Strangler fig

Gradually replace a legacy system by routing new or migrated capabilities to new implementations while the old system shrinks — like a fig tree strangling its host. Use facades, proxies, and feature flags to shift traffic capability by capability with rollback.

Trade-off: running two systems in parallel is expensive but safer than big-bang rewrite.

On interviews: migrate authentication from monolith to new service without downtime; define routing rules and success metrics.

Common pitfalls: unmaintained strangler layer forever; migrating data without sync verification; cutting over without observability parity.

Checklist:

• Route by feature or URL with measurable traffic split.
• Keep adapters thin and testable.
• Verify data parity between old and new paths.
• Decommission legacy modules with explicit milestones.