Architecture Deep Dive on a System You Built: Decisions, Trade-offs and Lessons

You are interviewing for an engineering manager role, and this system design round is about your own work rather than a new problem. Pick a project that you or your team built, explain its system design, and then expect the interviewer to dig deeply into the decisions behind it, the technical trade-offs, and what you would change. The bar is technical depth: managers in this loop are expected to discuss architecture in detail, not only the process around it. ### Constraints and Clarifications - The project is your choice; pick one whose architecture you understand well enough to go several levels deep. - Keep confidential details out: describe the design in general terms where needed (no internal names, customer data or figures you may not share), but keep the reasoning concrete. - Expect the interviewer to interrupt and steer toward whatever looks interesting or weak. ### Clarifying Questions - Would you prefer a system I designed hands-on, or one my team designed under my leadership where I owned the key decisions? - Should I start with a whiteboard overview, or go straight to the most interesting decision? - How much time should I spend on context compared with the deep dive? ### Part 1 — Present the system Explain what problem the system solved, its requirements and scale, and its architecture: the main components, data stores, how data flows through it, and how it is deployed and operated. ```hint Lead with the problem Start from who used the system and what it had to guarantee, then let the architecture follow from those requirements and from the scale numbers you can share. ``` #### What This Part Should Cover - The problem, the users, and the functional and non-functional requirements - Scale and load characteristics in shareable terms - A component diagram, the data model, and the main read and write paths - Your role compared with the team's, stated plainly ### Part 2 — Defend the key decisions Choose two or three of the most important design decisions. For each, describe the alternatives you considered, why you chose as you did, what the choice cost, and what evidence supported it. ```hint Every choice had a runner-up For each decision, be ready to name the option you rejected and the conditions under which it would have been the better choice. ``` #### What This Part Should Cover - Concrete alternatives with honest trade-offs: consistency, latency, cost, complexity, team skills - Data or experiments that informed each decision - How the decision was made and communicated, given your role as manager or technical lead - Failure modes and scaling limits, and how they were handled ### Part 3 — Reflect and evolve What went wrong in production or did not age well? Knowing what you know now, what would you change, and how would the design need to evolve at a much larger scale? ```hint Show the scars Pick a real incident or limitation and connect it to a specific design change, not to a generic lesson. ``` #### What This Part Should Cover - A real failure or limitation and its root cause in the design - Concrete changes you would make now, and why - How the system would scale further and where it would break first - What you learned as a leader about making and revisiting technical decisions ### What a Strong Answer Covers - A crisp, well-structured overview before the deep dive - Depth on demand: able to go several levels into storage, consistency, failure handling and operations - Decisions explained as trade-offs between real alternatives, backed by data - Honest ownership: clear about the candidate's contribution and the team's - A leadership perspective on how technical decisions were made, reviewed and revisited ### Follow-up Questions - Which part of this system would you redesign first if you started today, and why? - How did you handle a design decision where your senior engineers disagreed with you? - Which metrics or alerts told you the system was healthy, and which incident taught you to add them? - How would you onboard a new engineer to this architecture?