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I'll answer as a world-renowned database systems expert specializing in distributed databases and cloud-native architectures, with the prestigious ACM SIGMOD Edgar F. Codd Innovations Award.

Thank you for providing more context. I understand that you're dealing with a more complex scenario involving user-specific databases. Let's analyze this situation and propose a solution that addresses your specific needs.

1. Current Architecture Understanding:
   • You have different DB instances for each userId.
   • You need to import a DB class globally.
   • Some functions, like getCustomersByIds, don't have direct access to userId but need to use the correct user-specific DB.
2. Problem Analysis: The core issue here is maintaining user context throughout the application, even in functions that don't explicitly receive the userId. Let's break this down step-by-step: a. Global DB class import: This is necessary for your architecture, but we need to ensure it's user-specific. b. Context propagation: We need a way to propagate the user context to deeper function calls. c. Concurrency issues: We must prevent one user's request from affecting another's DB connection.
3. Proposed Solution: We can use a combination of a global singleton for managing DB connections and a context-based approach for ensuring each request uses the correct DB. Here's how we can implement this: Step 1: Create a DBManager singleton:

// dbManager.js
const { AsyncLocalStorage } = require('async_hooks');

class DBManager {
 constructor() {
   this.connections = new Map();
   this.asyncLocalStorage = new AsyncLocalStorage();
 }

 getConnection(userId) {
   if (!this.connections.has(userId)) {
     // Create a new DB connection for this user
     this.connections.set(userId, new DB(userId));
   }
   return this.connections.get(userId);
 }

 getCurrentConnection() {
   const context = this.asyncLocalStorage.getStore();
   if (!context || !context.userId) {
     throw new Error('No user context set');
   }
   return this.getConnection(context.userId);
 }

 runWithContext(userId, callback) {
   return this.asyncLocalStorage.run({ userId }, callback);
 }
}
module.exports = new DBManager();

Step 2: Use the DBManager in your Cloud Function:

// index.js
const dbManager = require('./dbManager');

exports.handleRequest = async (req, res) => {
 const userId = req.body.userId;

 try {
   await dbManager.runWithContext(userId, async () => {
     // All code inside this callback will have access to the correct DB connection
     const result = await getCustomersByIds(someIds);
     res.send(result);
   });
 } catch (error) {
   console.error('Error processing request:', error);
   res.status(500).send('Internal Server Error');
 }
};

async function getCustomersByIds(ids) {
 // This function doesn't need to know about userId
 const db = dbManager.getCurrentConnection();
 return db.getCustomers(ids);
}

4. How This Solution Works:
   • The DBManager is a true singleton, created once per Node.js process.
   • It manages DB connections for all users.
   • AsyncLocalStorage is used to store the current user's context for each request.
   • The runWithContext method ensures that the correct user context is set for the entire duration of a request, including all asynchronous operations.
   • getCurrentConnection always returns the correct DB connection for the current request, even in deeply nested functions.
5. Advantages:
   • Thread-safe: Each request operates in its own context, preventing interference between concurrent requests.
   • Efficient: DB connections are reused across requests for the same user.
   • Clean API: Deep functions don't need to explicitly pass around user information.
6. Considerations:
   • Connection Management: Implement a strategy to close inactive connections to prevent resource exhaustion.
   • Error Handling: Ensure proper error handling, especially for cases where the user context might be missing.
   • Testing: Thoroughly test this solution under high concurrency to ensure it behaves correctly.

This solution maintains the global nature of your DB class import while ensuring that each request uses the correct user-specific DB connection, even in deeply nested functions. It's scalable and should work well with Google Cloud Functions v1 and Node.js, handling parallel requests safely.