Scale an API Horizontally

A stateless REST API needs to handle 5× its current traffic. Add a load balancer and run multiple API instances.

Context

Horizontal scaling places multiple instances of a stateless service behind a load balancer, which distributes traffic and removes any single instance as a SPOF. The result: capacity grows linearly by adding instances instead of requiring larger hardware, and a downed instance does not interrupt service. Use it on stateless services under growing traffic — it is the cheapest way to scale; the prerequisite is that instances hold no local state, so sessions and any shared data must live in an external store.

Requirements

•Add a load balancer (e.g. Nginx, ALB) in front of the API
•Run 3+ API instances behind the load balancer
•Ensure session state is NOT stored on the API instance

Non-Functional Requirements

•Any API instance must be able to handle any request

Constraints

•Focus on the API layer only — skip the database and downstream services

Evaluation Criteria

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