Context

• We have a big old, obsolete running system, we want to replace it with new architecture, new tech stack. E.g: monolith => Micro service
• We don't have enough effort to replacing a whole new system at once, and that's also risky. We need a gradually migration.

Solution

• Incrementally replace specific pieces of functionality with new applications and services
• Create a façade that intercepts requests going to the backend legacy system, then routes requests to either to the legacy application or the new services. Existing features can be migrated to the new system gradually

Consideration

• At some point, when the migration is complete, the strangler fig façade will either go away or evolve into an adaptor for legacy clients.
• Make sure the façade doesn't become a single point of failure or a performance bottleneck

When not to use

• When requests to the back-end system cannot be intercepted.
• Smaller systems where the complexity of wholesale replacement is low.

Reference

• https://docs.microsoft.com/en-us/azure/architecture/patterns/strangler-fig
• https://dzone.com/articles/monolith-to-microservices-using-the-strangler-patt
• https://martinfowler.com/bliki/StranglerFigApplication.html