Serverless API on Azure Functions

Overview

This reference architecture delivers a REST API with no servers to manage, scaling from zero to high concurrency and back. Use it for spiky or unpredictable traffic, internal tools, webhook receivers, and APIs where you want to pay only for execution time. Azure Functions runs the compute, Cosmos DB stores data with single-digit-millisecond reads, and API Management provides the public, governed front door.

The isolated worker model lets you run current .NET, Node.js, Python, or Java with full control over the host.

Components

• Azure Functions: Stateless HTTP-triggered and queue-triggered functions that scale per request.
• API Management: Gateway for authentication, rate limiting, request validation against OpenAPI, and versioning.
• Cosmos DB: Globally distributed NoSQL store with tunable consistency and autoscale throughput.
• Azure Service Bus: Durable queue for asynchronous work offloaded from the request path.
• Azure Key Vault: Stores secrets and certificates, accessed via managed identity.
• Application Insights: Distributed tracing, metrics, and live diagnostics.

Data Flow

A client calls API Management, which validates the OAuth 2.0 token, enforces the rate limit, and forwards the request to an HTTP-triggered function. The function reads or writes Cosmos DB for synchronous data and, for longer tasks, drops a message on Service Bus. A queue-triggered function processes that message independently, decoupling slow work from the user-facing path. Functions retrieve secrets from Key Vault using a managed identity, so no credentials are stored in code or config.

Scaling and Resilience

The consumption or Flex Consumption plan scales function instances automatically with demand, including scale-to-zero when idle. Cosmos DB autoscale raises and lowers throughput within a configured ceiling. Service Bus buffers bursts and retries failed processing with dead-letter queues. For resilience, deploy across paired regions and enable Cosmos DB multi-region writes; API Management can run in a multi-region configuration with Traffic Manager routing.

Security

Managed identities eliminate stored credentials for service-to-service calls. API Management validates JWTs and applies the principle of least privilege through subscription keys and scopes. Private endpoints keep Cosmos DB and Key Vault off the public internet. All secrets live in Key Vault, and Application Insights captures security-relevant telemetry. Follow the Azure Well-Architected Framework security pillar for ongoing review.

Trade-offs and Alternatives

Serverless removes capacity planning and minimizes idle cost, but cold starts can add latency to infrequently called functions, and per-invocation pricing can exceed always-on compute at very high steady volume. Execution-time limits make long-running jobs awkward; offload them to Service Bus and durable functions. If you need fine-grained network control or run sustained high traffic, Azure Container Apps or AKS may be cheaper and more flexible. Choose this design when traffic is variable, operational overhead must stay low, and individual requests are short.