feat(NODE-3697): reduce serverSession allocation

src/cmap/connection.ts

@@ -114,7 +114,7 @@ export interface CommandOptions extends BSONSerializeOptions {
   // Applying a session to a command should happen as part of command construction,
   // most likely in the CommandOperation#executeCommand method, where we have access to
   // the details we need to determine if a txnNum should also be applied.
-  willRetryWrite?: true;
+  willRetryWrite?: boolean;
 
   writeConcern?: WriteConcern;
 }

src/operations/operation.ts

@@ -25,7 +25,7 @@ export interface OperationConstructor extends Function {
 export interface OperationOptions extends BSONSerializeOptions {
   /** Specify ClientSession for this command */
   session?: ClientSession;
-  willRetryWrites?: boolean;
+  willRetryWrite?: boolean;
 
   /** The preferred read preference (ReadPreference.primary, ReadPreference.primary_preferred, ReadPreference.secondary, ReadPreference.secondary_preferred, ReadPreference.nearest). */
   readPreference?: ReadPreferenceLike;
@@ -56,8 +56,7 @@ export abstract class AbstractOperation<TResult = any> {
   // BSON serialization options
   bsonOptions?: BSONSerializeOptions;
 
-  // TODO: Each operation defines its own options, there should be better typing here
-  options: Document;
+  options: OperationOptions;
 
   [kSession]: ClientSession | undefined;
 

src/sessions.ts

@@ -42,20 +42,6 @@ import {
 
 const minWireVersionForShardedTransactions = 8;
 
-function assertAlive(session: ClientSession, callback?: Callback): boolean {
-  if (session.serverSession == null) {
-    const error = new MongoExpiredSessionError();
-    if (typeof callback === 'function') {
-      callback(error);
-      return false;
-    }
-
-    throw error;
-  }
-
-  return true;
-}
-
 /** @public */
 export interface ClientSessionOptions {
   /** Whether causal consistency should be enabled on this session */
@@ -89,6 +75,8 @@ const kSnapshotTime = Symbol('snapshotTime');
 const kSnapshotEnabled = Symbol('snapshotEnabled');
 /** @internal */
 const kPinnedConnection = Symbol('pinnedConnection');
+/** @internal Accumulates total number of increments to add to txnNumber when applying session to command */
+const kTxnNumberIncrement = Symbol('txnNumberIncrement');
 
 /** @public */
 export interface EndSessionOptions {
@@ -123,13 +111,15 @@ export class ClientSession extends TypedEventEmitter<ClientSessionEvents> {
   defaultTransactionOptions: TransactionOptions;
   transaction: Transaction;
   /** @internal */
-  [kServerSession]?: ServerSession;
+  [kServerSession]: ServerSession | null;
   /** @internal */
   [kSnapshotTime]?: Timestamp;
   /** @internal */
   [kSnapshotEnabled] = false;
   /** @internal */
   [kPinnedConnection]?: Connection;
+  /** @internal */
+  [kTxnNumberIncrement]: number;
 
   /**
    * Create a client session.
@@ -172,7 +162,10 @@ export class ClientSession extends TypedEventEmitter<ClientSessionEvents> {
     this.sessionPool = sessionPool;
     this.hasEnded = false;
     this.clientOptions = clientOptions;
-    this[kServerSession] = undefined;
+
+    this.explicit = !!options.explicit;
+    this[kServerSession] = this.explicit ? this.sessionPool.acquire() : null;
+    this[kTxnNumberIncrement] = 0;
 
     this.supports = {
       causalConsistency: options.snapshot !== true && options.causalConsistency !== false
@@ -181,24 +174,29 @@ export class ClientSession extends TypedEventEmitter<ClientSessionEvents> {
     this.clusterTime = options.initialClusterTime;
 
     this.operationTime = undefined;
-    this.explicit = !!options.explicit;
     this.owner = options.owner;
     this.defaultTransactionOptions = Object.assign({}, options.defaultTransactionOptions);
     this.transaction = new Transaction();
   }
 
   /** The server id associated with this session */
   get id(): ServerSessionId | undefined {
-    return this.serverSession?.id;
+    return this[kServerSession]?.id;
   }
 
   get serverSession(): ServerSession {
-    if (this[kServerSession] == null) {
-      this[kServerSession] = this.sessionPool.acquire();
+    let serverSession = this[kServerSession];
+    if (serverSession == null) {
+      if (this.explicit) {
+        throw new MongoRuntimeError('Unexpected null serverSession for an explicit session');
+      }
+      if (this.hasEnded) {
+        throw new MongoRuntimeError('Unexpected null serverSession for an ended implicit session');
+      }
+      serverSession = this.sessionPool.acquire();
+      this[kServerSession] = serverSession;
     }
-
-    // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
-    return this[kServerSession]!;
+    return serverSession;
   }
 
   /** Whether or not this session is configured for snapshot reads */
@@ -267,9 +265,15 @@ export class ClientSession extends TypedEventEmitter<ClientSessionEvents> {
       const completeEndSession = () => {
         maybeClearPinnedConnection(this, finalOptions);
 
-        // release the server session back to the pool
-        this.sessionPool.release(this.serverSession);
-        this[kServerSession] = undefined;
+        const serverSession = this[kServerSession];
+        if (serverSession != null) {
+          // release the server session back to the pool
+          this.sessionPool.release(serverSession);
+          // Make sure a new serverSession never makes it on to the ClientSession
+          Object.defineProperty(this, kServerSession, {
+            value: ServerSession.clone(serverSession)
+          });
+        }
 
         // mark the session as ended, and emit a signal
         this.hasEnded = true;
@@ -279,7 +283,9 @@ export class ClientSession extends TypedEventEmitter<ClientSessionEvents> {
         done();
       };
 
-      if (this.serverSession && this.inTransaction()) {
+      if (this.inTransaction()) {
+        // If we've reached endSession and the transaction is still active
+        // by default we abort it
         this.abortTransaction(err => {
           if (err) return done(err);
           completeEndSession();
@@ -353,12 +359,16 @@ export class ClientSession extends TypedEventEmitter<ClientSessionEvents> {
     return this.id.id.buffer.equals(session.id.id.buffer);
   }
 
-  /** Increment the transaction number on the internal ServerSession */
+  /**
+   * Increment the transaction number on the internal ServerSession
+   *
+   * @privateRemarks
+   * This helper increments a value stored on the client session that will be
+   * added to the serverSession's txnNumber upon applying it to a command.
+   * This is because the serverSession is lazily acquired after a connection is obtained
+   */
   incrementTransactionNumber(): void {
-    if (this.serverSession) {
-      this.serverSession.txnNumber =
-        typeof this.serverSession.txnNumber === 'number' ? this.serverSession.txnNumber + 1 : 0;
-    }
+    this[kTxnNumberIncrement] += 1;
   }
 
   /** @returns whether this session is currently in a transaction or not */
@@ -376,7 +386,6 @@ export class ClientSession extends TypedEventEmitter<ClientSessionEvents> {
       throw new MongoCompatibilityError('Transactions are not allowed with snapshot sessions');
     }
 
-    assertAlive(this);
     if (this.inTransaction()) {
       throw new MongoTransactionError('Transaction already in progress');
     }
@@ -627,7 +636,7 @@ function attemptTransaction<TSchema>(
         throw err;
       }
 
-      if (session.transaction.isActive) {
+      if (session.inTransaction()) {
         return session.abortTransaction().then(() => maybeRetryOrThrow(err));
       }
 
@@ -641,11 +650,6 @@ function endTransaction(
   commandName: 'abortTransaction' | 'commitTransaction',
   callback: Callback<Document>
 ) {
-  if (!assertAlive(session, callback)) {
-    // checking result in case callback was called
-    return;
-  }
-
   // handle any initial problematic cases
   const txnState = session.transaction.state;
 
@@ -750,7 +754,6 @@ function endTransaction(
     callback(error, result);
   }
 
-  // Assumption here that commandName is "commitTransaction" or "abortTransaction"
   if (session.transaction.recoveryToken) {
     command.recoveryToken = session.transaction.recoveryToken;
   }
@@ -832,6 +835,30 @@ export class ServerSession {
 
     return idleTimeMinutes > sessionTimeoutMinutes - 1;
   }
+
+  /**
+   * @internal
+   * Cloning meant to keep a readable reference to the server session data
+   * after ClientSession has ended
+   */
+  static clone(serverSession: ServerSession): Readonly<ServerSession> {
+    const arrayBuffer = new ArrayBuffer(16);
+    const idBytes = Buffer.from(arrayBuffer);
+    idBytes.set(serverSession.id.id.buffer);
+
+    const id = new Binary(idBytes, serverSession.id.id.sub_type);
+
+    // Manual prototype construction to avoid modifying the constructor of this class
+    return Object.setPrototypeOf(
+      {
+        id: { id },
+        lastUse: serverSession.lastUse,
+        txnNumber: serverSession.txnNumber,
+        isDirty: serverSession.isDirty
+      },
+      ServerSession.prototype
+    );
+  }
 }
 
 /**
@@ -944,11 +971,11 @@ export function applySession(
   command: Document,
   options: CommandOptions
 ): MongoDriverError | undefined {
-  // TODO: merge this with `assertAlive`, did not want to throw a try/catch here
   if (session.hasEnded) {
     return new MongoExpiredSessionError();
   }
 
+  // May acquire serverSession here
   const serverSession = session.serverSession;
   if (serverSession == null) {
     return new MongoRuntimeError('Unable to acquire server session');
@@ -966,15 +993,16 @@ export function applySession(
   serverSession.lastUse = now();
   command.lsid = serverSession.id;
 
-  // first apply non-transaction-specific sessions data
-  const inTransaction = session.inTransaction() || isTransactionCommand(command);
-  const isRetryableWrite = options?.willRetryWrite || false;
+  const inTxnOrTxnCommand = session.inTransaction() || isTransactionCommand(command);
+  const isRetryableWrite = !!options.willRetryWrite;
 
-  if (serverSession.txnNumber && (isRetryableWrite || inTransaction)) {
+  if (isRetryableWrite || inTxnOrTxnCommand) {
+    serverSession.txnNumber += session[kTxnNumberIncrement];
+    session[kTxnNumberIncrement] = 0;
     command.txnNumber = Long.fromNumber(serverSession.txnNumber);
   }
 
-  if (!inTransaction) {
+  if (!inTxnOrTxnCommand) {
     if (session.transaction.state !== TxnState.NO_TRANSACTION) {
       session.transaction.transition(TxnState.NO_TRANSACTION);
     }

test/integration/sessions/sessions.spec.prose.test.ts

@@ -0,0 +1,51 @@
+import { expect } from 'chai';
+
+import { Collection } from '../../../src/index';
+
+describe('ServerSession', () => {
+  let client;
+  let testCollection: Collection<{ _id: number; a?: number }>;
+  beforeEach(async function () {
+    const configuration = this.configuration;
+    client = await configuration.newClient({ maxPoolSize: 1, monitorCommands: true }).connect();
+
+    // reset test collection
+    testCollection = client.db('test').collection('too.many.sessions');
+    await testCollection.drop().catch(() => null);
+  });
+
+  afterEach(async () => {
+    await client?.close(true);
+  });
+
+  /**
+   * TODO(NODE-4082): Refactor tests to align exactly with spec wording.
+   * Assert the following across at least 5 retries of the above test: (We do not need to retry in nodejs)
+   * Drivers MUST assert that exactly one session is used for all operations at least once across the retries of this test.
+   * Note that it's possible, although rare, for greater than 1 server session to be used because the session is not released until after the connection is checked in.
+   * Drivers MUST assert that the number of allocated sessions is strictly less than the number of concurrent operations in every retry of this test. In this instance it would less than (but NOT equal to) 8.
+   */
+  it('13. may reuse one server session for many operations', async () => {
+    const events = [];
+    client.on('commandStarted', ev => events.push(ev));
+
+    const operations = [
+      testCollection.insertOne({ _id: 1 }),
+      testCollection.deleteOne({ _id: 2 }),
+      testCollection.updateOne({ _id: 3 }, { $set: { a: 1 } }),
+      testCollection.bulkWrite([{ updateOne: { filter: { _id: 4 }, update: { $set: { a: 1 } } } }]),
+      testCollection.findOneAndDelete({ _id: 5 }),
+      testCollection.findOneAndUpdate({ _id: 6 }, { $set: { a: 1 } }),
+      testCollection.findOneAndReplace({ _id: 7 }, { a: 8 }),
+      testCollection.find().toArray()
+    ];
+
+    const allResults = await Promise.all(operations);
+
+    expect(allResults).to.have.lengthOf(operations.length);
+    expect(events).to.have.lengthOf(operations.length);
+
+    // This is a guarantee in node, unless you are performing a transaction (which is not being done in this test)
+    expect(new Set(events.map(ev => ev.command.lsid.id.toString('hex'))).size).to.equal(1);
+  });
+});

test/integration/sessions/sessions.test.ts

@@ -367,4 +367,37 @@ describe('Sessions Spec', function () {
       });
     });
   });
+
+  describe('Session allocation', () => {
+    let client;
+    let testCollection;
+
+    beforeEach(async function () {
+      client = await this.configuration
+        .newClient({ maxPoolSize: 1, monitorCommands: true })
+        .connect();
+      // reset test collection
+      testCollection = client.db('test').collection('too.many.sessions');
+      await testCollection.drop().catch(() => null);
+    });
+
+    afterEach(async () => {
+      await client?.close();
+    });
+
+    it('should only use one session for many operations when maxPoolSize is 1', async () => {
+      const documents = Array.from({ length: 50 }).map((_, idx) => ({ _id: idx }));
+
+      const events = [];
+      client.on('commandStarted', ev => events.push(ev));
+      const allResults = await Promise.all(
+        documents.map(async doc => testCollection.insertOne(doc))
+      );
+
+      expect(allResults).to.have.lengthOf(documents.length);
+      expect(events).to.have.lengthOf(documents.length);
+
+      expect(new Set(events.map(ev => ev.command.lsid.id.toString('hex'))).size).to.equal(1);
+    });
+  });
 });

test/tools/cluster_setup.sh

@@ -17,7 +17,7 @@ if [[ $1 == "replica_set" ]]; then
     echo "mongodb://bob:pwd123@localhost:31000,localhost:31001,localhost:31002/?replicaSet=rs"
 elif [[ $1 == "sharded_cluster" ]]; then
     mkdir -p $SHARDED_DIR
-    mlaunch init --dir $SHARDED_DIR --auth --username "bob" --password "pwd123" --replicaset --nodes 3 --arbiter --name rs --port 51000 --enableMajorityReadConcern --setParameter enableTestCommands=1 --sharded 1 --mongos 2
+    mlaunch init --dir $SHARDED_DIR --auth --username "bob" --password "pwd123" --replicaset --nodes 3 --name rs --port 51000 --enableMajorityReadConcern --setParameter enableTestCommands=1 --sharded 1 --mongos 2
     echo "mongodb://bob:pwd123@localhost:51000,localhost:51001"
 elif [[ $1 == "server" ]]; then
     mkdir -p $SINGLE_DIR

test/tools/spec-runner/index.js

@@ -459,6 +459,8 @@ function validateExpectations(commandEvents, spec, savedSessionData) {
   const rawExpectedEvents = spec.expectations.map(x => x.command_started_event);
   const expectedEvents = normalizeCommandShapes(rawExpectedEvents);
 
+  expect(actualEvents).to.have.lengthOf(expectedEvents.length);
+
   for (const [idx, expectedEvent] of expectedEvents.entries()) {
     const actualEvent = actualEvents[idx];