feat: add saving last seen topology description COMPASS-4568 (#291)

Author: Anemy

lib/data-service.js

@@ -15,6 +15,12 @@ class DataService extends EventEmitter {
    */
   constructor(model) {
     super();
+
+    // Stores the most recent topology description from
+    // the server's SDAM events:
+    // https://github.com/mongodb/specifications/blob/master/source/server-discovery-and-monitoring/server-discovery-and-monitoring-monitoring.rst#events
+    this.lastSeenTopology = null;
+
     this.client = null;
     const NativeClient = require('./native-client');
     this.client = new NativeClient(model)
@@ -24,7 +30,10 @@ class DataService extends EventEmitter {
       .on('serverClosed', (evt) => this.emit('serverClosed', evt))
       .on('topologyOpening', (evt) => this.emit('topologyOpening', evt))
       .on('topologyClosed', (evt) => this.emit('topologyClosed', evt))
-      .on('topologyDescriptionChanged', (evt) => this.emit('topologyDescriptionChanged', evt));
+      .on('topologyDescriptionChanged', (evt) => {
+        this.lastSeenTopology = evt.newDescription;
+        this.emit('topologyDescriptionChanged', evt);
+      });
   }
 
   getConnectionOptions() {
@@ -470,6 +479,16 @@ class DataService extends EventEmitter {
     this.client.currentOp(includeAll, callback);
   }
 
+  /**
+   * Returns the most recent topology description from the server's SDAM events.
+   * https://github.com/mongodb/specifications/blob/master/source/server-discovery-and-monitoring/server-discovery-and-monitoring-monitoring.rst#events
+   *
+   * @returns {null | TopologyDescription} If the data service is connected to a mongos.
+   */
+  getLastSeenTopology() {
+    return this.lastSeenTopology;
+  }
+
   /**
    * Returns the result of serverStats.
    *

test/data-service.test.js

@@ -1,16 +1,16 @@
-var helper = require('./helper');
+const helper = require('./helper');
 
-var assert = helper.assert;
-var expect = helper.expect;
-var eventStream = helper.eventStream;
-var ObjectId = require('bson').ObjectId;
+const assert = helper.assert;
+const expect = helper.expect;
+const eventStream = helper.eventStream;
+const ObjectId = require('bson').ObjectId;
 
-var DataService = require('../lib/data-service');
+const DataService = require('../lib/data-service');
 
 describe('DataService', function() {
   this.slow(10000);
   this.timeout(20000);
-  var service = new DataService(helper.connection);
+  const service = new DataService(helper.connection);
 
   before(function(done) {
     service.connect(done);
@@ -756,6 +756,32 @@ describe('DataService', function() {
     });
   });
 
+  describe('#getLastSeenTopology', function() {
+    it('returns the server\'s toplogy description', function() {
+      const topology = service.getLastSeenTopology();
+
+      expect(topology.servers.has('127.0.0.1:27018')).to.equal(true);
+
+      expect(topology).to.deep.include({
+        compatibilityError: null,
+        compatible: true,
+        heartbeatFrequencyMS: 10000,
+        localThresholdMS: 15,
+        logicalSessionTimeoutMinutes: 30,
+        maxElectionId: null,
+        maxSetVersion: null,
+        stale: false,
+        type: 'Single',
+        setName: null
+      });
+    });
+
+    it('it returns null when a topology description event hasn\'t yet occured', function() {
+      const testService = new DataService(helper.connection);
+      expect(testService.getLastSeenTopology()).to.equal(null);
+    });
+  });
+
   describe('#updateMany', function() {
     after(function(done) {
       helper.deleteTestDocuments(service.client, function() {