Five foundations for building complex Rails apps

Takeaways

Vanilla Rails suffices until ~100–150k LOC; beyond that, maintainability needs new technical and mental foundations.
Domain-Driven Design first—prioritize business understanding over Jira-as-truth; shifts engineers toward purposeful modeling.
Mutation testing validates test quality on complex logic—not widespread in Ruby but strong signal for critical paths.
Event sourcing and CQRS appear as layers for apps that must mirror business growth and auditability.
AI listed as fifth foundation—tooling to navigate complexity, not a replacement for DDD discipline.

Career/coding reference for large Rails systems; indirect for vault ops unless building Rails-based agent backends.

Where is the complexity threshold where DDD pays off in smaller greenfield apps?