Hands-On Lab & Phase Exit Request matrix, exercises, oral exam, and completion checklist
Phase 12 - DTOs, Zod & Input Validation Volume 04 - Professional Backend
Architecture milestone install
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wire
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test invalid inputs
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verify persistence
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explain architecture
Phase 12 introduces an explicit runtime validation boundary before controller and service logic.
Hands-on lab Complete the lab against the running Phase 11 TaskFlow API. The goal is not only to make valid requests succeed; you must prove invalid data is rejected at the intended layer.
Lab A - Install and wire validation • From apps/api install zod@4.6.5.
• Register StandardSchemaValidationPipe globally in main.ts.
• Create tasks.schemas.ts.
• Remove the temporary tasks.types.ts once controller/service imports have been migrated.
• Build the API before running it.
Lab B - Create schema tests Payload Expected
valid projectId + title 201
missing title 400
title with only spaces 400
title > 200 chars 400
projectId not UUID 400
unknown admin field 400
valid UUID for missing Project 404
Lab C - Update schema tests Payload Expected
{"status":"DONE"} success for existing Task{"status":"ALMOST_DONE"} 400{"description":null} success; clear description{} 400{"projectId":"..."} 400 unknown fieldLab D - Path parameter tests • GET /tasks/not-a-uuid should return 400 without querying the service for a Task.
• GET /tasks/ should pass validation and then return 404 from application logic.
• PATCH and DELETE should apply the same UUID contract.
1 - Follow Reproduce the reference schemas exactly and pass the matrix.
2 - Modify Lower title max length temporarily and observe the boundary behavior, then restore it.
3 - Implement Add a validated dueAt transport field with an intentional accepted format; map it safely into the service.
4 - Engineer Design whether TaskFlow should reject or strip unknown fields in public API v1, and defend the decision with compatibility and debugging trade-offs.
Architecture oral exam • Why can a TypeScript interface not validate JSON arriving over HTTP?
• What does StandardSchemaValidationPipe do, and what does the route decorator schema option do?
• Why does a valid UUID for a missing Project become 404 rather than 400?
• What is the semantic difference between omitted description and description: null in PATCH?
• Why should TaskFlow not automatically move all Zod schemas into packages/validators today?
• Which layer should verify that an assignee belongs to the same Workspace as the Task?
Phase exit checklist • zod is installed and locked at 4.6.5.
• global StandardSchemaValidationPipe is active.
• create/update/path schemas are attached to controller inputs.
• unknown body fields are rejected.
• valid data still persists through Prisma.
• invalid shape does not reach TasksService.
• API build passes.
• you can explain validation vs business rule vs database constraint without looking at the book.
Figure 1 - Phase 12 complete architecture Untrusted HTTP input
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TaskFlow now has a clean request-shape boundary before application logic.
Phase 13 will make errors professional across the whole API: consistent error structure, exception boundaries, Prisma conflict translation, request correlation, and what should or should not be exposed to clients.
Hands On Lab and Phase Exit 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.
Restate the goal of “Hands On Lab and Phase Exit” as one observable behavior. Write the success signal before touching code.
Trace the boundary map: untrusted input → runtime schema → validated value → business rule → safe operation. 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: Reusable create/update/query schemas integrated into NestJS request handling with useful validation errors.. 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.
const CreateTaskSchema = z.object({
title: z.string().trim().min(1).max(160),
priority: z.enum(["LOW", "MEDIUM", "HIGH"]),
});
const input = CreateTaskSchema.parse(requestBody);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.
Return to the nearest observable boundary: input, network, process log, database, or rendered UI. Compare actual evidence with the predicted path.
Change one variable at a time and reproduce the original failure after the fix; otherwise you have correlation, not diagnosis.
Without copying the final source, create a second small scenario that applies “Hands On Lab and Phase Exit” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Create a partial update schema carefully and reason about omitted vs nullable fields.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Duplicating client and server validation rules without a sharing strategy
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.