codebase-onboarding

What this skill does

# Codebase Onboarding

**Tier:** POWERFUL  
**Category:** Engineering  
**Domain:** Documentation / Developer Experience  

---

## Overview

Analyze a codebase and generate comprehensive onboarding documentation tailored to your audience. Produces architecture overviews, key file maps, local setup guides, common task runbooks, debugging guides, and contribution guidelines. Outputs to Markdown, Notion, or Confluence.

## Core Capabilities

- **Architecture overview** — tech stack, system boundaries, data flow diagrams
- **Key file map** — what's important and why, with annotations
- **Local setup guide** — step-by-step from clone to running tests
- **Common developer tasks** — how to add a route, run migrations, create a component
- **Debugging guide** — common errors, log locations, useful queries
- **Contribution guidelines** — branch strategy, PR process, code style
- **Audience-aware output** — junior, senior, or contractor mode

---

## When to Use

- Onboarding a new team member or contractor
- After a major refactor that made existing docs stale
- Before open-sourcing a project
- Creating a team wiki page for a service
- Self-documenting before a long vacation

---

## Codebase Analysis Commands

Run these before generating docs to gather facts:

```bash
# Project overview
cat package.json | jq '{name, version, scripts, dependencies: (.dependencies | keys), devDependencies: (.devDependencies | keys)}'

# Directory structure (top 2 levels)
find . -maxdepth 2 -not -path '*/node_modules/*' -not -path '*/.git/*' -not -path '*/.next/*' | sort | head -60

# Largest files (often core modules)
find src/ -name "*.ts" -not -path "*/test*" -exec wc -l {} + | sort -rn | head -20

# All routes (Next.js App Router)
find app/ -name "route.ts" -o -name "page.tsx" | sort

# All routes (Express)
grep -rn "router\.\(get\|post\|put\|patch\|delete\)" src/routes/ --include="*.ts"

# Recent major changes
git log --oneline --since="90 days ago" | grep -E "feat|refactor|breaking"

# Top contributors
git shortlog -sn --no-merges | head -10

# Test coverage summary
pnpm test:ci --coverage 2>&1 | tail -20
```

---

## Generated Documentation Template

### README.md — Full Template

```markdown
# [Project Name]

> One-sentence description of what this does and who uses it.

[![CI](https://github.com/org/repo/actions/workflows/ci.yml/badge.svg)](https://github.com/org/repo/actions/workflows/ci.yml)
[![Coverage](https://codecov.io/gh/org/repo/branch/main/graph/badge.svg)](https://codecov.io/gh/org/repo)

## What is this?

[2-3 sentences: problem it solves, who uses it, current state]

**Live:** https://myapp.com  
**Staging:** https://staging.myapp.com  
**Docs:** https://docs.myapp.com

---

## Quick Start

### Prerequisites

| Tool | Version | Install |
|------|---------|---------|
| Node.js | 20+ | `nvm install 20` |
| pnpm | 8+ | `npm i -g pnpm` |
| Docker | 24+ | [docker.com](https://docker.com) |
| PostgreSQL | 16+ | via Docker (see below) |

### Setup (5 minutes)

```bash
# 1. Clone
git clone https://github.com/org/repo
cd repo

# 2. Install dependencies
pnpm install

# 3. Start infrastructure
docker compose up -d   # Starts Postgres, Redis

# 4. Environment
cp .env.example .env
# Edit .env — ask a teammate for real values or see Vault

# 5. Database setup
pnpm db:migrate        # Run migrations
pnpm db:seed           # Optional: load test data

# 6. Start dev server
pnpm dev               # → http://localhost:3000

# 7. Verify
pnpm test              # Should be all green
```

### Verify it works

- [ ] `http://localhost:3000` loads the app
- [ ] `http://localhost:3000/api/health` returns `{"status":"ok"}`
- [ ] `pnpm test` passes

---

## Architecture

### System Overview

```
Browser / Mobile
    │
    ▼
[Next.js App] ←──── [Auth: NextAuth]
    │
    ├──→ [PostgreSQL] (primary data store)
    ├──→ [Redis] (sessions, job queue)
    └──→ [S3] (file uploads)
         
Background:
[BullMQ workers] ←── Redis queue
    └──→ [External APIs: Stripe, SendGrid]
```

### Tech Stack

| Layer | Technology | Why |
|-------|-----------|-----|
| Frontend | Next.js 14 (App Router) | SSR, file-based routing |
| Styling | Tailwind CSS + shadcn/ui | Rapid UI development |
| API | Next.js Route Handlers | Co-located with frontend |
| Database | PostgreSQL 16 | Relational, RLS for multi-tenancy |
| ORM | Drizzle ORM | Type-safe, lightweight |
| Auth | NextAuth v5 | OAuth + email/password |
| Queue | BullMQ + Redis | Background jobs |
| Storage | AWS S3 | File uploads |
| Email | SendGrid | Transactional email |
| Payments | Stripe | Subscriptions |
| Deployment | Vercel (app) + Railway (workers) | |
| Monitoring | Sentry + Datadog | |

---

## Key Files

| Path | Purpose |
|------|---------|
| `app/` | Next.js App Router — pages and API routes |
| `app/api/` | API route handlers |
| `app/(auth)/` | Auth pages (login, register, reset) |
| `app/(app)/` | Protected app pages |
| `src/db/` | Database schema, migrations, client |
| `src/db/schema.ts` | **Drizzle schema — single source of truth** |
| `src/lib/` | Shared utilities (auth, email, stripe) |
| `src/lib/auth.ts` | **Auth configuration — read this first** |
| `src/components/` | Reusable React components |
| `src/hooks/` | Custom React hooks |
| `src/types/` | Shared TypeScript types |
| `workers/` | BullMQ background job processors |
| `emails/` | React Email templates |
| `tests/` | Test helpers, factories, integration tests |
| `.env.example` | All env vars with descriptions |
| `docker-compose.yml` | Local infrastructure |

---

## Common Developer Tasks

### Add a new API endpoint

```bash
# 1. Create route handler
touch app/api/my-resource/route.ts
```

```typescript
// app/api/my-resource/route.ts
import { NextRequest, NextResponse } from 'next/server'
import { auth } from '@/lib/auth'
import { db } from '@/db/client'

export async function GET(req: NextRequest) {
  const session = await auth()
  if (!session) {
    return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
  }
  
  const data = await db.query.myResource.findMany({
    where: (r, { eq }) => eq(r.userId, session.user.id),
  })
  
  return NextResponse.json({ data })
}
```

```bash
# 2. Add tests
touch tests/api/my-resource.test.ts

# 3. Add to OpenAPI spec (if applicable)
pnpm generate:openapi
```

### Run a database migration

```bash
# Create migration
pnpm db:generate     # Generates SQL from schema changes

# Review the generated SQL
cat drizzle/migrations/0001_my_change.sql

# Apply
pnpm db:migrate

# Roll back (manual — inspect generated SQL and revert)
psql $DATABASE_URL -f scripts/rollback_0001.sql
```

### Add a new email template

```bash
# 1. Create template
touch emails/my-email.tsx

# 2. Preview in browser
pnpm email:preview

# 3. Send in code
import { sendEmail } from '@/lib/email'
await sendEmail({
  to: user.email,
  subject: 'Subject line',
  template: 'my-email',
  props: { name: "username"
})
```

### Add a background job

```typescript
// 1. Define job in workers/jobs/my-job.ts
import { Queue, Worker } from 'bullmq'
import { redis } from '@/lib/redis'

export const myJobQueue = new Queue('my-job', { connection: redis })

export const myJobWorker = new Worker('my-job', async (job) => {
  const { userId, data } = job.data
  // do work
}, { connection: redis })

// 2. Enqueue
await myJobQueue.add('process', { userId, data }, {
  attempts: 3,
  backoff: { type: 'exponential', delay: 1000 },
})
```

---

## Debugging Guide

### Common Errors

**`Error: DATABASE_URL is not set`**
```bash
# Check your .env file exists and has the var
cat .env | grep DATABASE_URL

# Start Postgres if not running
docker compose up -d postgres
```

**`PrismaClientKnownRequestError: P2002 Unique constraint failed`**
```
User already exists with that email. Check: is this a duplicate registration?
Run: SELECT * FROM users WHERE email = '[email protected]';
```

**`Error: JWT expired`**
```bash
# Dev: extend token TTL in .env
JWT_EXPIRES_IN=30d

# Check cloc