Project deep dive: full architecture diagram, design decisions and business impact

Several rounds of this interview loop were built around the candidate's own work. The phone screen opened with a detailed walkthrough of a past project, and the technical rounds each began with a discussion of the current project. The hiring manager round went deep into the current project's architecture: the candidate had to draw the complete system architecture diagram, then discuss the business impact of past projects. A final round probed the architectural decisions behind the current and past projects. Prepare and deliver this deep dive for a project of your own. Treat the three parts below as one conversation with an interviewer who keeps asking "why?" and "what happens if...?". ### Clarifying Questions - Should the diagram cover the whole system, or only the parts my team owns and the boundaries around them? - How long should the walkthrough take, and would you like me to pause for questions as I go? - Some names and figures are confidential. Is it acceptable to generalize them while keeping the structure and orders of magnitude? - Should the architectural decisions come from the current project only, or may I also use past projects? ### Part 1 — The complete architecture diagram Draw the full architecture of your current project and walk the interviewer through it. ```hint Follow one request Pick one representative request or data record and trace it through every component it touches. A box it never touches either belongs to another flow worth showing, or does not belong in the diagram. ``` #### What This Part Should Cover - The system boundary: users, upstream and downstream systems, external dependencies - Every component and data store with its responsibility, and how they communicate (synchronous calls, queues, scheduled jobs) - The scale and reliability figures that explain the design, such as traffic, data size and latency targets - Which parts you designed or built yourself ### Part 2 — Architectural decisions For two or three significant decisions in your current project or a past one, explain the context, what you chose, what you rejected and why, and how the choice has held up since. ```hint A decision needs a rejected alternative A choice is only worth discussing if a reasonable engineer could have chosen differently. For each one, be ready to name the option you rejected and what it would have cost. ``` #### What This Part Should Cover - The requirements and constraints that forced each decision - The alternatives considered and the criteria used to choose between them - Consequences observed after launch, including costs and mistakes - What you would decide differently today, and what would trigger a change ### Part 3 — Business impact Explain what your past projects changed for the business, and how you know. ```hint Connect a number to the project Choose a measure a business owner cares about, and be ready to explain how much of its change came from this project rather than from something else happening at the same time. ``` #### What This Part Should Cover - A business-facing metric with a baseline and an outcome - How the effect was measured and attributed to the project - Your own contribution versus the team's - The costs, risks or trade-offs that came with the impact ### What a Strong Answer Covers - A diagram that is complete at one consistent level of abstraction, including data stores, asynchronous paths and external dependencies - Precise ownership: what you designed, built or decided, versus what the team did - Decisions defended with constraints, alternatives and real consequences, including what went wrong - Impact stated with numbers and an honest account of how it was measured - Composure under probing on failure modes, scaling limits and alternatives, without bluffing ### Follow-up Questions - If traffic grew tenfold tomorrow, which component in your diagram would fail first, and what would you change? - Pick the most critical data store in your diagram. What do users experience if it is unavailable for ten minutes? - Which decision would you reverse today, and what would reversing it cost? - How did you get agreement on a design that others initially opposed?