ADR 5: Backend Framework Selection

Context

Having decided on Node.js/TypeScript, we need a framework to orchestrate the API, AI services, and database interactions. The framework must support modularity, high testability, and a clear separation of concerns to maintain the project's integrity.

Comparison (Market 2026)

Feature	NestJS	Fastify	Express	Elysia (Bun)
Architecture	Highly Modular (DI)	Plugin-based	Unstructured	Minimalist
Integrity	High (Opinionated)	Medium	Low	Medium
Scalability	Excellent	Excellent	Medium	High
Learning Curve	High	Low	Lowest	Medium

Decision

NestJS using the Fastify underlying HTTP engine.

Rationale

NestJS was chosen as the backbone of TrustVote AI, leveraging the Fastify adapter for optimal performance:

1. Dependency Injection (DI): Essential for mocking AI services and databases during testing, ensuring the core logic can be audited without side effects.
2. Architecture: It forces a modular structure (Modules, Controllers, Services). This is critical for a "Trust" project where we need to isolate the "Voting Engine" from the "AI Analysis Engine".
3. Performance (Fastify Engine): By opting for Fastify as the underlying platform instead of the default Express engine, we achieve significantly higher throughput and lower latency, which is vital for high-integrity, real-time voting scenarios.
4. Ecosystem: Native support for Microservices and WebSockets, which will be needed when we implement real-time voting updates.
5. Documentation: Being the most used enterprise framework for TS, finding solutions for complex security patterns is much faster.

Consequences

• Boilerplate: NestJS requires more files initially compared to Express.
• Strictness: We must follow the NestJS way (decorators, modules), which adds a small overhead in development time but saves weeks of refactoring later.
• Fastify Compatibility: While Fastify is faster, we must ensure that any third-party plugins used are compatible with the NestJS Fastify adapter.