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Use the 45 minutes to protect time for a complete design, one or two meaningful deep dives, and a final evaluation—not to follow a rigid script. Start by clarifying the system’s purpose and boundaries, make only the scale estimates that affect architecture, sketch the full design, then explore its most consequential components and trade-offs.
A flexible 45-minute practice plan
This agenda is a starting template, not a universal interview format. Published guides divide the time differently: System Design Prep’s framework allocates 5 minutes to scope, 5 to numbers, 15 to high-level design, 15 to deep dives, and 5 to wrap-up. The System Design Interview Handbook instead suggests separate ranges for requirements and estimation, data modeling, APIs, detailed design, and evaluation. Both approaches aim to leave room for a coherent design and meaningful analysis.
| Time | Focus | What to produce |
|---|---|---|
| 0–5 minutes | Clarify scope | Core requirements, boundaries, and visible assumptions |
| 5–10 minutes | Estimate selectively | Rounded scale assumptions that influence architecture |
| 10–20 minutes | Model and sketch the whole system | Core entities and access patterns, major components, and request/data flow |
| 20–35 minutes | Deepen one or two areas | Component behavior, failure handling, and justified trade-offs |
| 35–45 minutes | Evaluate and adapt | Requirements check, bottlenecks, failure cases, and a concise close |
API and schema decisions do not need a fixed slot: introduce them while sketching the system or set aside a brief block if they shape the design. The key checkpoint is to have begun the high-level design by about minute 15; the handbook specifically advises moving on if it has not started by then.
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Minutes 0–5: Clarify the problem
Open-ended prompts are part of the exercise. Ask who uses the system, what its essential behavior is, and which features are outside scope. Identify relevant quality goals—such as latency, availability, consistency, or durability—rather than listing every possible requirement. Write assumptions down so you can connect later design choices to them.
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Minutes 5–10: Estimate only what changes the design
Consider users, request rates, storage, or bandwidth when those figures help distinguish between architectural options. Use rounded orders of magnitude; precision is less useful than understanding whether a choice needs to handle a particular scale. The handbook recommends keeping estimation to five minutes or less. If an estimate does not affect a decision, move on.
Minutes 10–20: Make the whole design legible
Identify the main entities and how the system reads or writes them. Then draw the major pieces—such as clients, entry points, services, and storage—and trace a representative request or data flow through them. Explain which requirement motivates each important component. A clear end-to-end picture gives the discussion a foundation before you spend time on internals.
Minutes 20–35: Choose consequential deep dives
Pick one or two parts that are most constrained by the requirements or most likely to become bottlenecks. Explain how each works, what can fail, and why your approach is reasonable. State the cost of a choice alongside its benefit: for example, a design decision that improves one quality may add operational complexity or affect another. Ask the interviewer where additional depth would be most useful, then adapt.
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Return to the requirements you established at the beginning. Point out likely bottlenecks, describe relevant failure behavior, and discuss a plausible next step if scale or constraints change. End with any important limitation that remains. Keep the close analytical: it is a chance to show that the design meets its stated goals, not to add unrelated components at the last moment.
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How to run a useful practice round
- Choose an open-ended prompt. Familiar examples include “Design Twitter” and “Design a URL shortener”; a new prompt can be useful too. These examples illustrate the prompt style, not how often any particular company asks them.
- Set a 45-minute timer and work visibly. Use a blank page or whiteboard to record assumptions, architecture, and data flow.
- Narrate your reasoning. Say what you are deciding and why, ask clarifying questions, and pause for feedback or redirection. Treat the exercise as a conversation rather than a memorized presentation.
- Use requirements to justify components. For an unfamiliar problem, break it into known building blocks, but select components only when the stated needs support them.
- Reserve a deliberate review. When practicing alone, narrate aloud and review the round after the timer. Do not use a successful completion of one practice prompt as evidence of a guaranteed interview result.
Review the round and choose one improvement
After time is up, check your process against these questions:
- Did I clarify the prompt before naming technologies?
- Did I make assumptions visible and connect design choices to them?
- Did my estimates help distinguish architectural needs, or take time without changing the design?
- Did I show the request path and major data stores before detailing internals?
- Did I choose one or two meaningful deep dives instead of scattering attention?
- Did I explain the costs and trade-offs of major choices as well as their benefits?
- Did I respond collaboratively to questions or changing constraints?
- Did I leave time to check requirements and discuss failure cases?
Pick the clearest missed behavior as the goal for your next round—for example, completing the whole-system diagram sooner, explaining an access pattern more clearly, or stating the downside of a major component choice. This checklist is a coaching aid, not a validated scoring system.
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Adjust the depth to the role and conversation
Use the interviewer’s questions and constraints to decide where to spend remaining time. A more senior or staff-level discussion may call for broader operational reasoning and trade-off analysis; the handbook describes wider expectations for those roles. Do not treat any one timing breakdown as a company-wide standard: the available practitioner guides offer different budgets, and neither establishes a universal format.
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