API & Database Connection Pool Limits

// 1. Proper database/sql Connection Pool Sizing in Go
db, err := sql.Open("postgres", dsn)
if err != nil {
    log.Fatal(err)
}

// Bounded pool to prevent PostgreSQL context-switch thrashing
db.SetMaxOpenConns(50)                  // Never allow more than 50 simultaneous sockets
db.SetMaxIdleConns(25)                  // Keep warm idle pool for burst smoothing
db.SetConnMaxLifetime(5 * time.Minute)  // Recycle connections to prevent RDS memory leaks
db.SetConnMaxIdleTime(1 * time.Minute)

// 2. Production HTTP Middleware: Pool Acquire Deadline & Adaptive 503 Fast-Fail
func WithPoolGuard(db *sql.DB, acquireTimeout time.Duration, next http.HandlerFunc) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        // Fast-fail context for checking out a pool connection
        acquireCtx, cancel := context.WithTimeout(r.Context(), acquireTimeout)
        defer cancel()

        conn, err := db.Conn(acquireCtx)
        if err != nil {
            // Pool is starved! Fast-fail in 25ms rather than waiting 5 seconds for a 504
            w.Header().Set("Retry-After", "2")
            w.WriteHeader(http.StatusServiceUnavailable)
            w.Write([]byte(`{"error":"db_pool_exhausted","retry_in_sec":2}`))
            return
        }
        defer conn.Close() // Return connection to pool immediately

        // Execute request with remaining client deadline
        next(w, r)
    }
}