fix(NODE-4188): default localThresholdMS to 15ms (#3207)

Author: baileympearson

src/sdam/topology_description.ts

@@ -63,7 +63,7 @@ export class TopologyDescription {
     this.stale = false;
     this.compatible = true;
     this.heartbeatFrequencyMS = options.heartbeatFrequencyMS ?? 0;
-    this.localThresholdMS = options.localThresholdMS ?? 0;
+    this.localThresholdMS = options.localThresholdMS ?? 15;
 
     if (setName) {
       this.setName = setName;

test/integration/server-discovery-and-monitoring/topology_description.test.ts

@@ -0,0 +1,32 @@
+import { expect } from 'chai';
+
+import { MongoClient, MongoClientOptions } from '../../../src/mongo_client';
+import { getTopology } from '../../../src/utils';
+
+describe('TopologyDescription (integration tests)', function () {
+  let client: MongoClient;
+
+  afterEach(async function () {
+    await client.close();
+  });
+
+  context('options', function () {
+    context('localThresholdMS', function () {
+      it('should default to 15ms', async function () {
+        const options: MongoClientOptions = {};
+        client = await this.configuration.newClient(options).connect();
+        const topologyDescription = getTopology(client).description;
+        expect(topologyDescription).to.have.ownProperty('localThresholdMS').to.equal(15);
+      });
+
+      it('should be set to the localThresholdMS option when it is passed in', async function () {
+        const options: MongoClientOptions = {
+          localThresholdMS: 30
+        };
+        client = await this.configuration.newClient(options).connect();
+        const topologyDescription = getTopology(client).description;
+        expect(topologyDescription).to.have.ownProperty('localThresholdMS').to.equal(30);
+      });
+    });
+  });
+});

test/unit/assorted/server_selection.spec.test.ts

@@ -11,11 +11,6 @@ describe('Server Selection Logic (spec)', function () {
       this.currentTest.skipReason = 'Nodejs driver does not support PossiblePrimary';
       this.skip();
     }
-
-    if (this.currentTest.title.match(/nearest_multiple/i)) {
-      this.currentTest.skipReason = 'TODO(NODE-4188): localThresholdMS should default to 15ms';
-      this.skip();
-    }
   });
 
   const selectionSpecDir = join(__dirname, '../../spec/server-selection/server_selection');

test/unit/sdam/server_selection.test.js

@@ -435,5 +435,78 @@ describe('server selection', function () {
         });
       });
     });
+
+    context('localThresholdMS is respected as an option', function () {
+      let serverDescription1, serverDescription2, serverDescription3, serverDescriptions;
+      beforeEach(() => {
+        serverDescription1 = new ServerDescription(
+          '127.0.0.1:27017',
+          {
+            setName: 'test',
+            isWritablePrimary: true,
+            ok: 1
+          },
+          { roundTripTime: 15 }
+        );
+        serverDescription2 = new ServerDescription(
+          '127.0.0.1:27018',
+          {
+            setName: 'test',
+            secondary: true,
+            ok: 1
+          },
+          { roundTripTime: 25 }
+        );
+        serverDescription3 = new ServerDescription(
+          '127.0.0.1:27019',
+          {
+            setName: 'test',
+            secondary: true,
+            ok: 1
+          },
+          { roundTripTime: 35 }
+        );
+        serverDescriptions = new Map();
+        serverDescriptions.set(serverDescription1.address, serverDescription1);
+        serverDescriptions.set(serverDescription2.address, serverDescription2);
+        serverDescriptions.set(serverDescription3.address, serverDescription3);
+      });
+      it('includes servers inside the latency window with default localThresholdMS', function () {
+        const topologyDescription = new TopologyDescription(
+          TopologyType.Single,
+          serverDescriptions,
+          'test',
+          MIN_SECONDARY_WRITE_WIRE_VERSION,
+          new ObjectId(),
+          MIN_SECONDARY_WRITE_WIRE_VERSION
+        );
+        const selector = secondaryWritableServerSelector();
+        const servers = selector(topologyDescription, Array.from(serverDescriptions.values()));
+        expect(servers).to.have.lengthOf(2);
+        const selectedAddresses = new Set(servers.map(({ address }) => address));
+        expect(selectedAddresses.has(serverDescription1.address)).to.be.true;
+        expect(selectedAddresses.has(serverDescription2.address)).to.be.true;
+        expect(selectedAddresses.has(serverDescription3.address)).to.be.false;
+      });
+
+      it('includes servers inside the latency window with custom localThresholdMS', function () {
+        const topologyDescription = new TopologyDescription(
+          TopologyType.Single,
+          serverDescriptions,
+          'test',
+          MIN_SECONDARY_WRITE_WIRE_VERSION,
+          new ObjectId(),
+          MIN_SECONDARY_WRITE_WIRE_VERSION,
+          { localThresholdMS: 5 }
+        );
+        const selector = secondaryWritableServerSelector();
+        const servers = selector(topologyDescription, Array.from(serverDescriptions.values()));
+        expect(servers).to.have.lengthOf(1);
+        const selectedAddresses = new Set(servers.map(({ address }) => address));
+        expect(selectedAddresses.has(serverDescription1.address)).to.be.true;
+        expect(selectedAddresses.has(serverDescription2.address)).to.be.false;
+        expect(selectedAddresses.has(serverDescription3.address)).to.be.false;
+      });
+    });
   });
 });