🔗 1 million DB updates, 3 days, 200 lines of Go: A legacy rescue story

A legacy PHP application faced daily failures, causing delays in marketing campaigns. Instead of a complete rewrite, the team opted for a minimal intervention using Go and RabbitMQ. By decoupling the request lifecycle, they managed to process user updates asynchronously without overwhelming the database. The solution, implemented in just 200 lines of Go, led to zero failures over 18 months, demonstrating the power of simplicity and idempotency in rescue scenarios. This article details the challenges faced, the design choices made, and the significant outcomes achieved. It's a practical case study that underscores the need for adaptive, low-risk solutions in legacy systems.

package main

import (
 "database/sql"
 "time"
)

func processBatch(messages []StatusMessage) error {
 // 1. Fixed batch size of 50,000 messages.
 batch := messages[:50000]
 // 2. Begin an atomic transaction. (ALL or NOTHING)
 tx, err := db.Begin()
 if err != nil {
 return err
 }
 defer tx.Rollback()
 // 3. Process updates within the transaction.
 for _, msg := range batch {
 _, err := tx.Exec(
 ` UPDATE users SET status = 'processed' WHERE id = $1 AND status = 'pending' `,
 msg.UserID)
 if err != nil {
 return err
 }
 }
 // 4. Commit the batch atomically.
 if err := tx.Commit(); err != nil {
 return err
 }
 // 5. Acknowledge messages to RabbitMQ ONLY after commit.
 for _, msg := range batch {
 msg.Ack()
 }
 // 6. Static Backpressure: Sleep for 2 seconds.
 time.Sleep(2 * time.Second)
 return nil
}