Software Architecture

Modularity

• Build small, independent units that each do one job. Prefer composition of small pieces over large multi-purpose modules.

Coupling & interfaces

• Favor loose coupling: modules interact through explicit, narrow interfaces, not shared mutable state or reaching into each other's internals.

Monolith vs services

• Modular-monolith is the default for this app: organize by clear internal module boundaries inside one deployable. Peel out a separate service only when there's a concrete justification (independent scaling, independent deploy cadence, org boundary) — avoid premature microservices.

Observability

• Design in logs, metrics, and traces for any new subsystem, not bolted on after the fact. A feature isn't done until it's debuggable in production.

Security posture

• Shift-left: security review happens at design time (DevSecOps), not only at the code-review gate.
• Apply Zero Trust and least privilege between modules/services — a component should only have the access it needs, nothing implicit.
• Practice data minimization: don't store or pass more data than the use case needs, especially for financial/PII data.

External dependencies

• Put an abstraction layer around external/broker/market-data/AI dependencies (e.g. IBKR/Freedom Finance data feeds, AI providers) so the app depends on an internal interface, not directly on a third-party SDK/shape. This keeps provider swaps and testing tractable.

Applied to this stack

These principles apply concretely to: Next.js 15 (App Router) + Drizzle ORM on Neon Postgres + better-auth + Vercel. Keep the module boundary between services/ (external integration + data access) and app//components/ (presentation) intact; don't let Drizzle queries leak into components.

---

Source: wondermentapps.com/blog/software-architecture-best-practices