Architecting a Modular Monolith: Fast Flow Without the Microservice Overhead

A modular monolith strikes a balance between simplicity and long‑term evolvability. Instead of prematurely distributing complexity, we encode domain boundaries inside a single deployable artifact—making refactors and reshaping features cheaper while avoiding the blast radius of a distributed system too early.

Why Not Jump Straight To Microservices?

Operational Tax: Every additional deployable adds CI/CD, infra surface, secrets, runtime tuning.
Premature Boundaries: Domain seams guessed up‑front frequently drift from actual change & collaboration patterns.
Debugging Cost: Early teams need fast feedback loops; distributed tracing + network hops slow learning.

Defining Modular Boundaries

We map high-change user journeys and domain concepts to internal modules with explicit contracts:

Create a capability catalog (verbs + nouns).
Group into cohesive domains with low external chatter.
Enforce boundaries via folder structure + TypeScript project references.
Expose a small public surface: factories, service facades, DTOs.

Example Module Layout

src/
	billing/
		domain/ (value objects, entities)
		app/ (use cases & orchestrators)
		infra/ (db adapters, gateways)
		api/ (handlers / resolvers)
	accounts/
	shared/

Evolution Path

When a module exhibits independent scaling / deployment needs (different latency SLO, team ownership), we extract behind a stable contract:

Introduce an internal interface + anti-corruption layer.
Shadow implementation as an internal service (same tests).
Route a small percentage of traffic (progressive cutover).

Metrics That Guide Extraction

Deployment coupling (modules changing together > 70% of the time).
Change fail rate localized to a module.
Hot paths saturating resource profiles differently.

Key Takeaways

Delay distribution until friction appears. Use modularity to create seams early, then externalize only when economics justify the split.