Tell me about a time you simplified a complex legacy architecture or refactored a monolith.

In Simple Words: Use the Strangler Fig Pattern: extract functionality incrementally around the edges, verify correctness with feature flags and dual-writing, and avoid risky "big bang" rewrites.

The STAR Response:

• Situation: Our core e-commerce monolith had a 3,500-line OrderProcessor file that handled validation, inventory reservation, credit card charging, and email receipts in a single synchronous database transaction, causing frequent deadlocks.
• Task: Decouple the order processing pipeline into maintainable, independently scalable modules without disrupting active sales.
• Action:
  1. Mapped out the domain boundaries and introduced an asynchronous event-driven architecture using the Strangler Fig Pattern.
  2. Extracted inventory checks and email receipts into asynchronous background consumers listening to OrderCreated domain events.
  3. Implemented the Dual-Writing & Dark Launch technique: ran the new event pipeline alongside the old code in shadow mode to verify that outputs matched 100%.
  4. Switched production traffic over using feature flags with zero downtime.
• Result: Reduced primary checkout latency by 65% (from 1,200ms to 420ms), eliminated transaction deadlocks completely, and reduced the codebase cognitive load so new developers could ship inventory features safely.

Practical Script & Summary Template

### Strangler Fig Migration Phases:
1. **Identify & Isolate**: Define clean domain boundaries and interfaces.
2. **Shadow Execution (Dark Launch)**: Execute new path concurrently and compare results without user exposure.
3. **Canary Cutover**: Shift 1% -> 10% -> 50% -> 100% of traffic via dynamic feature flags.
4. **Decommission**: Safely delete deprecated legacy code blocks.