Modular Monolith First: Domain Boundaries A growing system should first become modular in code and data ownership. A modular monolith keeps one deployment while giving each business capability an explicit public interface.
- - - - - HR Payroll Procurement Inventory Accounting Recruitment > > > > > Figure 2 - Business capabilities become modules, not miscellaneous folders
Module rule • Group controllers, services, policies, repositories, schemas, and events around a business capability rather than technical type alone.
• Expose only the providers/contracts another module genuinely needs. NestJS modules encapsulate providers by default and exported providers form the module public interface.
• Avoid making every helper global; controlled imports keep dependency direction visible as the team grows.
When to consider extraction • Independent scaling profile, independent release cadence, strong data ownership, separate team responsibility, regulatory isolation, or persistent reliability pressure.
• Do not extract a microservice merely because a module has many files. apps/api/src/modules/ people/ payroll/ leave/ procurement/ inventory/ accounting/ recruitment/
Each module owns its rules and exports a small public API.
A clean module boundary is valuable even if every module runs in the same NestJS process. Official reference: NestJS Modules - https://docs.nestjs.com/modules
Enterprise systems are difficult because business invariants, approvals, auditability, effective dates, and permissions interact over long-lived data.
Restate the goal of “Modular Monolith and Domain Boundaries” as one observable behavior. Write the success signal before touching code.
Trace the boundary map: business workflow → invariants → permissions → transaction/audit → operational outcome. 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: Enterprise expansion blueprint for an HRIS/procurement/inventory/payroll-style system with modules, workflows, events, audit, and scaling triggers.. 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.
Requisition -> Approval -> Purchase Order -> Goods Receipt -> Invoice
Invariant examples:
- received quantity <= ordered quantity
- finalized approvals are auditable
- closed orders reject new receiptsDo 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 “Modular Monolith and Domain Boundaries” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Choose one enterprise domain and identify business capabilities before choosing services/databases.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Microservices before module boundaries are understood
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.