feat(NODE-5197): add server monitoring mode

src/connection_string.ts

@@ -29,6 +29,7 @@ import {
 } from './mongo_logger';
 import { ReadConcern, type ReadConcernLevel } from './read_concern';
 import { ReadPreference, type ReadPreferenceMode } from './read_preference';
+import { ServerMonitoringModes } from './sdam/monitor';
 import type { TagSet } from './sdam/server_description';
 import {
   DEFAULT_PK_FACTORY,
@@ -1055,6 +1056,17 @@ export const OPTIONS = {
   serializeFunctions: {
     type: 'boolean'
   },
+  serverMonitoringMode: {
+    default: 'auto',
+    transform({ values: [value] }) {
+      if (!ServerMonitoringModes.includes(value as string)) {
+        throw new MongoParseError(
+          'serverMonitoringMode must be one of `auto`, `poll`, or `stream`'
+        );
+      }
+      return value;
+    }
+  },
   serverSelectionTimeoutMS: {
     default: 30000,
     type: 'uint'

src/index.ts

@@ -489,7 +489,9 @@ export type {
   MonitorOptions,
   MonitorPrivate,
   RTTPinger,
-  RTTPingerOptions
+  RTTPingerOptions,
+  ServerMonitoringMode,
+  ServerMonitoringModes
 } from './sdam/monitor';
 export type { Server, ServerEvents, ServerOptions, ServerPrivate } from './sdam/server';
 export type {

src/mongo_client.ts

@@ -32,6 +32,7 @@ import { executeOperation } from './operations/execute_operation';
 import { RunAdminCommandOperation } from './operations/run_command';
 import type { ReadConcern, ReadConcernLevel, ReadConcernLike } from './read_concern';
 import { ReadPreference, type ReadPreferenceMode } from './read_preference';
+import type { ServerMonitoringMode } from './sdam/monitor';
 import type { TagSet } from './sdam/server_description';
 import { readPreferenceServerSelector } from './sdam/server_selection';
 import type { SrvPoller } from './sdam/srv_polling';
@@ -257,6 +258,8 @@ export interface MongoClientOptions extends BSONSerializeOptions, SupportedNodeC
   proxyUsername?: string;
   /** Configures a Socks5 proxy password when the proxy in proxyHost requires username/password authentication. */
   proxyPassword?: string;
+  /** Instructs the driver monitors to use a specific monitoring mode */
+  serverMonitoringMode?: ServerMonitoringMode;
 
   /** @internal */
   srvPoller?: SrvPoller;
@@ -816,6 +819,7 @@ export interface MongoOptions
   proxyPort?: number;
   proxyUsername?: string;
   proxyPassword?: string;
+  serverMonitoringMode: ServerMonitoringMode;
 
   /** @internal */
   connectionType?: typeof Connection;

src/sdam/monitor.ts

@@ -3,6 +3,7 @@ import { clearTimeout, setTimeout } from 'timers';
 import { type Document, Long } from '../bson';
 import { connect } from '../cmap/connect';
 import { Connection, type ConnectionOptions } from '../cmap/connection';
+import { getFAASEnv } from '../cmap/handshake/client_metadata';
 import { LEGACY_HELLO_COMMAND } from '../constants';
 import { MongoError, MongoErrorLabel, MongoNetworkTimeoutError } from '../error';
 import { CancellationToken, TypedEventEmitter } from '../mongo_types';
@@ -44,6 +45,11 @@ function isInCloseState(monitor: Monitor) {
   return monitor.s.state === STATE_CLOSED || monitor.s.state === STATE_CLOSING;
 }
 
+/** @public */
+export const ServerMonitoringModes = ['auto', 'poll', 'stream'];
+/** @public */
+export type ServerMonitoringMode = (typeof ServerMonitoringModes)[number];
+
 /** @internal */
 export interface MonitorPrivate {
   state: string;
@@ -55,6 +61,7 @@ export interface MonitorOptions
   connectTimeoutMS: number;
   heartbeatFrequencyMS: number;
   minHeartbeatFrequencyMS: number;
+  serverMonitoringMode: ServerMonitoringMode;
 }
 
 /** @public */
@@ -73,9 +80,16 @@ export class Monitor extends TypedEventEmitter<MonitorEvents> {
   s: MonitorPrivate;
   address: string;
   options: Readonly<
-    Pick<MonitorOptions, 'connectTimeoutMS' | 'heartbeatFrequencyMS' | 'minHeartbeatFrequencyMS'>
+    Pick<
+      MonitorOptions,
+      | 'connectTimeoutMS'
+      | 'heartbeatFrequencyMS'
+      | 'minHeartbeatFrequencyMS'
+      | 'serverMonitoringMode'
+    >
   >;
   connectOptions: ConnectionOptions;
+  isRunningInFaasEnv: boolean;
   [kServer]: Server;
   [kConnection]?: Connection;
   [kCancellationToken]: CancellationToken;
@@ -103,8 +117,10 @@ export class Monitor extends TypedEventEmitter<MonitorEvents> {
     this.options = Object.freeze({
       connectTimeoutMS: options.connectTimeoutMS ?? 10000,
       heartbeatFrequencyMS: options.heartbeatFrequencyMS ?? 10000,
-      minHeartbeatFrequencyMS: options.minHeartbeatFrequencyMS ?? 500
+      minHeartbeatFrequencyMS: options.minHeartbeatFrequencyMS ?? 500,
+      serverMonitoringMode: options.serverMonitoringMode
     });
+    this.isRunningInFaasEnv = getFAASEnv() != null;
 
     const cancellationToken = this[kCancellationToken];
     // TODO: refactor this to pull it directly from the pool, requires new ConnectionPool integration
@@ -207,27 +223,37 @@ function resetMonitorState(monitor: Monitor) {
   monitor[kConnection] = undefined;
 }
 
+function useStreamingProtocol(monitor: Monitor, topologyVersion: TopologyVersion | null): boolean {
+  // If we have no topology version we always poll no matter
+  // what the user provided.
+  if (topologyVersion == null) return false;
+
+  const serverMonitoringMode = monitor.options.serverMonitoringMode;
+  if (serverMonitoringMode === 'poll') return false;
+  if (serverMonitoringMode === 'stream') return true;
+
+  // If we are in auto mode, we need to figure out if we're in a FaaS
+  // environment or not and choose the appropriate mode.
+  if (monitor.isRunningInFaasEnv) return false;
+  return true;
+}
+
 function checkServer(monitor: Monitor, callback: Callback<Document | null>) {
   let start = now();
   const topologyVersion = monitor[kServer].description.topologyVersion;
-  const isAwaitable = topologyVersion != null;
+  const isAwaitable = useStreamingProtocol(monitor, topologyVersion);
   monitor.emit(
     Server.SERVER_HEARTBEAT_STARTED,
     new ServerHeartbeatStartedEvent(monitor.address, isAwaitable)
   );
 
-  function failureHandler(err: Error) {
+  function failureHandler(err: Error, awaited: boolean) {
     monitor[kConnection]?.destroy({ force: true });
     monitor[kConnection] = undefined;
 
     monitor.emit(
       Server.SERVER_HEARTBEAT_FAILED,
-      new ServerHeartbeatFailedEvent(
-        monitor.address,
-        calculateDurationInMs(start),
-        err,
-        isAwaitable
-      )
+      new ServerHeartbeatFailedEvent(monitor.address, calculateDurationInMs(start), err, awaited)
     );
 
     const error = !(err instanceof MongoError)
@@ -274,7 +300,7 @@ function checkServer(monitor: Monitor, callback: Callback<Document | null>) {
 
     connection.command(ns('admin.$cmd'), cmd, options, (err, hello) => {
       if (err) {
-        return failureHandler(err);
+        return failureHandler(err, isAwaitable);
       }
 
       if (!('isWritablePrimary' in hello)) {
@@ -286,15 +312,12 @@ function checkServer(monitor: Monitor, callback: Callback<Document | null>) {
       const duration =
         isAwaitable && rttPinger ? rttPinger.roundTripTime : calculateDurationInMs(start);
 
-      const awaited = isAwaitable && hello.topologyVersion != null;
       monitor.emit(
         Server.SERVER_HEARTBEAT_SUCCEEDED,
-        new ServerHeartbeatSucceededEvent(monitor.address, duration, hello, awaited)
+        new ServerHeartbeatSucceededEvent(monitor.address, duration, hello, isAwaitable)
       );
 
-      // if we are using the streaming protocol then we immediately issue another `started`
-      // event, otherwise the "check" is complete and return to the main monitor loop
-      if (awaited) {
+      if (isAwaitable) {
         monitor.emit(
           Server.SERVER_HEARTBEAT_STARTED,
           new ServerHeartbeatStartedEvent(monitor.address, true)
@@ -316,7 +339,7 @@ function checkServer(monitor: Monitor, callback: Callback<Document | null>) {
     if (err) {
       monitor[kConnection] = undefined;
 
-      failureHandler(err);
+      failureHandler(err, false);
       return;
     }
 
@@ -337,7 +360,7 @@ function checkServer(monitor: Monitor, callback: Callback<Document | null>) {
           monitor.address,
           calculateDurationInMs(start),
           conn.hello,
-          false
+          useStreamingProtocol(monitor, conn.hello?.topologyVersion)
         )
       );
 
@@ -370,7 +393,7 @@ function monitorServer(monitor: Monitor) {
       }
 
       // if the check indicates streaming is supported, immediately reschedule monitoring
-      if (hello && hello.topologyVersion) {
+      if (useStreamingProtocol(monitor, hello?.topologyVersion)) {
         setTimeout(() => {
           if (!isInCloseState(monitor)) {
             monitor[kMonitorId]?.wake();

src/sdam/topology.ts

@@ -65,6 +65,7 @@ import {
   TopologyDescriptionChangedEvent,
   TopologyOpeningEvent
 } from './events';
+import type { ServerMonitoringMode } from './monitor';
 import { Server, type ServerEvents, type ServerOptions } from './server';
 import { compareTopologyVersion, ServerDescription } from './server_description';
 import { readPreferenceServerSelector, type ServerSelector } from './server_selection';
@@ -143,6 +144,7 @@ export interface TopologyOptions extends BSONSerializeOptions, ServerOptions {
   directConnection: boolean;
   loadBalanced: boolean;
   metadata: ClientMetadata;
+  serverMonitoringMode: ServerMonitoringMode;
   /** MongoDB server API version */
   serverApi?: ServerApi;
   [featureFlag: symbol]: any;

test/lambda/mongodb/app.mjs

@@ -1,3 +1,5 @@
+import * as assert from 'node:assert/strict';
+
 import { MongoClient } from 'mongodb';
 
 // Creates the client that is cached for all requests, subscribes to
@@ -30,18 +32,21 @@ mongoClient.on('commandFailed', (event) => {
 
 mongoClient.on('serverHeartbeatStarted', (event) => {
   console.log('serverHeartbeatStarted', event);
+  assert.strictEqual(event.awaited, false);
 });
 
 mongoClient.on('serverHeartbeatSucceeded', (event) => {
   heartbeatCount++;
   totalHeartbeatDuration += event.duration;
   console.log('serverHeartbeatSucceeded', event);
+  assert.strictEqual(event.awaited, false);
 });
 
 mongoClient.on('serverHeartbeatFailed', (event) => {
   heartbeatCount++;
   totalHeartbeatDuration += event.duration;
   console.log('serverHeartbeatFailed', event);
+  assert.strictEqual(event.awaited, false);
 });
 
 mongoClient.on('connectionCreated', (event) => {