feat(NODE-7121): prevent connection churn on backpressure errors when establishing connections

src/cmap/connect.ts

@@ -35,6 +35,11 @@ import {
 /** @public */
 export type Stream = Socket | TLSSocket;
 
+function applyBackpressureLabels(error: MongoError) {
+  error.addErrorLabel(MongoErrorLabel.SystemOverloadedError);
+  error.addErrorLabel(MongoErrorLabel.RetryableError);
+}
+
 export async function connect(options: ConnectionOptions): Promise<Connection> {
   let connection: Connection | null = null;
   try {
@@ -103,6 +108,8 @@ export async function performInitialHandshake(
   const authContext = new AuthContext(conn, credentials, options);
   conn.authContext = authContext;
 
+  // If we encounter an error preparing the handshake document, do NOT apply backpressure labels.  Errors
+  // encountered building the handshake document are all client-side, and do not indicate an overloaded server.
   const handshakeDoc = await prepareHandshakeDocument(authContext);
 
   // @ts-expect-error: TODO(NODE-5141): The options need to be filtered properly, Connection options differ from Command options
@@ -163,12 +170,15 @@ export async function performInitialHandshake(
     try {
       await provider.auth(authContext);
     } catch (error) {
+      // NOTE: If we encounter an error authenticating a connection, do NOT apply backpressure labels.
+
       if (error instanceof MongoError) {
         error.addErrorLabel(MongoErrorLabel.HandshakeError);
         if (needsRetryableWriteLabel(error, response.maxWireVersion, conn.description.type)) {
           error.addErrorLabel(MongoErrorLabel.RetryableWriteError);
         }
       }
+
       throw error;
     }
   }
@@ -189,6 +199,11 @@ export async function performInitialHandshake(
       if (error instanceof MongoError) {
         error.addErrorLabel(MongoErrorLabel.HandshakeError);
       }
+      // If we encounter a network error executing the initial handshake, apply backpressure labels.
+      if (error instanceof MongoNetworkError) {
+        applyBackpressureLabels(error);
+      }
+
       throw error;
     }
   }
@@ -424,6 +439,10 @@ export async function makeSocket(options: MakeConnectionOptions): Promise<Stream
     socket = await connectedSocket;
     return socket;
   } catch (error) {
+    // If we encounter an error while establishing a socket, apply the backpressure labels to it.  We cannot
+    // differentiate between DNS, TLS errors and network errors without refactoring our connection establishment to
+    // handle all three steps separately.
+    applyBackpressureLabels(error);
     socket.destroy();
     throw error;
   } finally {

src/error.ts

@@ -99,7 +99,9 @@ export const MongoErrorLabel = Object.freeze({
   ResetPool: 'ResetPool',
   PoolRequestedRetry: 'PoolRequestedRetry',
   InterruptInUseConnections: 'InterruptInUseConnections',
-  NoWritesPerformed: 'NoWritesPerformed'
+  NoWritesPerformed: 'NoWritesPerformed',
+  RetryableError: 'RetryableError',
+  SystemOverloadedError: 'SystemOverloadedError'
 } as const);
 
 /** @public */

src/sdam/server.ts

@@ -400,7 +400,8 @@ export class Server extends TypedEventEmitter<ServerEvents> {
       error instanceof MongoNetworkError && !(error instanceof MongoNetworkTimeoutError);
     const isNetworkTimeoutBeforeHandshakeError =
       error instanceof MongoNetworkError && error.beforeHandshake;
-    const isAuthHandshakeError = error.hasErrorLabel(MongoErrorLabel.HandshakeError);
+    const isAuthOrEstablishmentHandshakeError = error.hasErrorLabel(MongoErrorLabel.HandshakeError);
+    const isSystemOverloadError = error.hasErrorLabel(MongoErrorLabel.SystemOverloadedError);
 
     // Perhaps questionable and divergent from the spec, but considering MongoParseErrors like state change errors was legacy behavior.
     if (isStateChangeError(error) || error instanceof MongoParseError) {
@@ -424,8 +425,12 @@ export class Server extends TypedEventEmitter<ServerEvents> {
     } else if (
       isNetworkNonTimeoutError ||
       isNetworkTimeoutBeforeHandshakeError ||
-      isAuthHandshakeError
+      isAuthOrEstablishmentHandshakeError
     ) {
+      // Do NOT clear the pool if we encounter a system overloaded error.
+      if (isSystemOverloadError) {
+        return;
+      }
       // from the SDAM spec: The driver MUST synchronize clearing the pool with updating the topology.
       // In load balanced mode: there is no monitoring, so there is no topology to update.  We simply clear the pool.
       // For other topologies: the `ResetPool` label instructs the topology to clear the server's pool in `updateServer()`.

test/integration/server-discovery-and-monitoring/server_discovery_and_monitoring.prose.test.ts

@@ -1,13 +1,18 @@
 import { expect } from 'chai';
 import { once } from 'events';
 
-import { type MongoClient } from '../../../src';
+import {
+  type ConnectionCheckOutFailedEvent,
+  type ConnectionPoolClearedEvent,
+  type MongoClient
+} from '../../../src';
 import {
   CONNECTION_POOL_CLEARED,
   CONNECTION_POOL_READY,
   SERVER_HEARTBEAT_FAILED,
   SERVER_HEARTBEAT_SUCCEEDED
 } from '../../../src/constants';
+import { sleep } from '../../tools/utils';
 
 describe('Server Discovery and Monitoring Prose Tests', function () {
   context('Monitors sleep at least minHeartbeatFrequencyMS between checks', function () {
@@ -187,4 +192,93 @@ describe('Server Discovery and Monitoring Prose Tests', function () {
       }
     });
   });
+
+  context('Connection Pool Backpressure', function () {
+    let client: MongoClient;
+    const checkoutFailedEvents: Array<ConnectionCheckOutFailedEvent> = [];
+    const poolClearedEvents: Array<ConnectionPoolClearedEvent> = [];
+
+    beforeEach(async function () {
+      // 1. Create a test client that listens to CMAP events, with maxConnecting=100.
+      client = this.configuration.newClient({}, { maxConnecting: 100 });
+
+      client.on('connectionCheckOutFailed', e => checkoutFailedEvents.push(e));
+      client.on('connectionPoolCleared', e => poolClearedEvents.push(e));
+
+      await client.connect();
+
+      // 2. Run the following commands to set up the rate limiter.
+      // ```python
+      // client.admin.command("setParameter", 1, ingressConnectionEstablishmentRateLimiterEnabled=True)
+      // client.admin.command("setParameter", 1, ingressConnectionEstablishmentRatePerSec=20)
+      // client.admin.command("setParameter", 1, ingressConnectionEstablishmentBurstCapacitySecs=1)
+      // client.admin.command("setParameter", 1, ingressConnectionEstablishmentMaxQueueDepth=1)
+      // ```
+      const admin = client.db('admin').admin();
+      await admin.command({
+        setParameter: 1,
+        ingressConnectionEstablishmentRateLimiterEnabled: true
+      });
+      await admin.command({
+        setParameter: 1,
+        ingressConnectionEstablishmentRatePerSec: 20
+      });
+      await admin.command({
+        setParameter: 1,
+        ingressConnectionEstablishmentBurstCapacitySecs: 1
+      });
+      await admin.command({
+        setParameter: 1,
+        ingressConnectionEstablishmentMaxQueueDepth: 1
+      });
+
+      // 3. Add a document to the test collection so that the sleep operations will actually block:
+      // `client.test.test.insert_one({})`.
+      await client.db('test').collection('test').insertOne({});
+    });
+
+    afterEach(async function () {
+      // 7. Sleep for 1 second to clear the rate limiter.
+      await sleep(1000);
+
+      // 8. Ensure that the following command runs at test teardown even if the test fails.
+      // `client.admin("setParameter", 1, ingressConnectionEstablishmentRateLimiterEnabled=False)`.
+      const admin = client.db('admin').admin();
+      await admin.command({
+        setParameter: 1,
+        ingressConnectionEstablishmentRateLimiterEnabled: false
+      });
+
+      await client.close();
+    });
+
+    it(
+      'does not clear the pool when connections are closed due to connection storms',
+      {
+        requires: {
+          // This test requires MongoDB 7.0+.
+          mongodb: '>=7.0' // rate limiting added in 7.0
+        }
+      },
+      async function () {
+        // 4. Run the following find command on the collection in 100 parallel threads/coroutines. Run these commands concurrently
+        // but block on their completion, and ignore errors raised by the command.
+        // `client.test.test.find_one({"$where": "function() { sleep(2000); return true; }})`
+        await Promise.allSettled(
+          Array.from({ length: 100 }).map(() =>
+            client
+              .db('test')
+              .collection('test')
+              .findOne({ $where: 'function() { sleep(2000); return true; }' })
+          )
+        );
+
+        // 5. Assert that at least 10 `ConnectionCheckOutFailedEvent` occurred.
+        expect(checkoutFailedEvents.length).to.be.at.least(10);
+
+        // 6. Assert that 0 `PoolClearedEvent` occurred.
+        expect(poolClearedEvents).to.be.empty;
+      }
+    );
+  });
 });

test/spec/connection-monitoring-and-pooling/cmap-format/pool-create-min-size-error.json

@@ -17,7 +17,7 @@
         "isMaster",
         "hello"
       ],
-      "closeConnection": true,
+      "errorCode": 91,
       "appName": "poolCreateMinSizeErrorTest"
     }
   },

test/spec/connection-monitoring-and-pooling/cmap-format/pool-create-min-size-error.yml

@@ -11,7 +11,7 @@ failPoint:
   mode: { times: 50 }
   data:
     failCommands: ["isMaster","hello"]
-    closeConnection: true
+    errorCode: 91
     appName: "poolCreateMinSizeErrorTest"
 poolOptions:
   minPoolSize: 1

test/spec/load-balancers/sdam-error-handling.json

@@ -1,6 +1,6 @@
 {
   "description": "state change errors are correctly handled",
-  "schemaVersion": "1.3",
+  "schemaVersion": "1.4",
   "runOnRequirements": [
     {
       "topologies": [
@@ -263,7 +263,7 @@
       "description": "errors during the initial connection hello are ignored",
       "runOnRequirements": [
         {
-          "minServerVersion": "4.9"
+          "minServerVersion": "4.4.7"
         }
       ],
       "operations": [
@@ -282,7 +282,7 @@
                   "isMaster",
                   "hello"
                 ],
-                "closeConnection": true,
+                "errorCode": 11600,
                 "appName": "lbSDAMErrorTestClient"
               }
             }
@@ -297,7 +297,7 @@
             }
           },
           "expectError": {
-            "isClientError": true
+            "isError": true
           }
         }
       ],

test/spec/load-balancers/sdam-error-handling.yml

@@ -1,6 +1,6 @@
 description: state change errors are correctly handled
 
-schemaVersion: '1.3'
+schemaVersion: '1.4'
 
 runOnRequirements:
   - topologies: [ load-balanced ]
@@ -141,9 +141,8 @@ tests:
   # to the same mongos on which the failpoint is set.
   - description: errors during the initial connection hello are ignored
     runOnRequirements:
-      # Server version 4.9+ is needed to set a fail point on the initial
-      # connection handshake with the appName filter due to SERVER-49336.
-      - minServerVersion: '4.9'
+      # Require SERVER-49336 for failCommand + appName on the initial handshake.
+      - minServerVersion: '4.4.7'
     operations:
       - name: failPoint
         object: testRunner
@@ -154,14 +153,14 @@ tests:
             mode: { times: 1 }
             data:
               failCommands: [isMaster, hello]
-              closeConnection: true
+              errorCode: 11600
               appName: *singleClientAppName
       - name: insertOne
         object: *singleColl
         arguments:
           document: { x: 1 }
         expectError:
-          isClientError: true
+          isError: true
     expectEvents:
       - client: *singleClient
         eventType: cmap

test/spec/server-discovery-and-monitoring/errors/error_handling_handshake.json

@@ -97,14 +97,22 @@
       "outcome": {
         "servers": {
           "a:27017": {
-            "type": "Unknown",
-            "topologyVersion": null,
+            "type": "RSPrimary",
+            "setName": "rs",
+            "topologyVersion": {
+              "processId": {
+                "$oid": "000000000000000000000001"
+              },
+              "counter": {
+                "$numberLong": "1"
+              }
+            },
             "pool": {
-              "generation": 1
+              "generation": 0
             }
           }
         },
-        "topologyType": "ReplicaSetNoPrimary",
+        "topologyType": "ReplicaSetWithPrimary",
         "logicalSessionTimeoutMinutes": null,
         "setName": "rs"
       }

test/spec/server-discovery-and-monitoring/errors/error_handling_handshake.yml

@@ -43,13 +43,4 @@ phases:
     when: beforeHandshakeCompletes
     maxWireVersion: 9
     type: timeout
-  outcome:
-    servers:
-      a:27017:
-        type: Unknown
-        topologyVersion: null
-        pool:
-          generation: 1
-    topologyType: ReplicaSetNoPrimary
-    logicalSessionTimeoutMinutes: null
-    setName: rs
+  outcome: *outcome