Part 3: Scalability¶

Overview: From Modular to Scalable Architecture¶

“Modularity gives you structure.
Scalability gives you strategy.”

Modular architecture is a powerful starting point. It gives you:

But as your app evolves—more features, more shared services, more contributors—you’ll hit structural friction that modularity alone can't solve.

That’s when you shift to scalable architecture.

Not because modular is wrong. But because:

Modules start depending on shared services
Global concerns like auth, logging, or analytics spread across features
Your folder structure stops answering the question: How do I grow this app with 10 developers and 100 routes?

Scalability is about evolving your structure to match team scale, system complexity, and domain boundaries.

A scalable folder structure does four things well:

Property	Description
Separation of layers	Clearly splits routing, services, models, and infra
Global service integration	Supports logging, error handling, AI pipelines, permissions
Developer parallelism	Allows multiple teams to work without merge conflicts
Evolvability	You can replace or extend parts (e.g., swap GPT for Claude, or SQL for vector DB) without rewriting half the system

It’s not about having more folders. It’s about creating a structure that scales with both technical load and team complexity.

Concern	Modular Focus	Scalable Focus
Feature ownership	Each module owns everything	Global services must be shared cleanly
Code reuse	Optional and local	Mandatory and enforced
Testing	Unit-first, colocated	Unit + integration + CI pipelines
Dev velocity	1–2 devs/module	2+ teams, overlapping features
Cross-cutting concerns	Minimal	Core (auth, observability, versioning) must be layered
App orchestration	Simple request→response	Background jobs, queues, task orchestration, streams

Scalability introduces architectural patterns: services, adapters, repositories, core layers, shared infrastructure.

You move from “just building features” → to engineering systems.

This part gives you everything you need to transition into a scalable architecture.

We’ll walk through: