stock-bot/CONNECTION-POOL-IMPLEMENTATION.md

Connection Pool Implementation Summary

What We've Implemented

1. Removed Singleton Pattern ✅

• Deleted libs/mongodb-client/src/singleton.ts
• Deleted libs/postgres-client/src/singleton.ts
• Removed singleton exports from both index files
• Forces all new code to use the connection factory pattern

2. Added Connection Pool Monitoring ✅

Both MongoDB and PostgreSQL clients now support:

• getPoolMetrics() method returning:
  • totalConnections
  • activeConnections
  • idleConnections
  • waitingRequests
  • errors count
  • lastError message
  • avgResponseTime
  • created timestamp
  • lastUsed timestamp

3. Implemented Connection Lifecycle Events ✅

Both clients now support event callbacks:

interface ConnectionEvents {
  onConnect?: () => void | Promise<void>;
  onDisconnect?: () => void | Promise<void>;
  onError?: (error: Error) => void | Promise<void>;
  onPoolCreated?: () => void | Promise<void>;
}

4. Dynamic Pool Sizing ✅

New feature for automatically adjusting pool size based on load:

interface DynamicPoolConfig {
  enabled: boolean;
  minSize: number;
  maxSize: number;
  scaleUpThreshold: number;    // % utilization to trigger scale up
  scaleDownThreshold: number;  // % utilization to trigger scale down
  scaleUpIncrement: number;     // connections to add
  scaleDownIncrement: number;   // connections to remove
  evaluationInterval: number;   // ms between evaluations
}

5. Pool Warmup Strategy ✅

• warmupPool() method pre-creates minimum connections
• Runs parallel queries/pings to establish connections
• Reduces cold start latency
• Validates connections before marking pool as ready

Enhanced Components

Connection Factory (@stock-bot/connection-factory)

• Manages connection pools across services
• Provides consistent configuration
• Handles lifecycle events
• Supports pool metrics collection

Enhanced OperationContext

• Backward compatible with singleton pattern
• Supports dependency injection via ServiceContainer
• Lazy loads database connections
• Proper resource disposal with dispose() method

Service Container

• Lightweight dependency injection
• Scoped containers for request isolation
• Automatic resource cleanup
• Service lifecycle management

Usage Examples

Basic Setup

// Setup service container with connection pools
const container = await setupServiceContainer();

// Create operation context with container
const context = OperationContext.create('handler', 'operation', {
  container
});

// Use databases normally
await context.mongodb.insertOne(data);
await context.postgres.query('SELECT...');

// Clean up when done
await context.dispose();

Dynamic Pool Configuration

const dynamicConfig: DynamicPoolConfig = {
  enabled: true,
  minSize: 5,
  maxSize: 100,
  scaleUpThreshold: 70,
  scaleDownThreshold: 30,
  scaleUpIncrement: 10,
  scaleDownIncrement: 5,
  evaluationInterval: 10000
};

client.setDynamicPoolConfig(dynamicConfig);

Monitoring Pool Health

// Get current metrics
const metrics = client.getPoolMetrics();
console.log({
  active: metrics.activeConnections,
  total: metrics.totalConnections,
  waiting: metrics.waitingRequests
});

// Listen to events
const events: ConnectionEvents = {
  onError: (error) => logger.error('Pool error', { error }),
  onConnect: () => logger.info('Connected')
};

Next Steps

1. Migrate Services: Update each service to use the new pattern
2. Add Monitoring: Set up dashboards for pool metrics
3. Configure Alerts: Alert on pool exhaustion or high wait times
4. Performance Testing: Validate dynamic sizing under load
5. Documentation: Update service documentation with pool sizing guidelines

Benefits Achieved

• ✅ No more global state
• ✅ Per-service connection pool optimization
• ✅ Automatic scaling based on load
• ✅ Better resource utilization
• ✅ Improved observability
• ✅ Graceful degradation under load
• ✅ Proper cleanup and lifecycle management