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azure
System Design
intermediate
B2B SaaS with per-tenant billing

Multi-Tenant SaaS Platform

Generic SaaS

A multi-tenant SaaS platform must balance cost efficiency (shared infrastructure) with security (tenant isolation). This Azure-native architecture uses Azure AD B2C for identity with tenant-scoped JWTs, row-level security in Azure SQL for data isolation, Redis Cache for instant feature flag lookups, and Azure Functions for asynchronous usage metering that feeds billing integration. Ideal for SaaS founders building multi-tenant platforms that need per-tenant isolation, usage metering, and subscription billing.

Data Flow

Front Door + WAF
Azure AD B2C
Tenant API
Async Jobs
App Service
Billing Service
Usage Metering
Tenant DB (SQL)
Feature Flag Cache

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Service Breakdown (9 services)

Other9 services
Front Door + WAF
  • •Routes application traffic with L7 rules
  • •Supports WAF and SSL offloading
  • •Integrates with backend pools for scaling
Azure AD B2C
  • •Authenticates tenants with customizable sign-in flows
  • •Supports social and enterprise identity federation
  • •Issues scoped tokens per tenant for API access
Tenant API
  • •Exposes tenant-scoped endpoints with rate limiting
  • •Routes requests to the correct tenant context
  • •Enforces subscription tier limits per tenant
App Service
  • •Hosts the core application logic for all tenants
  • •Auto-scales based on concurrent request volume
  • •Supports deployment slots for zero-downtime updates
Billing Service
  • •Tracks usage metrics per tenant for invoicing
  • •Integrates with payment gateways for subscriptions
  • •Enforces plan limits and triggers upgrade prompts
Tenant DB (SQL)
  • •Stores tenant-specific data with row-level isolation
  • •Supports elastic pools for cost-efficient scaling
  • •Handles automated backups and point-in-time restore
Feature Flag Cache
  • •Caches frequently accessed data in-memory
  • •Reduces database round-trips and latency
  • •Supports TTL-based expiration policies
Usage Metering
  • •Aggregates API call counts and resource consumption
  • •Emits usage events for billing reconciliation
  • •Runs on schedule to minimize compute cost
Async Jobs
  • •Provides reliable enterprise message brokering
  • •Supports topics and queue-based messaging
  • •Guarantees at-least-once delivery

Scaling Strategy

App Service auto-scales horizontally behind API Management with tenant-aware routing. Azure SQL uses elastic pools for tenant DB isolation with shared compute. Redis Cache stores feature flags with sub-millisecond lookups. Usage metering runs asynchronously via Functions to avoid impacting request latency, and Service Bus handles billing event processing with at-least-once delivery guarantees.

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