Scalability & Capacity Planning Questions

Analyzing how a system's performance changes as load grows and planning the resources to keep it healthy. Covers horizontal vs vertical scaling, throughput vs latency under load, headroom and saturation, load modeling, and forecasting capacity for expected traffic. Includes identifying the scaling bottleneck that will bind first as demand increases.

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Medium: Following a capacity-related outage, describe how you would perform capacity planning to prevent recurrence. Include how you would estimate headroom, pick metrics to monitor, define safety margins, implement autoscaling or throttling, and communicate capacity-related risks to product teams.

Sample Answer

Approach overview:- Treat the outage as a signal to build a quantitative capacity plan: measure actual demand and headroom, define safety margins tied to SLOs, implement automated scaling and graceful throttling, and communicate risk and mitigations to product teams.

1) Estimate headroom (how much spare capacity we need)- Gather recent telemetry: peak and rolling p50/p95/p99 RPS, concurrent connections, request size distribution, and resource usage (CPU, memory, network, I/O) per request from profiling and APM.- Build a capacity model: capacity_per_instance = requests_per_sec_handled_at_target_utilization (e.g., RPS@70% CPU). Required_instances = ceil(peak_RPS * (1 + growth_factor) / capacity_per_instance).- Include growth and burst assumptions (e.g., 10% weekly growth, 2x short-term burst). Validate with load tests that replay production traffic patterns (including p99 latencies and long-tail effects).

2) Pick metrics to monitor (what to alert on)- Load signals: RPS, concurrent requests, queue depth.- Performance: latency p50/p95/p99, tail latencies, error rates (4xx/5xx), DB/ downstream latencies.- Saturation: CPU utilization, memory headroom, disk I/O, socket/file descriptor usage, GC pause metrics.- Infrastructure: instance counts, autoscaler health, scaling cooldowns, pod restarts.- Business/SLO signals: user transactions/sec, feature-specific error rates, error budget burn rate.- Alerting: multi-signal alerts that combine increased latency + error rate + saturation, to reduce false positives.

3) Define safety margins- Set target utilization (e.g., 60–75% CPU) rather than 90–100% to preserve burst headroom. For critical services, aim for larger margins (50–60%).- Define short-term burst headroom (e.g., able to handle 2x observed peak for 1–5 minutes) and steady-state headroom (e.g., 25–50% spare capacity).- Tie margins to SLOs and error budgets: if error budget is low, increase margins or temporarily restrict features.

4) Implement autoscaling and throttling- Autoscaling: - Use horizontal autoscaling (HPA with custom metrics or cluster autoscaler) based on target utilization (CPU or custom RPS-per-pod metric) and queue length. - Configure conservative cooldowns and predictive/scheduled scaling for known traffic patterns (e.g., morning spike). - Add a buffer: minimum instances = baseline to cover expected burst; maximum = budgeted cap. - Test scaling end-to-end (scale-up latency, cold start times).- Throttling and graceful degradation: - Implement rate limits per user/API key and global guards to protect backend. - Use circuit breakers and priority queues: degrade non-critical features when saturated. - Backpressure: queueing with bounded queues and clear retry-after headers. - Fallbacks: serve cached responses or degraded UI rather than failing hard.- Safety nets: autoscaler override alerts, runaway-scaling protection to avoid cascading cost/latency issues.

5) Validation and runbooks- Load test the system with realistic traffic including p99 paths and downstream slowdowns; verify autoscaler behavior and throttling responses.- Create runbooks: how to read dashboards, emergency scale commands, how to flip throttles/feature flags, escalation path.

6) Communicate capacity-related risks to product teams- Create a concise risk summary: current capacity vs observed peak, safety margin, likely failure modes, cost/benefit trade-offs for increasing capacity or adding throttles.- Share SLOs, error budget state, and the planned mitigations with timelines (e.g., “We need +30% instances by Q3 or feature X will be throttled under 2x peak”).- Provide dashboards and playbook links, hold a short walk-through with product and PM to align on acceptable degradations and priorities.- Propose options with impacts: schedule capacity increase, optimize code/queries, or add feature-level rate limits; include estimated cost and user impact.

Why this works:- Quantitative modeling + testing reduces surprises. Metrics tied to SLOs and multi-signal alerts detect real risk. Autoscaling plus throttling provides both elasticity and defensive controls. Clear communication aligns product decisions with reliability trade-offs so recurrence is unlikely.

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