Service Monorepo: Full Architecture Guide

The Monorepo That Runs 29 Services

In the last post, I explained why I built this platform from scratch. Now let's open the hood.

This monorepo contains two applications — a FastAPI backend and a Next.js frontend — plus shared infrastructure configuration. Everything lives in one repository, deploys via one docker-compose.yml, and runs on one $12/month VPS.

Here's the directory structure, explained layer by layer.

Top-Level Layout

The two applications communicate exclusively through HTTP. The frontend never imports Python code, and the backend never references React components. The contract is the API schema — documented automatically by FastAPI's OpenAPI generation at /docs.

FastAPI Backend — fastapi_backend/

Entry Point — main.py

The application is assembled in main.py. Here's the skeleton:

The lifespan context manager initializes Redis and starts the background scheduler on startup, then tears them down on shutdown.

Settings — config.py

All configuration comes from environment variables via Pydantic's BaseSettings:

No hardcoded secrets. No .env files committed to git. Every deployment environment (dev, staging, production) supplies its own values through environment variables or Docker secrets.

Database — database.py

Async SQLAlchemy 2.0 with session-per-request pattern:

The expire_on_commit=False is intentional — it prevents lazy-loading issues after commit, which is a common pitfall in async SQLAlchemy.

The 29 Routers

Every API endpoint lives in app/routers/. Here's what each one does:

That's 29 routers serving approximately 120 individual endpoints. Each router is between 50 and 300 lines. The admin.py router is the largest at ~500 lines because it handles post CRUD with all the tag/series/difficulty associations.

The 21 Database Models

All models inherit from SQLAlchemy's DeclarativeBase and live in app/models/:

Key models and their relationships:

The most interesting table is RagChunk. It stores post content split into chunks, each with a 1536-dimensional vector embedding. The search query is: find chunks whose embedding is closest to the query embedding, filtered by the user's premium tier. This is the core of the "Ask AI" feature.

Redis as the Glue Layer

Redis isn't a cache in this architecture — it's a service bus. It handles five distinct concerns:

1. Rate limiting — slowapi uses Redis as its backing store. Each endpoint family has its own limit (100/hr for anonymous, 500/hr for authenticated). The key is rate_limit:{ip}:{route_group} with a sliding window counter.

2. OAuth state — Google OAuth uses a redirect-based flow. The state parameter (a random token) is stored in Redis with a 10-minute TTL. After the callback, the token is verified and deleted. This prevents CSRF on the OAuth handshake.

3. Embedding cache — When a user asks the RAG system a question, the query is first checked against a Redis set of recent queries. If found within 5 minutes, the cached embedding is reused. This saves ~200ms per query on repeated questions.

4. Task queue — Redis pub/sub dispatches background tasks: email digests, content revalidation (purging Cloudflare cache), and maintenance jobs. The publisher pushes to a channel, and the APScheduler subscriber picks it up.

5. Leaderboard — XP rankings use Redis sorted sets (ZADD, ZREVRANK, ZRANGE). The leaderboard is recomputed every 5 seconds from a materialized PostgreSQL view, then stored in Redis for fast reads. This avoids sorting 10,000+ users on every page load.

Background Scheduler

app/core/scheduler.py runs four recurring jobs:

• Daily digests — queries the Post table for posts published in the last 24 hours, assembles an HTML email, and sends via SMTP to subscribed users.
• Sitemap regeneration — queries all published posts and series, generates a fresh sitemap.xml, and pings Google/Bing.
• Token cleanup — deletes expired refresh tokens from the database.
• Stripe sync — checks for subscriptions that should have expired and marks them accordingly.

The scheduler runs inside the same Python process as the FastAPI app. No separate Celery worker needed.

Frontend — blog_frontend/

The API Client — src/lib/api.ts

The frontend communicates with the backend through a typed API client. Every endpoint is a function:

The apiFetch wrapper handles:

• Automatic JWT token injection from cookies or localStorage
• 401 → token refresh → retry (with debounce to avoid race conditions)
• Error normalization (FastAPI validation errors → user-friendly messages)
• Request deduplication for concurrent identical calls

Server Components vs. Client Components

Every page is a server component by default. Client components are only used where interactivity is required:

• PostEditor — markdown editing, tag selection, image upload
• RagChat — streaming chat UI with source citations
• KnowledgeGraph — D3.js force-directed graph
• Navbar/CommandPalette — user menu, keyboard shortcuts
• ThemeToggle — dark/light mode

Server components handle everything else: data fetching, metadata generation, static params, and most of the rendering. This means the average page ships ~40KB of HTML instead of ~200KB of JavaScript.

Shared Infrastructure — nginx/

Nginx handles:

• Routing — /blog → Next.js frontend, /api → FastAPI backend
• Caching — static assets (CSS, JS, images) cached for 1 year with hashed filenames
• Compression — brotli for modern browsers, gzip fallback
• Security headers — HSTS, X-Content-Type-Options, CSP, Referrer-Policy
• Cloudflare integration — real IP headers, cache purging via API

The key insight: Nginx is the only entry point. There's no Kubernetes ingress, no cloud load balancer, no API gateway. One Nginx config handles everything.

What I'd Change

Looking back, there are three things I'd do differently:

1. Use a task queue from day one. The Redis pub/sub approach works, but it's not durable. If the app crashes mid-job, the task is lost. A proper queue (ARQ, Celery with Redis broker) would give retries, dead-letter queues, and job persistence.

2. Split the admin router. app/routers/admin.py at 500+ lines handles too many concerns. Post CRUD, user management, analytics queries, and task management should be separate routers. The file grew organically and never got refactored.

3. Add OpenAPI types to the frontend. The API client in src/lib/api.ts is manually typed. There's no code generation from the FastAPI OpenAPI schema. This means when the backend adds a field, the frontend type needs a manual update. Using openapi-typescript or orval would eliminate this class of bugs.

What's Next

In the next post, I'll dive into the RAG chat system — how embeddings are generated, how hybrid search works, and how the streaming response pipeline is built.

Built with FastAPI, Next.js 16, PostgreSQL, Redis, and zero third-party CMS. Deployed on a $12/month VPS.

Next in this series: Building a RAG Chat System: From Zero to Production →