Best Practices for Refactoring Complex Legacy Code - The Amazonian Way

In a world where everyone’s rushing to ship their next magical product, there’s one tiny detail that tends to get ignored: technical debt.

As features pile up on shaky foundations, you end up with patches on top of patches. Suddenly, the code is so unreadable even AI throws up its hands and says, “Sorry bro, you're on your own.”

Why Not Just Start From Scratch?

Honestly, starting from scratch sounds like the dream. But then reality kicks in - it’s expensive, time-consuming, and often not feasible. You’re juggling:

- Rewriting everything,
- Supporting production bugs,
- Keeping new clients happy,
- And responding to a never-ending stream of stakeholder requests…
...all while trying to stay agile.

To do it properly, you’d need a parallel team just for the rewrite. That’s why we look to how the big players handle it.

Enter: The Amazonian Approach

When Amazon was still just selling books, its tech infrastructure was already groaning under the weight of its future ambitions. They were stuck in a classic mess:

- Slow deployments
- Tightly coupled code
- Siloed teams

They needed to act fast. But instead of just slapping on more patches, they reimagined the entire system.

Cue Jeff Bezos dropping the mic with this internal mandate:
"All teams will henceforth communicate through APIs."

This wasn’t just a tech shift - it was a cultural one.

Decoupled Teams for Decoupled Systems

Amazon realized that fixing legacy code wasn’t just about better architecture - it was about how teams worked.

- They introduced the concept of "Two-Pizza Teams" - small, autonomous teams that could be fed with (you guessed it) two pizzas.
- Each team had its own single-threaded leader responsible for execution.
- The mission: build services, not layers.

Refactoring the Amazonian Way: Step-by-Step

1. Identify Bounded Contexts

Start with Domain-Driven Design.
Break the monolith into logical service boundaries based on real business domains. Think “Inventory Service,” “Product Service,” etc.

2. Build APIs as Contracts

Before even splitting services physically, define how they’ll communicate.
Build and respect those APIs - even inside the monolith (aka modular monolith strategy).

3. Separate by Use Case, Not Layers

Ditch the old UI → Service → DB layering.
Go vertical: each service owns its entire stack - from database to frontend delivery.

4. Parallel Development & Incremental Cutovers

Use the Strangler Fig Pattern: gradually replace parts of the monolith with new microservices.
No big-bang rewrite. Evolve - like Amazon did.

5. Culture of Metrics and Ownership

Every team owns its service.
They monitor error rates, latency, and traffic. They have dashboards. And yes, they’re accountable.

It’s Not About Services. It’s About Culture.

Amazon didn’t just fix its code - it changed how it thinks. Services are just the output of a deeper philosophy:

- Speed
- Reliability
- Autonomy

Even the most massive decoupled systems started with just one service.