API Multi-tenancy Architecture

You are a SaaS architect specializing in multi-tenant systems and data isolation patterns.

## CONTEXT
I need to implement multi-tenancy in my API.

**Requirements:**
- Tenant Model: {{TENANT_MODEL}}
- Isolation Level: {{ISOLATION}}
- Customization Needs: {{CUSTOMIZATION}}
- Scale: {{SCALE}}

## TASK
Design multi-tenant API architecture:

### 1. TENANT IDENTIFICATION
**Subdomain:**
```
https://tenant1.api.example.com/users
```

**Header:**
```
X-Tenant-ID: tenant_123
```

**Path:**
```
/api/tenants/tenant_123/users
```

### 2. DATA ISOLATION STRATEGIES

**Shared Database, Shared Schema:**
- tenant_id column on all tables
- Row-level security

**Shared Database, Separate Schema:**
- Schema per tenant
- Connection pooling per schema

**Separate Database:**
- Complete isolation
- Connection management

### 3. MIDDLEWARE IMPLEMENTATION
```typescript
async function tenantMiddleware(req, res, next) {
  const tenantId = extractTenantId(req);
  const tenant = await getTenant(tenantId);

  if (!tenant) {
    return res.status(404).json({ error: 'Tenant not found' });
  }

  req.tenant = tenant;
  setTenantContext(tenantId);
  next();
}
```

### 4. CUSTOMIZATION
- Tenant-specific configuration
- Feature flags per tenant
- Custom branding/themes

### 5. RATE LIMITING
Per-tenant rate limits and quotas.

### 6. BILLING INTEGRATION
Usage tracking per tenant.