Chime·Software Engineer·Onsite - System Design / Architecture·Senior

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Apr 2026

Summary

System design round at Chime focused entirely on load balancers. Felt like a deep dive more than a broad architecture question, which I wasn't fully expecting.

Questions Asked (1)

Walk me through how load balancers fit into the network stack, specifically the differences between operating at layer 4 versus layer 7, and what routing and distribution strategies exist. When would you pick one approach over another and what are the trade-offs?

System DesignTechnical Trade-offs

Author's notes

I started with L4 vs L7 and felt okay there.

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AI HintsAI Generated

Suggested Approach

Start by defining load balancers and their role in the network stack, then clearly contrast Layer 4 and Layer 7 operation with examples. Discuss common routing and distribution strategies, and finally explain when to choose each approach based on trade-offs like performance, flexibility, and complexity.

Pro tip: Relate the concepts to Chime's scale and reliability needs—e.g., Layer 4 for high-throughput, low-latency traffic and Layer 7 for intelligent routing based on HTTP headers—to show practical insight.

1. Define Load Balancers and Their Role

Explain that load balancers distribute incoming network traffic across multiple servers to ensure availability, scalability, and reliability. Position them in the network stack between clients and backend servers.

2. Contrast Layer 4 vs Layer 7

Describe Layer 4 (transport layer) load balancing: operates on TCP/UDP, routes based on IP and port, no inspection of application data. Layer 7 (application layer) load balancing: operates on HTTP/HTTPS, can inspect headers, URLs, cookies, and make content-aware routing decisions.

3. Explain Routing and Distribution Strategies

Cover common algorithms: round robin, least connections, IP hash, weighted round robin, and for Layer 7, path-based or header-based routing. Mention health checks and session persistence.

4. Discuss Trade-offs and Selection Criteria

Compare performance (Layer 4 is faster, less overhead), flexibility (Layer 7 enables advanced routing, SSL termination, caching), and complexity (Layer 7 requires more resources and configuration). Consider use cases: Layer 4 for high-volume, low-latency; Layer 7 for microservices, API gateways, or content-based routing.

5. Tie to Real-World Scenarios

Give examples: Layer 4 for database or streaming traffic; Layer 7 for web applications needing URL-based routing or A/B testing. Mention hybrid approaches (e.g., Layer 4 at edge, Layer 7 internally).

Key Points to Mention

Layer 4 operates at transport layer (TCP/UDP) and routes based on IP/port; Layer 7 operates at application layer (HTTP/HTTPS) and can inspect content.
Common distribution strategies: round robin, least connections, IP hash, weighted round robin, and for Layer 7, path-based routing.
Trade-offs: Layer 4 offers higher performance and lower latency; Layer 7 provides more flexibility and advanced features like SSL termination and content-based routing.
Use cases: Layer 4 for high-throughput, low-latency services; Layer 7 for microservices, API gateways, and applications requiring intelligent routing.
Health checks and session persistence are important for both, but Layer 7 can do more granular health checks.
Hybrid architectures often combine both: Layer 4 at the edge for DDoS protection and Layer 7 for internal routing.

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