Modular Monolith Backend
Refactored a flat MVC Node.js codebase into a feature-based modular monolith with layered architecture, Zod validation, and production-grade logging.
01. Overview
This project involved restructuring an existing Node.js MVC backend into a clean, feature-based modular monolith. Each feature module encapsulates its own controller, service, repository, and validation layers with strict TypeScript typing and Zod-based schema validation.
02. Problem Statement
Unscalable MVC Sprawl
The original codebase used a classic MVC pattern where all controllers, models, and routes lived in flat directories. As features grew, the coupling between modules made it nearly impossible to modify one feature without risking regressions across others. Cross-cutting concerns like validation and error handling were duplicated across controllers.
03. Constraints
- 01Zero Downtime MigrationThe refactor had to be done incrementally without breaking existing API contracts or causing downtime.
- 02Maintain API CompatibilityAll existing REST endpoints had to remain backwards-compatible with frontend clients.
- 03Type Safety EnforcementEvery layer boundary must be typed — no implicit any or unvalidated data crossing module boundaries.
04. System Architecture
Controller Layer
Thin controllers that parse validated input and delegate to service layer. No business logic.
Service Layer
Core business logic. Each service encapsulates domain operations and coordinates between repositories.
Repository Layer
Data access abstraction. MongoDB operations are isolated behind typed interfaces for testability.
05. Key Decisions
Zod over Joi for Validation
Zod's TypeScript-first approach means validation schemas double as type definitions. This eliminated the class of bugs where a runtime validator and TypeScript type diverge.
Winston + Morgan for Logging
Production-grade structured logging with Morgan for HTTP request logs and Winston for application-level logging with log rotation and transport configuration.
06. Trade-Offs
More Files, Clearer Boundaries
The feature-based structure tripled the number of files. However, each file has a single responsibility, making the codebase navigable and each module independently testable.
Incremental Migration Overhead
Maintaining backwards compatibility during the migration required temporary adapter layers that were removed after full cutover.
07. Outcome
The refactored architecture allows feature teams to work independently within their module boundaries. New features can be added by creating a new feature directory without touching existing code. The Zod-based validation pipeline catches malformed data at the API boundary before it reaches business logic.