Multi-Tenant Architecture

The application tier must reliably identify tenant context for every request and enforce isolation. This starts with resolving the tenant ID via methods like subdomains or JWT claims and propagating it throughout the request lifecycle. Compute resources can be shared for cost-efficiency or dedicated for performance isolation, with container orchestration platforms like Kubernetes offering powerful tools like namespaces and resource quotas to prevent the "noisy neighbor" problem at the compute level.

Tenant Identification Strategies:

• Subdomain (`tenant-a.app.com`): Strong security isolation for cookies, professional branding. Requires DNS management.
• URL Path (`app.com/tenant-a`): Simpler implementation, no DNS changes. Can complicate routing.
• HTTP Header (`X-Tenant-ID`): Flexible for APIs, but insecure if used as the sole authorization mechanism.
• JWT Claim: Most secure method. The tenant ID is cryptographically signed within the token, preventing tampering.

Security is the bedrock of multi-tenancy. A "zero-trust" model is essential, where tenant boundaries are relentlessly enforced at every layer. This includes tenant-scoped Identity and Access Management (IAM), ensuring a user's roles and permissions are valid only within the context of a specific tenant. Data must be encrypted both in transit (TLS 1.3) and at rest. For shared schema models, database features like Row-Level Security (RLS) provide a critical last line of defense against application bugs that could cause data leaks.

Key Security Layers:

• Tenant-Scoped RBAC: User permissions are defined per-tenant, not globally.
• Federated Identity (SSO): Allowing tenants to use their own corporate identity providers (e.g., Okta, Entra ID).
• Encryption Everywhere: Using TLS for data in transit and provider-managed or customer-managed keys (CMK) for data at rest.
• Row-Level Security (RLS): A database-native control that automatically filters data based on the current tenant's ID, acting as a safety net for application code.

In a shared environment, fairness is paramount. Resource governance is the set of tools used to prevent the "noisy neighbor" problem, where one tenant's high usage degrades performance for others. This is achieved through rate limiting, which controls API request frequency, and quotas, which cap resource consumption (e.g., storage). These controls are not just for stability; they are the technical foundation for tiered pricing plans.

Query Optimization is Critical:

In shared databases, most indexes must be composite and start with the `TenantID` column. This allows the database to instantly narrow its search to the relevant tenant's data, which is the single most important optimization for multi-tenant query performance.

Successful SaaS platforms allow for tenant-specific experiences. Instead of custom code, this is best achieved with a metadata-driven UI, where tenant branding, enabled features, and layouts are stored as configuration data. For allowing custom data fields, `JSONB` columns in databases like PostgreSQL offer a flexible and efficient solution in shared-schema models. The entire tenant lifecycle, from automated onboarding to secure offboarding, must be managed through robust, automated workflows to enable scalable growth.

Feature Flags are Essential for:

• Tiered Functionality: Enabling features only for tenants on specific subscription plans.
• Canary Releases: Rolling out new features to a small subset of tenants for safe production testing.