Legacy systems aren't inherently bad — they’re often successful systems that adapted over time. But modernizing them is rarely straightforward.
The Strangler Pattern offers a gradual path to replace parts of the old system without a risky big bang rewrite. Inspired by how a strangler fig tree overtakes its host, this pattern lets you slowly route functionality to a new system while retiring old components.
Key Steps
- Identify Boundaries: Start with clearly defined parts that can be intercepted — typically user-facing endpoints or service layers.
- Implement New Flow: Use a proxy layer or API Gateway to route select requests to the new service.
- Gradually Expand: As confidence builds, more traffic or features are handed over to the new system.
In one of my projects, this helped us migrate authentication and billing modules separately over a 9-month period. This minimized risks, helped us validate performance early, and eventually eliminated the old codebase with minimal disruption.
Tooling
- API Gateway (e.g., Kong, AWS API Gateway)
- Feature toggles for routing
- Observability tools to compare metrics from both systems
Avoid rewriting complex logic blindly. Understand usage patterns and existing pitfalls first.
I’ve come to appreciate the strangler pattern as a pragmatic approach in fast-moving teams with high business pressure. It’s not just a technical strategy — it’s a way to gain stakeholder trust and drive change iteratively.