MongoDB Connection Issues in Vercel Serverless Environment

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Current Behavior

In Vercel Serverless production environment:

1. First request after deployment works normally
2. After handling complex database transactions:

• Subsequent requests fail with BSON cursor error
• Service becomes unresponsive for several minutes
• Requires instance “recovery” time before working again

3. Error occurs consistently after complex operations involving multiple collections

Expected Behavior

All database operations should work consistently:

• Successful connection maintenance
• Stable transaction handling
• No extended downtime after complex operations
• Quick recovery from any connection issues

Error Message

MongoUnexpectedServerResponseError: BSON element "cursor" is missing at CursorResponse.get (/var/task/node_modules/.pnpm/mongodb@6.10.0/node_modules/mongodb/lib/cmap/wire_protocol/responses.js:48:19)

Environment

• Platform: Vercel Serverless Functions
• MongoDB Driver: 6.10.0
• Node.js: 18.x
• Deployment Region: Singapore (SG)
• Production Environment (not occurring in local development)

Steps to Reproduce

1. Deploy application to Vercel
2. Execute a complex operation involving transactions across multiple collections:

// Database connection configuration
const config = {
  maxPoolSize: 5,
  minPoolSize: 1,
  retryWrites: true,
  retryReads: true,
  socketTimeoutMS: 20000,
  connectTimeoutMS: 10000,
  serverSelectionTimeoutMS: 10000
};

// Connection manager implementation
class DatabaseConnectionManager {
  private static instance: DatabaseConnectionManager;
  private client: MongoClient | null = null;

  async connect() {
    if (this.client) return this.client;
    
    this.client = await MongoClient.connect(MONGODB_URI, config);
    return this.client;
  }
}

// Example of transaction that triggers the issue
async function createActivity(activityData) {
  const client = await DatabaseConnectionManager.getInstance().connect();
  const session = client.startSession();

  try {
    await session.withTransaction(async () => {
      // 1. Validate user
      const user = await userCollection.findOne({ id: activityData.userId });
      if (!user) throw new Error('User not found');

      // 2. Create activity
      const activity = await activitiesCollection.insertOne(activityData);

      // 3. Update related collections
      await groupsCollection.updateOne(
        { id: activityData.groupId },
        { $push: { activities: activity.insertedId } }
      );
    });
  } finally {
    await session.endSession();
  }
}

3. First operation succeeds
4. Subsequent operations fail with BSON cursor error
5. Service remains unresponsive for several minutes

Key Observations

1. Issue only occurs in Vercel Serverless environment
2. Local development (vercel dev) works perfectly
3. Recovery requires several minutes of inactivity
4. Problem is consistent across multiple deployments

Attempted Solutions

1. Connection Pool Configuration:

• Tested various pool sizes (1-10)
• Adjusted timeout settings
• Modified connection validation

2. Transaction Management:

• Added retry logic
• Implemented transaction monitoring
• Optimized operation sequence

3. Error Handling:

• Added connection state verification
• Implemented retry mechanisms
• Enhanced logging

Questions

1. What causes the BSON cursor error in this Serverless context?
2. Why does the service require several minutes to recover?
3. What’s the recommended connection/pool configuration for Serverless environments?
4. Are there best practices for handling MongoDB transactions in Serverless functions?

Any guidance or suggestions would be greatly appreciated!