multi-tenant-architecture

Multi-Tenant Platform Architecture (Cloudflare or Vercel)

• IS: domain strategy, tenant identification and isolation, subdomain routing, custom domains, white-label setup, and plan/limit mapping on Cloudflare or Vercel.
• IS NOT: general app folder structure or module boundaries (use define-architecture), or scaffolding a new repo (use scaffold-nextjs).

Contents

• Platform dispatch (decide first)
• Workflow (order matters)
• Gotchas
• Deliverables
• Pre-commit checklist
• Related skills

Platform dispatch (decide first)

• Pick one platform and commit; do not mix hosting. Hybrid setups create routing complexity that compounds.
• Load only the chosen platform's references unless explicitly comparing the two.
• Always load psl.md when deciding domain strategy (step 1).
• Load limits-and-quotas.md before mapping limits to pricing (step 8).

Workflow (order matters)

1. Choose domain strategy

• Use a dedicated tenant domain (separate from the brand domain) for all subdomains and custom hostnames. Reputation does not isolate; a phishing site on random.acme.com damages the whole domain.
• Register a separate TLD for tenant workloads (e.g. acme.app for tenants, acme.com for brand).
• Consider PSL for browser cookie isolation; it does not protect reputation. See psl.md.
• Start PSL submission early; review can take weeks.

1. Choose tenant identification strategy

• Subdomain-based: tenant.yourdomain.com. Requires wildcard DNS. Simplest for many tenants.
• Custom domain: tenant brings their own domain and CNAMEs to your platform. Best for serious or paying tenants.
• Path-based: yourdomain.com/tenant-slug. No per-tenant DNS/SSL, but limits branding and complicates cookie isolation.
• Pick one primary strategy; offer custom domain as an upgrade path.

1. Define isolation model

• Cloudflare: per-tenant Workers via dispatch namespaces for untrusted code. Avoid shared-tenant branching unless you fully control code and data.
• Vercel: single shared Next.js app with tenant_id scoping. Middleware resolves tenant from hostname; every data query includes tenant context. Use Postgres RLS for defence-in-depth.

1. Route traffic deterministically

• Cloudflare: the platform Worker owns routing; hostname -> tenant id -> dispatch namespace -> tenant Worker. 404 when no mapping exists.
• Vercel: Middleware extracts the hostname and rewrites to a /domains/[domain] dynamic segment. Edge Config for sub-millisecond tenant lookups. 404 when no mapping exists.
• Tenants never control routing or see each other on either platform.

1. Pass tenant context through the stack

• Cloudflare: the platform Worker resolves the tenant and injects headers or bindings before dispatching to the tenant Worker.
• Vercel: Middleware sets x-tenant-id, x-tenant-slug, x-tenant-plan on the forwarded request headers (not the response). Server Components read via headers(); API routes read from request headers:

  // middleware.ts
  import { NextRequest, NextResponse } from "next/server";
  export function middleware(request: NextRequest) {
    const hostname = request.headers.get("host") ?? "";
    const tenant = hostname.split(".")[0]; // resolve from Edge Config/DB in production
    const requestHeaders = new Headers(request.headers);
    requestHeaders.set("x-tenant-id", tenant);
    return NextResponse.next({ request: { headers: requestHeaders } });
  }

• The Middleware or platform Worker is the single authority; never trust client-supplied tenant identity.

1. Bind only what is needed

• Cloudflare: least-privilege bindings per tenant (DB/storage/limited platform API), no shared global state. Treat new bindings as explicit changes; redeploy to grant access.
• Vercel: Edge Config for tenant config (domain mappings, feature flags, plan info). @vercel/sdk for domain management. Database connections scoped by tenant_id, or database-per-tenant (Neon).

1. Support custom domains and per-tenant static files

• Provide a DNS target, verify ownership, store the mapping, route by hostname.
• Cloudflare: Cloudflare for SaaS custom hostnames with managed certs. See cloudflare-platform.md.
• Vercel: @vercel/sdk for programmatic domain CRUD plus automatic Let's Encrypt SSL; wildcard subdomains require Vercel nameservers. See vercel-domains.md.
• Custom domains shift reputation to the tenant and create natural user segments (casual on platform domain, serious on their own domain).
• robots.txt, sitemap.xml, llms.txt must vary by tenant; never serve them from /public. Cloudflare: generate in the tenant Worker. Vercel: route handlers under the domain segment (see vercel-platform.md).

1. Surface limits as plans

• Map platform limits to pricing tiers; expose them in the API and UI.
• Do not run long jobs in requests; use queues or workflows.
• See limits-and-quotas.md for limits snapshots and source links; re-check official docs before final architecture or pricing decisions.

1. Make the API the product

• Everything works over HTTP; the UI is for ops, incidents, and billing.
• Platform logic stays in the routing layer (dispatch Worker or Middleware); tenant content serves requests.
• If it only works in the UI, the platform is leaking.

1. Extend without breaking boundaries

• Add queues, workflows, or containers as optional modes.
• Keep routing explicit and isolation intact.

Gotchas

• Don't use the brand domain for tenant subdomains: a phishing site on random.acme.com damages the entire acme.com reputation. Register a separate TLD for tenant workloads.
• Don't skip PSL submission when subdomains host untrusted content: review takes weeks, not days, and the cookie-isolation timeline slips with it.
• Don't trust client-supplied tenant identity, even behind auth. The Middleware or platform Worker is the single authority for tenant resolution.
• Don't set tenant headers on the Middleware response object: headers() in Server Components reads forwarded request headers, so use NextResponse.next({ request: { headers } }) or the tenant id never arrives.
• Don't mix hosting platforms: pick Cloudflare or Vercel and commit. Hybrid setups create routing complexity that compounds.
• Don't start with path-based tenancy if custom domains are on the roadmap: migrating later requires URL rewrites, cookie changes, and DNS migration.
• Don't share database connections across tenants without RLS or tenant_id scoping: one missing WHERE clause leaks another tenant's data.
• Don't block /.well-known/acme-challenge/* with Middleware or redirects: Let's Encrypt HTTP-01 validation fails and custom-domain SSL never issues.
• Don't treat Edge Config writes as instant: propagation takes up to 10 seconds, so a "domain connected" UI that reads Edge Config immediately shows stale state.

Deliverables

• Platform choice rationale: Cloudflare vs Vercel with justification
• Tenant identification strategy: subdomain, custom domain, or path-based
• Domain map: brand vs tenant domain, PSL plan, custom domain flow
• Isolation plan: per-tenant Workers or shared-app with tenant scoping
• Routing plan: hostname lookup, dispatch/rewrite logic, fallback behavior
• Tenant context flow: how tenant identity propagates through Middleware/headers/DB
• Binding/config matrix: per-tenant capabilities and data access
• Limits-to-pricing map: CPU/memory/request/domain budgets per tier
• API surface plus ops UI scope

Pre-commit checklist

• [ ] Platform chosen with clear rationale documented
• [ ] Tenant workloads off the brand domain; PSL decision and timeline set
• [ ] Tenant identification strategy chosen; custom domain upgrade path defined
• [ ] Isolation model defined: per-tenant Workers (Cloudflare) or shared-app plus RLS (Vercel)
• [ ] Routing authoritative and tenant-blind; dispatch or Middleware handles all traffic
• [ ] Tenant context flows through Middleware/platform Worker only; no client-supplied identity trusted
• [ ] Custom domain onboarding defined with DNS target, verification, and cert provisioning
• [ ] Per-tenant static files (robots.txt, sitemap.xml, llms.txt) served dynamically
• [ ] Limits tied to billing; API parity with UI
• [ ] Limits snapshot refreshed from official docs and dated in planning notes

Related skills

• define-architecture: folder structure, module contracts, and middleware pipelines for the application itself.
• scaffold-nextjs: bootstrap the Next.js turborepo before applying these tenancy patterns.
• optimise-seo: per-tenant sitemaps, canonical URLs, and structured data once routing works.