Software Architecture for Solo Developers
The Modular Monolith
Building maintainable systems alone requires different tradeoffs than team development. This post covers modular monoliths, decision logs, architectural fitness functions, and when to accept tech debt as a solo founder.
The Modular Monolith
Microservices require DevOps overhead, network debugging, distributed tracing, and team coordination — all costs a solo dev shouldn't pay. A modular monolith divides code into bounded contexts (packages/modules) with strict internal APIs, keeping deployment simplicity while enforcing architectural boundaries. Extract to services only when scaling forces it.
Architecture Decision Records
ADRs document every significant architectural choice: context, decision, consequences. A one-page ADR saves hours of rediscovering why you chose PostgreSQL over MongoDB, or why you went with polling over WebSockets. When you revisit a component months later, the ADR tells you the reasoning — and whether the constraints have changed.
Fitness Functions
Automated guards that protect architectural decisions: circular dependency checks, package-level API surface tracking, database query analysis (no N+1), and response time budgets. CI running these checks catches regressions that a solo dev would miss until deployment. Examples: forbid direct database access from controller modules, enforce max 200 lines per file.
Strategic vs Tactical Tech Debt
Tactical tech debt (quick fix now, clean later) is necessary for solo devs with limited time. Strategic tech debt (architectural shortcuts that compound) must be avoided. The distinction: can the shortcut be fixed in 30 minutes without rewriting surrounding code? If yes, tactical. If it affects the entire module's interface, strategic.
Testing Without a QA Team
Integration tests beat unit tests for solo projects — they cover real user workflows and catch more bugs per test written. The testing pyramid for solo devs: 60% integration (API routes, DB queries), 30% unit (core business logic, complex calculations), 10% E2E (critical user paths). Prioritize tests that would wake you up at 3 AM if they broke.
Documentation That Stays Fresh
Code comments rot. READMEs become fiction. The only documentation that stays accurate: the code itself (readable with good naming), integration tests (show how components connect), and ADRs (why decisions were made). Everything else — docstrings, inline comments explaining the obvious — is noise.
Solo development architecture is about maximizing impact per unit of complexity. Modular monoliths, ADRs, and strategic testing practices let one person build and maintain systems that would otherwise require a team.