Scalability & Capacity Planning Questions
Analyzing how a system's resource needs change as demand grows, and deciding what scaling strategy and headroom to plan for ahead of that growth. Covers translating a stated growth trajectory, for example doubling in six months, a 10x seasonal spike, or a step-change in active users, into a headroom and saturation forecast: how much spare capacity exists today, when it runs out, and what safety margin to hold under forecast uncertainty. Covers the horizontal-versus-vertical scaling decision, including why horizontal scaling itself has diminishing returns as coordination overhead grows with node count, and identifying which resource becomes the binding constraint first as demand increases. Also covers weighing that plan against cost and communicating the trade-off to non-technical stakeholders such as finance or product. This is forward-looking planning, not execution: it does not cover provisioning or autoscaling mechanics, live incident triage, profiling an already-slow code path for hotspots, tail-latency measurement and tracing, or caching and concurrency design patterns, each its own discipline.
A retail client expects 10x their baseline traffic for a 72-hour holiday sale. Walk through the capacity plan you'd put together: how much headroom you'd provision for, how you'd decide between holding that capacity ready in advance versus scaling into it as traffic ramps, and how you'd balance the cost of over-provisioning against the risk of falling over mid-sale. Propose three measurable commitments (SLAs or business metrics) you'd be comfortable promising the client.
Your product is moving from 100K to 1M monthly active users over the next three months. Walk through the capacity ramp plan you'd put together: how you'd model the resource growth at each stage, what you'd test before each stage to validate your assumptions, and how you'd manage cost so expenses stay predictable rather than spiking ahead of the growth.
Your architecture needs to handle a sudden 100k RPS burst, and finance is asking why the infrastructure budget needs to grow to support it. Walk through how you would translate that burst-capacity requirement into a concrete plan, including the headroom you'd hold and the metrics and thresholds you'd point to as evidence, and how you would present the capacity-versus-cost trade-off to finance stakeholders in terms they would accept.
Quantify cost versus performance when choosing between larger instance types (vertical scaling) with high single-instance throughput and horizontal scaling across many smaller instances. Outline a benchmark plan: what workloads you'd run, what metrics you'd collect (cost per request, latency percentiles, ops per second), how you'd compute amortized cost, and what business or technical thresholds would tip your recommendation toward one approach.
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