Mission Rebuild the capability of Phase 04 without following the exact code line-by-line. The goal is transfer: can you use the ideas when the variable names, UI, or route shape changes?
Required outcome Build a NestJS 12 API on Fastify and understand modules, controllers, providers, dependency injection, configuration, and request lifecycle.
Required artifact A modular API with health endpoint and a small feature module whose controller and service responsibilities are separate.
Constraints • Start from the previous phase checkpoint or a fresh branch. • Use the course architecture boundaries, but do not copy the reference implementation until after your first attempt. • Keep changes scoped to the capability. • Preserve validation, authorization, error handling, and observability expectations already earned in earlier phases. • Record any design decision where more than one implementation is reasonable.
Acceptance criteria • Can explain provider resolution. • Can create a feature module from scratch. • Can distinguish adapter and framework responsibilities. • Can debug a dependency injection error.
Independent build sequence 1. Generate the API workspace and replace the default HTTP adapter with Fastify deliberately. 2. Create a HealthModule with controller and service instead of putting everything in AppController. 3. Inject the service through the constructor and explain how Nest resolves it. 4. Add configuration through Nest Config rather than reading process.env everywhere. 5. Trace one request with logs through bootstrap, controller, and service. 6. Break provider registration intentionally, read the dependency-resolution error, and repair the module graph.
Self-review Compare your implementation to the reference only after the acceptance criteria pass. Differences are not automatically mistakes. For each difference, ask: does it preserve the same contract, security boundary, correctness, maintainability, and operational behavior? Keep the alternative when you can justify it with evidence.
Stretch challenge Change one assumption: more users, a second tenant, a slow network, an expired session, a concurrent mutation, a deployment restart, or a failed dependency. Predict whether your design still behaves correctly. If not, document the next architectural improvement rather than adding complexity blindly.
Phase exit Run the relevant checks, inspect Git diff, and write a three-sentence summary: what the system can do now, which boundary owns the new responsibility, and which failure you now know how to diagnose. Only then mark the phase complete.
NestJS structure becomes valuable when the request pipeline and responsibility of controllers, services, guards, pipes, and providers are explicit rather than magical.
Restate the goal of “Phase Synthesis & Independent Challenge” as one observable behavior. Write the success signal before touching code.
Trace the boundary map: HTTP request → Fastify adapter → Nest pipeline → controller → service → response. Mark the one boundary this lesson changes.
Inspect the current repository state and record the exact files, route, table, or runtime involved. Avoid creating an abstraction before you can name the existing owner.
Implement the smallest vertical slice. Add one coherent change, run the narrowest verification command, and read the output before continuing.
Connect the slice to TaskFlow’s phase artifact: A modular API with health endpoint and a small feature module whose controller and service responsibilities are separate.. Verify behavior from the outside as a user or API client would.
Create one controlled failure that is safe to reproduce. Predict the symptom first, capture evidence, locate the earliest broken boundary, then fix only that cause.
Close the reference and rebuild or explain the core change from memory. Record what you would monitor or test before trusting the same change in production.
@Controller("tasks")
export class TasksController {
constructor(private readonly tasks: TasksService) {}
@Get()
findAll() {
return this.tasks.findAll();
}
}Do not copy this as a finished answer. Predict what each boundary does, type the smallest useful slice, run it, then compare the result with your prediction.
Read the constructor index in the error, then check provider registration, module imports, and exports in that order.
Confirm method/path, controller registration, global prefix, and guard/pipe behavior using request logs.
Without copying the final source, create a second small scenario that applies “Phase Synthesis & Independent Challenge” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Break provider registration intentionally, read the dependency-resolution error, and repair the module graph.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Business logic embedded in decorators/controllers
Predict the symptom before causing the failure. Then locate the earliest boundary where reality diverges from expectation.Why does this responsibility live here, what input does it trust, what output proves success, and what would go wrong if another layer owned it?
If your explanation is vague, repeat the boundary trace before moving on.Close the reference. Reproduce the core idea from memory or explain the exact request/data flow aloud. Then check your diff or notes for accidental shortcuts that bypass validation, authorization, error handling, or verification earned in earlier phases.