Technical Deep Dive: Present and Defend the Design of a Service You Own

The interviewer asks you to explain the architecture of a service you own and then take them through its technical details. There is no new problem to solve: your own system is the subject, and the interviewer decides where to dig. Expect them to pick components and ask how they work, why they were built that way, what breaks, and what you would change. Prepare this answer for a service you know deeply. Assume a whiteboard or shared drawing tool is available. ```hint Lead with purpose Before drawing any boxes, be able to say in two sentences what the service does, who calls it, and what happens to those callers if it disappears. ``` ```hint Know your numbers and your incidents Interviewers dig hardest into scale, data flow and past failures; have concrete figures ready, and at least one incident or redesign you can explain end to end. ``` ### Constraints and Clarifications - Choose a service whose main design decisions you made or can defend; a system you only consumed is a weak choice. - Do not disclose confidential details. Generalize names and exact figures where necessary while keeping the technical substance. ### Clarifying Questions - How long should the overview be before the interviewer starts asking questions? - Should the walkthrough cover the whole service or concentrate on the part you personally built? - Is the interviewer familiar with the domain, or should the business context come first? ### What a Strong Answer Covers - A crisp statement of the service's purpose, callers, scale and key requirements before any architecture - A clear high-level diagram with the data flow of the main read and write paths - Depth on demand: data model and storage choices, interfaces, consistency, scaling and failure handling for whichever component the interviewer picks - The reasoning behind the main design decisions, including rejected alternatives and known weaknesses - Operational reality: monitoring, incidents, on-call lessons, and how the design evolved - The candidate's specific role and decisions, separated from the team's ### Follow-up Questions - If traffic grew tenfold tomorrow, which component would fail first, and what would you do? - What is the worst incident this service has had, and what changed afterward? - Which design decision would you reverse if you were starting again? - How do you know the service is healthy right now?