This one ate up a lot of time because I kept wanting to jump straight into the execution sandbox piece, which is the fun part, but the interviewer kept pulling me back to basics like auth flows and problem storage.
Start by clarifying functional and non-functional requirements, then sketch a high-level architecture covering the main components: web app, API, judge, and data stores. Dive deep into the judge's sandboxing and scaling, and discuss trade-offs and evolution. Use back-of-the-envelope calculations to justify design choices.
Pro tip: Emphasize security and isolation in the code execution sandbox, as it's the most critical and challenging part. Also, proactively discuss how you'd handle malicious code and resource limits, showing you think like a production engineer.
Ask questions to understand scale, supported languages, execution time limits, and expected user base. Define functional (submit code, run tests, view results) and non-functional (latency, availability, security) requirements.
Outline the main components: client apps, API gateway, user service, problem service, submission service, judge service, and data stores. Explain how they interact and the flow of a code submission.
Design schemas for users, problems, submissions, and test cases. Discuss SQL vs NoSQL choices, indexing, and how to store code and results efficiently.
Detail the judge's execution environment: containerization (Docker), resource limits (CPU, memory, time), isolation (namespaces, seccomp), and security measures to prevent escapes and abuse.
Explain how to scale the judge horizontally with a queue and worker pool, use caching for problems and results, and evolve the design for higher concurrency, new languages, and real-time feedback.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.