

serverHeartbeatStarted

{ connectionId: 'localhost:27017' }

serverHeartbeatSucceeded

{ durationMS: 20,
  reply: {
    setName: "rs", setVersion: 1, electionId: new ObjectId(),
    maxBsonObjectSize : 16777216, maxMessageSizeBytes : 48000000,
    maxWriteBatchSize : 1000, localTime : new Date(),
    maxWireVersion : 4, minWireVersion : 0, ok : 1,
    hosts: ["localhost:32000", "localhost:32001"],
    arbiters: ["localhost:32002"]
  },
  connectionId: 'localhost:27017' }

serverHeartbeatFailed

{ durationMS: 20,
  err: new MongoError('some error'),
  connectionId: 'localhost:27017' }

serverOpening

{ topologyId: 0, name: 'localhost:27017' }

serverClosed

{ topologyId: 0, name: 'localhost:27017' }

topologyOpening

{ topologyId: 0 }

topologyClosed

{ topologyId: 0 }

topologyDescriptionChanged

{
  topologyId: 0,
  previousDescription: {
    topologyType: "ReplicaSetNoPrimary",
    setName: "rs",
    servers: [
      {
        type: "RSSecondary",
        address: "localhost:32001",
        hosts: [
          "localhost:32000",
          "localhost:32001"
        ],
        arbiters: [
          "localhost:32002"
        ],
        setName: "rs"
      },
      {
        type: "RSSecondary",
        address: "localhost:32000",
        hosts: [
          "localhost:32000",
          "localhost:32001"
        ],
        arbiters: [
          "localhost:32002"
        ],
        setName: "rs"
      },
      {
        type: "RSArbiter",
        address: "localhost:32002",
        hosts: [
          "localhost:32000",
          "localhost:32001"
        ],
        arbiters: [
          "localhost:32002"
        ],
        setName: "rs"
      }
    ]
  },
  newDescription: {
    topologyType: "ReplicaSetWithPrimary",
    setName: "rs",
    servers: [
      {
        type: "RSPrimary",
        address: "localhost:32001",
        hosts: [
          "localhost:32000",
          "localhost:32001"
        ],
        arbiters: [
          "localhost:32002"
        ],
        setName: "rs"
      },
      {
        type: "RSSecondary",
        address: "localhost:32000",
        hosts: [
          "localhost:32000",
          "localhost:32001"
        ],
        arbiters: [
          "localhost:32002"
        ],
        setName: "rs"
      },
      {
        type: "RSArbiter",
        address: "localhost:32002",
        hosts: [
          "localhost:32000",
          "localhost:32001"
        ],
        arbiters: [
          "localhost:32002"
        ],
        setName: "rs"
      }
    ]
  },
  diff: {
    servers: [{
        "address": "localhost:32000",
        "from": "RSSecondary",
        "to": "RSPrimary"
      }
    ]
  }  
}

The type field in the server array documents can be one of the following values:

Type Description
RSPrimary Primary server
RSSecondary Secondary server
RSArbiter Arbiter
Standalone Standalone server
Unknown Unknown server

The topologyType field can be one of the following values:

Type Description
ReplicaSetWithPrimary Replica set with a primary
ReplicaSetNoPrimary Replica set with no primary
Unknown Unknown topology