SaaS Architecture: Scenario-Based Questions

Multi-tenant SaaS architectures allow multiple customers (tenants) to share infrastructure while maintaining logical isolation. Designing it well requires balancing scalability, security, customization, and observability.

🏗️ Core Design Patterns

• Shared Everything: All tenants use the same database and compute layer, distinguished by tenant ID.
• Shared-nothing: Each tenant has its own isolated stack — highest isolation but least efficient.
• Hybrid: Shared app layer, isolated data layer (e.g., one schema or DB per tenant).

🔐 Security and Isolation

• Use tenant-aware access controls in the application layer (filter every request by tenant context).
• Encrypt tenant data at rest and in transit (use per-tenant keys for higher compliance).
• Implement RBAC or ABAC to manage roles across and within tenants.

⚙️ Scalability Considerations

• Use horizontal scaling and stateless services to support tenant growth.
• Auto-scale read replicas or cache layers for high-volume tenants.
• Support multi-region deployments with geo-routing based on tenant location.

🧰 Customization and Configuration

• Use feature flags and configuration-as-data to support tenant-specific behavior.
• Store branding, limits, or preferences in a tenant metadata table or config service.

📊 Monitoring and Billing

• Log and trace all activity with tenant ID context.
• Expose metering and usage dashboards per tenant.
• Support billing models (e.g., per seat, per usage) based on telemetry.

✅ Best Practices

• Tenant ID must be enforced at every boundary (API, DB, cache).
• Use tenancy-aware data models with indexing and partitioning strategies.
• Set quotas per tenant to protect against abuse or noisy neighbors.

🚫 Common Pitfalls

• Skipping access controls — exposing one tenant’s data to another.
• Hardcoding tenant-specific logic in app code — reduces maintainability.
• Inconsistent monitoring or alerting across tenants.

📌 Real-World Insight

Successful SaaS platforms like Shopify, Salesforce, and Slack rely on scalable multi-tenant designs with rigorous controls. They build abstractions for config, identity, and observability — treating every tenant as a VIP.