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Phase 35 · Lesson 02

Dependency Direction and Accidental Coupling

≈ 2 min core reading · 198 source/companion words
Learning contract

What you should be able to do after this

  • Explain Dependency Direction and Accidental Coupling without relying on framework buzzwords.
  • Connect the lesson to the phase outcome: Review and refactor TaskFlow architecture using evidence: dependency direction, duplication, tests, complexity, security, and operational boundaries.
  • Reproduce the relevant behavior and verify it with evidence.
  • Diagnose at least one representative failure instead of guessing.
Figure 35.2 — TaskFlow boundary map
observe architecture
identify smell/risk
establish tests
small refactor
verify behavior
document decision
This lesson belongs to Phase 35. Keep the full request/data path visible even when today's code changes only one box.
Instructor workflow: predict first → implement the smallest slice → run it → inspect evidence → break one assumption → diagnose → explain it back. Do not copy the whole code block before understanding the first failing boundary.

Dependency Direction and Accidental Coupling Dependencies should point toward the layer that owns the contract. A useful review looks for framework details leaking upward, business decisions leaking downward, and shared packages importing application-specific code.

- - Persistence - Infrastructure HTTP / UI Application intent > > contract > implementation

Figure 3 - Desired direction: outer mechanisms depend on inner contracts

Healthy examples • TasksService depends on TaskRepository contract, not Prisma query shapes.

• Controllers depend on application services, not database clients.

• The web consumes API contracts, not Prisma-generated types.

• packages/validators contains transport-neutral shared validation, not NestJS decorators or React components.

Coupling smells • A UI component imports generated Prisma enums directly.

• A repository throws NestJS HTTP exceptions.

• A shared package imports from apps/web or apps/api.

• A service accepts Fastify Request/Reply objects.

• Infrastructure credentials become arguments to domain/business helpers. # Review question for every import Who owns this contract?

If A imports B: - Does A genuinely depend on B's responsibility? - Or did a convenient type/location create accidental coupling?

Do not chase zero coupling. Chase intentional coupling with clear ownership. Source basis: integrated TaskFlow architecture and engineering decisions established in Phases 00-34; this chapter is an internal review/refactoring synthesis rather than a new framework feature.

Executable lesson studio · architecture

Now turn the explanation into engineering skill.

Dependency Direction and Accidental Coupling matters because it changes a real boundary in TaskFlow. The goal is to understand the inputs, owner, observable output, and failure modes before adding more abstraction.

By the end, you should be able to

  • Explain Dependency Direction and Accidental Coupling in terms of concrete inputs, ownership, outputs, and evidence.
  • Implement or reproduce the smallest TaskFlow slice that demonstrates this architecture concept.
  • Diagnose one representative architecture failure using observable evidence instead of trial-and-error.
Figure 35.2B — Mental model
User intent
UI boundary
API contract
business rule
persistence
observable result
Read left-to-right while implementing. When debugging, walk right-to-left until the first observed fact differs from your prediction.
Code-along sequence

One coherent change at a time

  1. 01

    Restate the goal of “Dependency Direction and Accidental Coupling” as one observable behavior. Write the success signal before touching code.

  2. 02

    Trace the boundary map: User intent → UI boundary → API contract → business rule → persistence → observable result. Mark the one boundary this lesson changes.

  3. 03

    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.

  4. 04

    Implement the smallest vertical slice. Add one coherent change, run the narrowest verification command, and read the output before continuing.

  5. 05

    Connect the slice to TaskFlow’s phase artifact: Architecture review report plus one behavior-preserving refactor backed by tests and an ADR.. Verify behavior from the outside as a user or API client would.

  6. 06

    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.

  7. 07

    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.

txtBoundary trace
Input -> owner -> transformation -> observable output -> failure evidence

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.

Verification

Prove the happy path

  1. Run the narrowest relevant command or request and require a clean exit/status.
  2. Inspect the boundary-specific evidence: terminal output, Network panel, database row/query plan, test report, container health, or deployment logs.
  3. Repeat the behavior once after the change so a one-off cached or stale result cannot masquerade as success.
Break it on purpose

Build debugging memory

  1. Refactor with no behavioral safety net
  2. Change one assumption in the architecture example—identity, input, configuration, ordering, connectivity, or state—and predict the exact symptom before running it.
SymptomInvestigation path
Expected result does not appear

Return to the nearest observable boundary: input, network, process log, database, or rendered UI. Compare actual evidence with the predicted path.

A shortcut appears to fix it

Change one variable at a time and reproduce the original failure after the fix; otherwise you have correlation, not diagnosis.

Closed-reference challenge

Can you transfer the idea?

Without copying the final source, create a second small scenario that applies “Dependency Direction and Accidental Coupling” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.

Explain-back

Answer these without reopening the solution

  1. What problem does Dependency Direction and Accidental Coupling solve in TaskFlow, and what would be observably wrong without it?
  2. Which part of the architecture boundary owns the decision, and which neighboring layer must not own it?
  3. What evidence proves the happy path instead of merely suggesting it worked?
  4. Name one realistic failure or abuse case and the first place you would inspect.
  5. If requirements doubled tomorrow, which part of this design should stay stable and which part is allowed to change?
Lesson studio

Turn the reading into skill

01

Do it

Find one duplication or accidental abstraction whose cost is visible in real code.

Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.
02

Break it safely

Refactor with no behavioral safety net

Predict the symptom before causing the failure. Then locate the earliest boundary where reality diverges from expectation.
03

Explain it back

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.
Exit ticket

Before marking complete

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.

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