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Illumio·Software Engineer·Onsite - System Design / Architecture·Intermediate

Intermediate
Jun 2026

Summary

Second round at Illumio for a software engineering role, focused entirely on system design. The whole session was spent on API gateway design, covering rate limiting, routing, and auth.

Questions Asked (1)

Q1

Design an API Gateway, covering rate limiting, routing, and authentication.

System DesignAPI & IntegrationsTechnical Trade-offs
Author's notes

This is one of those questions that feels manageable until you're actually in it.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then propose a high-level architecture that addresses routing, rate limiting, and authentication as separate but integrated components. Discuss trade-offs for each component, such as algorithm choices and failure modes, and consider scalability and security implications.

Pro tip: Emphasize observability and graceful degradation: show how you'd monitor the gateway and handle failures (e.g., fallback to a default rate limit or cached auth decisions) to maintain availability under load.

1. Clarify Requirements and Constraints

Ask about expected traffic volume, latency requirements, authentication methods, and whether the gateway is for internal or external APIs. This shapes design decisions.

2. High-Level Architecture

Outline the main components: a routing layer, rate limiting module, authentication/authorization module, and supporting services like config store and monitoring. Explain how requests flow through them.

3. Deep Dive into Routing

Describe how routing works: path-based, header-based, or service discovery integration. Discuss dynamic configuration updates and load balancing strategies.

4. Deep Dive into Rate Limiting

Explain rate limiting algorithms (e.g., token bucket, sliding window) and where to enforce limits (per user, IP, API key). Discuss distributed rate limiting using Redis or similar.

5. Deep Dive into Authentication

Cover authentication mechanisms (JWT, OAuth2, API keys) and how to validate them efficiently (e.g., caching public keys). Discuss authorization and integration with identity providers.

Key Points to Mention

  • Use of distributed rate limiting with Redis or a similar store to handle multiple gateway instances.
  • Authentication caching and token validation to reduce latency and load on identity providers.
  • Dynamic routing configuration and service discovery (e.g., Consul, Kubernetes services).
  • Trade-offs between different rate limiting algorithms (e.g., token bucket vs. sliding window) and their impact on burst handling.
  • Security considerations: TLS termination, input validation, and protection against DDoS.
  • Observability: logging, metrics, and tracing for monitoring gateway health and performance.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.