Multi-Tenant SaaS Architecture Designer

You are a SaaS platform architect. Help me design a multi-tenant architecture.

Product: [PRODUCT]
Tenant types: [TENANT_TYPES] (SMB, mid-market, enterprise)
Expected tenant count: [TENANT_COUNT]
Data sensitivity: [DATA_SENSITIVITY]
Compliance requirements: [COMPLIANCE]
Performance isolation needs: [ISOLATION_NEEDS]
Current architecture: [CURRENT_ARCHITECTURE]

Design the multi-tenant architecture:

**1. Isolation Model Selection**

| Model | Data Isolation | Cost | Complexity | Compliance |
| Shared DB, shared schema | Low | Lowest | Low | Difficult |
| Shared DB, schema per tenant | Medium | Low | Medium | Moderate |
| Database per tenant | High | Higher | Higher | Easiest |
| Infrastructure per tenant | Highest | Highest | Highest | Full |

- Recommendation for YOUR requirements with rationale
- Hybrid approach: Different tiers for different customer segments

**2. Data Architecture**
- Tenant identifier strategy (tenant_id column, schema, separate DB)
- Row-Level Security (RLS) implementation
- Cross-tenant query prevention (defense in depth)
- Tenant-specific customizations and configuration
- Data migration between isolation levels (SMB → enterprise)

**3. Application Architecture**
- Tenant context propagation (how does every layer know the tenant?)
- Middleware/interceptor design for tenant resolution
- Feature flags per tenant
- Custom branding and white-labeling
- Tenant-specific integrations and webhooks

**4. Noisy Neighbor Mitigation**
- Rate limiting per tenant
- Resource quotas (storage, API calls, compute)
- Queue isolation strategy
- Database connection pool management
- Throttling and fair-use policies

**5. Scaling Strategy**
- Horizontal scaling with tenant-aware routing
- Shard-per-tenant for large tenants
- Auto-scaling based on tenant activity
- Tenant placement and rebalancing

**6. Onboarding & Provisioning**
- Tenant provisioning automation
- Data seeding for new tenants
- Trial-to-paid conversion path
- Tenant deprovisioning and data retention

**7. Operational Excellence**
- Per-tenant monitoring and health scores
- Tenant-scoped debugging and log analysis
- Maintenance windows and zero-downtime deployments
- Per-tenant backup and restore

**8. Billing & Metering**
- Usage metering architecture
- Billing integration points
- Overage handling