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# Vercel Deployment Guidelines

## Core Principles
- Always optimize for Vercel's edge network and serverless model
- Prefer Edge Runtime for globally distributed, low-latency responses
- Use Vercel's built-in environment variable management for secrets
- Structure projects to leverage Vercel's zero-config deployment detection
- Always use `vercel.json` for advanced routing, headers, and redirects configuration

## Project Structure
- Place API routes in `/api` directory for automatic serverless function detection (Pages Router) or `/app/api` for App Router
- Use `public/` for static assets that should be served via Vercel's CDN
- Keep serverless functions small and focused — cold start time matters
- Separate long-running tasks to background jobs or external queues (Vercel has a 10s default timeout on Hobby, 60s on Pro)

## Environment Variables
- Never hard-code secrets; always use `process.env.VARIABLE_NAME`
- Prefix client-side env vars with `NEXT_PUBLIC_` (Next.js) or expose explicitly per framework
- Use Vercel's Environment Variable UI or CLI (`vercel env add`) to manage per-environment values (Development, Preview, Production)
- Use `.env.local` for local development; never commit it
- Reference `vercel.json` `env` field only for build-time non-secret values

## vercel.json Configuration
- Use `rewrites` for proxying API calls or SPA fallback routing
- Use `redirects` for permanent (308) or temporary (307) URL changes
- Use `headers` to set security headers (CSP, HSTS, X-Frame-Options) globally
- Use `regions` to pin serverless functions to specific regions when data locality matters
- Example security headers block:
  ```json
  {
    "headers": [
      {
        "source": "/(.*)",
        "headers": [
          { "key": "X-Content-Type-Options", "value": "nosniff" },
          { "key": "X-Frame-Options", "value": "DENY" },
          { "key": "X-XSS-Protection", "value": "1; mode=block" },
          { "key": "Referrer-Policy", "value": "strict-origin-when-cross-origin" }
        ]
      }
    ]
  }
  ```

## Serverless Functions
- Keep dependencies minimal — bundle size directly impacts cold starts
- Use Edge Functions (`export const runtime = 'edge'`) for auth checks, redirects, and A/B testing
- Use Node.js runtime for database connections, heavy computation, or Node-only packages
- Always handle errors gracefully and return proper HTTP status codes
- Use streaming responses for LLM or large data outputs

## Edge Middleware
- Place middleware in `middleware.ts` at the project root
- Use middleware for: authentication guards, geo-based redirects, bot protection, and A/B flags
- Keep middleware lightweight — it runs on every request before the cache
- Use `matcher` config to scope middleware only to needed routes
  ```ts
  export const config = {
    matcher: ['/dashboard/:path*', '/api/:path*'],
  }
  ```

## Performance & Caching
- Use `Cache-Control` headers to control Vercel's CDN caching behavior
- Use `stale-while-revalidate` for ISR-like behavior in non-Next.js apps
- Set `s-maxage` for CDN cache TTL and `max-age` for browser cache
- Avoid over-fetching in serverless functions — reuse DB connections with connection pooling (PgBouncer, Prisma Accelerate)
- Use `vercel/og` for dynamic OG image generation at the edge

## CI/CD & Preview Deployments
- Use Vercel's GitHub/GitLab/Bitbucket integration for automatic preview deployments per PR
- Use `vercel pull` + `vercel build` + `vercel deploy --prebuilt` in custom CI pipelines
- Set branch protection rules to require passing preview deployment checks
- Use `VERCEL_ENV` environment variable to differentiate behavior across preview/production

## Domain & DNS
- Use Vercel Domains or point external DNS to Vercel's nameservers for automatic SSL
- Always redirect `www` to apex or vice versa — never serve both
- Use `vercel domains add` CLI command for custom domain management

## Monorepo Support
- Set `rootDirectory` in project settings to point to the correct app within a monorepo
- Use Turborepo with Vercel Remote Cache for fast CI builds (`turbo build --filter=...`)
- Scope environment variables per project even in shared monorepos

## Databases & Storage
- Prefer Vercel-native storage (Vercel KV, Vercel Postgres, Vercel Blob) for zero-config integration
- For external databases, always use connection pooling — serverless functions do not maintain persistent connections
- Store large files in Vercel Blob or S3-compatible storage, not in the function bundle

## Logging & Observability
- Use `console.log` / `console.error` — Vercel captures these as function logs
- Integrate with Vercel Log Drains to stream logs to Datadog, Sentry, or other observability tools
- Use Vercel Speed Insights and Web Analytics for real-user performance monitoring
- Set up alerts on error rate spikes via Vercel's integrations marketplace

## Security Best Practices
- Enable Vercel's DDoS protection (included on all plans)
- Use Vercel Firewall rules to block malicious IPs and patterns
- Rotate secrets regularly using Vercel's environment variable versioning
- Never log sensitive data (tokens, passwords, PII) in serverless function output
- Use `VERCEL_OIDC_TOKEN` for secure machine-to-machine auth between Vercel and cloud providers

## Deployment Workflow
- Run `vercel --prod` only from protected branches (main/master)
- Use `vercel deploy` (without `--prod`) for staging/preview deployments
- Alias stable preview URLs with `vercel alias` for QA sign-off
- Use deployment protection (password or Vercel Access) on preview deployments for private projects