Legacy-First Design (LFD): A Temporal Approach to Software Sustainability

Legacy-First Design (LFD) is a conceptual architectural methodology that treats time as a primary architectural constraint. It addresses a recurring problem in contemporary software systems: structural degradation caused by short-term decision-making, technological volatility, and the absence of long-term architectural governance.

Rather than prescribing implementation details or specific architectural patterns, LFD operates at the architectural decision level, introducing explicit criteria for distinguishing between permanent and transient system elements. The method is built around six fundamental principles, including the novel concept of Survival in Abandoned State, which frames prolonged absence of maintenance as a predictable phase in the software lifecycle rather than an exceptional failure.

LFD is proposed as a normative and decision-oriented methodology, grounded in qualitative analysis of recurring failure patterns observed in long-lived software systems. It does not seek to optimize short-term outcomes such as delivery speed or technological adoption, but instead aims to preserve system identity, conceptual coherence, and readability across extended time horizons.

The methodology is compatible with existing architectural styles and modeling approaches, including Clean Architecture, Domain-Driven Design, and Hexagonal Architecture, while providing a temporal governance layer that these approaches do not explicitly address.

This publication presents the canonical definition (v1.0) of Legacy-First Design and is intended to serve as a stable reference for future discussion, application, and evolution of the method.