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 perform to txnNumber */
+const kTxnNumberIncrement = Symbol('txnNumberIncrement');
 
 /** @public */
 export interface EndSessionOptions {
@@ -130,6 +118,8 @@ export class ClientSession extends TypedEventEmitter<ClientSessionEvents> {
   [kSnapshotEnabled] = false;
   /** @internal */
   [kPinnedConnection]?: Connection;
+  /** @internal Accumulates total number of increments to perform to txnNumber */
+  [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 = Boolean(options.explicit);
+    this[kServerSession] = this.explicit ? this.sessionPool.acquire() : undefined;
+    this[kTxnNumberIncrement] = 0;
 
     this.supports = {
       causalConsistency: options.snapshot !== true && options.causalConsistency !== false
@@ -181,24 +174,27 @@ 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;
+    const serverSession = this[kServerSession];
+    if (serverSession == null) {
+      return undefined;
+    }
+    return serverSession.id;
   }
 
   get serverSession(): ServerSession {
-    if (this[kServerSession] == null) {
-      this[kServerSession] = this.sessionPool.acquire();
+    let serverSession = this[kServerSession];
+    if (serverSession == null) {
+      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 +263,15 @@ 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 +281,9 @@ 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();
@@ -355,10 +359,7 @@ export class ClientSession extends TypedEventEmitter<ClientSessionEvents> {
 
   /** Increment the transaction number on the internal ServerSession */
   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 +377,6 @@ 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 +627,7 @@ function attemptTransaction<TSchema>(
         throw err;
       }
 
-      if (session.transaction.isActive) {
+      if (session.inTransaction()) {
         return session.abortTransaction().then(() => maybeRetryOrThrow(err));
       }
 
@@ -641,11 +641,6 @@ 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 +745,6 @@ function endTransaction(
     callback(error, result);
   }
 
-  // Assumption here that commandName is "commitTransaction" or "abortTransaction"
   if (session.transaction.recoveryToken) {
     command.recoveryToken = session.transaction.recoveryToken;
   }
@@ -832,6 +826,30 @@ 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 +962,11 @@ 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');
@@ -967,14 +985,16 @@ export function applySession(
   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 = Boolean(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 +1,50 @@
+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(DRIVERS-2218): Refactor tests to align exactly with spec wording. Preliminarily implements:
+   * Drivers MAY assert that exactly one session is used for all the concurrent operations listed in the test, however this is a race condition if the session isn't released before checkIn (which SHOULD NOT be attempted)
+   * Drivers SHOULD assert that after repeated runs they are able to achieve the use of exactly one session, this will statistically prove we've reduced the allocation amount
+   * Drivers MUST assert that the number of allocated sessions never exceeds the number of concurrent operations executing
+   */
+
+  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);
+
+    expect(new Set(events.map(ev => ev.command.lsid.id.toString('hex'))).size).to.equal(1); // This is a guarantee in node
+  });
+});

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 = new Array(50).fill(null).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];