Multi-Tenant Architecture

Isolation Strategies

Strategy	Isolation	Complexity	Cost
Database per tenant	Highest	High	High
Schema per tenant	High	Medium	Medium
Shared schema (tenant_id column)	Medium	Low	Low
Row-level security (RLS)	Medium-High	Medium	Low

Shared Schema with Tenant ID (most common)

// Middleware: resolve tenant from subdomain or header
function tenantMiddleware(req: Request, res: Response, next: NextFunction) {
  const host = req.hostname; // acme.myapp.com
  const subdomain = host.split('.')[0];
  const tenant = await tenantRepo.findBySubdomain(subdomain);
  if (!tenant) return res.status(404).json({ error: 'Tenant not found' });
  req.tenantId = tenant.id;
  next();
}

// Always filter by tenant
app.get('/api/products', async (req, res) => {
  const products = await db.product.findMany({
    where: { tenantId: req.tenantId },
  });
  res.json(products);
});

Prisma with Tenant Scoping

// Extension to auto-apply tenant filter
const prisma = new PrismaClient().$extends({
  query: {
    $allOperations({ args, query, operation }) {
      if (['findMany', 'findFirst', 'count', 'updateMany', 'deleteMany'].includes(operation)) {
        args.where = { ...args.where, tenantId: getCurrentTenantId() };
      }
      if (['create', 'createMany'].includes(operation)) {
        args.data = { ...args.data, tenantId: getCurrentTenantId() };
      }
      return query(args);
    },
  },
});

PostgreSQL Row-Level Security

-- Enable RLS
ALTER TABLE products ENABLE ROW LEVEL SECURITY;

-- Policy: users see only their tenant's data
CREATE POLICY tenant_isolation ON products
  USING (tenant_id = current_setting('app.tenant_id')::uuid);

-- Set tenant context per request
SET app.tenant_id = 'tenant-uuid-here';
SELECT * FROM products; -- auto-filtered

// Set tenant context on each request
pool.on('connect', async (client) => {
  // Set after getting connection from pool
});

async function withTenant<T>(tenantId: string, fn: () => Promise<T>): Promise<T> {
  const client = await pool.connect();
  try {
    await client.query(`SET app.tenant_id = $1`, [tenantId]);
    return await fn();
  } finally {
    await client.query('RESET app.tenant_id');
    client.release();
  }
}

Schema-Per-Tenant

// Dynamic schema selection
function getTenantSchema(tenantId: string): string {
  return `tenant_${tenantId.replace(/-/g, '_')}`;
}

async function createTenantSchema(tenantId: string) {
  const schema = getTenantSchema(tenantId);
  await db.query(`CREATE SCHEMA IF NOT EXISTS ${schema}`);
  await db.query(`SET search_path TO ${schema}`);
  await runMigrations(); // Apply schema migrations
}

Tenant Resolution Strategies

Strategy	Example	Best For
Subdomain	acme.myapp.com	B2B SaaS
Path prefix	myapp.com/acme/...	Simpler setup
Custom header	X-Tenant-ID: acme	API-first
JWT claim	{ tenantId: "acme" }	Authenticated APIs

Anti-Patterns

Anti-Pattern	Fix
No tenant filter on queries	Use middleware or ORM extension to auto-apply
Tenant ID from client without validation	Derive from auth token or subdomain
No tenant data isolation testing	Write tests that verify cross-tenant isolation
Shared cache without tenant prefix	Prefix all cache keys with tenant ID
No tenant-aware rate limiting	Rate limit per tenant, not globally

Production Checklist

• Tenant resolution middleware on all routes
• Data isolation verified with automated tests
• Cache keys prefixed with tenant ID
• Rate limiting per tenant
• Tenant-scoped background jobs
• Tenant provisioning and deprovisioning flow
• Cross-tenant query prevention (RLS or ORM enforcement)