They literally handed me a whiteboard marker and said draw it.
Choose a project where you had significant ownership and can speak to both high-level architecture and low-level details. Structure your answer as a narrative: start with the problem and constraints, then walk through the architecture and your specific contributions, highlighting key design decisions and trade-offs. Be prepared to dive deep into any area the interviewer probes, and explicitly connect your choices to scalability and failure handling.
Pro tip: Don't just list technologies—explain the 'why' behind each decision, including alternatives you rejected and the trade-offs you accepted. Interviewers at Decagon value depth over breadth, so pick one or two areas to go really deep rather than skimming everything.
Briefly describe the project's purpose, your role, the team size, and the key constraints (e.g., latency, throughput, budget, deadlines). This frames your later decisions.
Give a high-level diagram of the system: major components, their responsibilities, and how they interact. Mention the technologies used and why they were chosen.
Focus on the parts you personally built. For each, explain the problem, the options you considered, the decision you made, and the rationale (including trade-offs).
Trace a typical request or data item through the system. Then discuss how you thought about scaling: what metrics you monitored, how you identified bottlenecks, and what you did (or would do) to address them.
Describe the failure scenarios you anticipated or encountered, how you handled them (e.g., retries, circuit breakers, graceful degradation), and what you learned or would do differently.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.