How to fix MongoExpiredSessionError: Cannot use a session that has ended in MongoDB

// BAD: Per-request connection
app.post('/users', async (req, res) => {
  const client = await MongoClient.connect(uri);
  const db = client.db('myapp');
  await db.collection('users').insertMany(users);
  await client.close(); // Closes before insertMany completes!
  res.send('OK');
});

// GOOD: Persistent connection
let client;
let db;

async function startApp() {
  client = await MongoClient.connect(uri);
  db = client.db('myapp');
  app.listen(3000);
}

app.post('/users', async (req, res) => {
  await db.collection('users').insertMany(users);
  res.send('OK');
});

startApp();

// BAD: Session closed before operation completes
const session = client.startSession();
collection.insertMany(docs, { session });
session.endSession(); // Called immediately, operation still running!

// GOOD: Await operation completion
const session = client.startSession();
try {
  await collection.insertMany(docs, { session });
} finally {
  await session.endSession();
}

// Using withTransaction helper
const session = client.startSession();
try {
  await session.withTransaction(async () => {
    await collection1.insertOne(doc1, { session });
    await collection2.updateOne(filter, update, { session });
    // Session automatically managed, no manual endSession needed
  });
} finally {
  await session.endSession();
}

// For Mongoose users
await mongoose.connection.transaction(async (session) => {
  await User.create([{ name: 'John' }], { session });
  await Order.create([{ userId: '123' }], { session });
  // Mongoose handles session lifecycle
});

const client = new MongoClient(uri, {
  maxIdleTimeMS: 120000, // 2 minutes instead of default 30 seconds
  serverSelectionTimeoutMS: 30000,
});

// Better: Set operation-level timeout
await collection.find(query)
  .maxTimeMS(10000) // 10 seconds for this query
  .toArray();

// BAD: Silent error handling breaks retry logic
session.withTransaction(async () => {
  try {
    await collection.insertOne(doc, { session });
  } catch (err) {
    console.log('Error occurred'); // Swallowed!
    // Session state now inconsistent
  }
});

// GOOD: Let errors propagate
session.withTransaction(async () => {
  await collection.insertOne(doc, { session });
  // Errors propagate to withTransaction retry logic
});

// BAD: Shared global session
let globalSession = client.startSession();
app.post('/data', async (req, res) => {
  await collection.insertOne(doc, { session: globalSession });
});

// GOOD: Per-operation session
app.post('/data', async (req, res) => {
  const session = client.startSession();
  try {
    await collection.insertOne(doc, { session });
  } finally {
    await session.endSession();
  }
});