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Part 3: Scalability

Overview: From Modular to Scalable Architecture

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


Why Modularity Isn’t Enough

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

  • Feature isolation
  • Testability
  • Clear developer ownership

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.


What Makes an Architecture “Scalable”?

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.


What Changes When You Scale?

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.


How This Part Is Structured

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

We’ll walk through:


📘 Chapter 11: Scalable Architecture Overview

  • How to recognize scale pressure in your project
  • When modular breaks down
  • The shift toward layered thinking

📘 Chapter 12: Scalable Folder Structure for React + Vite

  • Moving from features/ to layers/ (api/, services/, components/, etc.)
  • Handling cross-feature UI and state
  • Building reusable component libraries
  • Team-oriented React structures

📘 Chapter 13: Scalable Folder Structure for FastAPI

  • Layering your backend: api/, services/, schemas/, infra/, core/
  • Handling async background jobs (e.g., Celery, FastAPI background tasks)
  • Auth, role-based access, and scalable routing
  • Repository pattern and clean architecture principles
  • How to structure integrations with vector DBs, LLM clients, and queues

📘 Chapter 14: Deploying and Scaling React + FastAPI Projects

  • Folder structure impact on Docker, CI/CD, and deployment flow
  • Secrets, environment configs, and managing .env files across environments
  • Asset management and public files
  • Scaling horizontally vs. vertically
  • How folder structure supports Infrastructure-as-Code (IaC)