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System design fundamentals

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System Design Fundamentals Deep Dive

Overview

System design interviews test whether you can move from ambiguous requirements to a practical architecture with clear tradeoffs.

Core Concepts

  • Start with requirements, constraints, and success metrics.
  • Define scale assumptions early (QPS, data size, latency targets).
  • Prioritize correctness and operability over novelty.

Internal Architecture

  • Client -> edge/API gateway -> stateless services -> data stores.
  • Separate read-heavy and write-heavy paths where possible.
  • Keep boundaries explicit for ownership and failure isolation.

Data and Request Flow

  • Request enters gateway, is authenticated, then routed to service.
  • Service reads cache first, then storage fallback on miss.
  • Write path records durable state before async fan-out.

Scalability and Reliability

  • Horizontal scaling for stateless services.
  • Backpressure + rate limiting at ingress.
  • Timeouts, retries with jitter, and circuit breakers.

Code Examples

Mobile App -> API Gateway -> BFF -> Services -> DB
                               \-> Queue -> Workers

Common Interview Questions

  • Q: How do you structure an answer in 35-45 minutes? A: Structure the answer as constraints then tradeoffs: SLOs, capacity assumptions, bottlenecks, failure modes, and mitigation plans with clear triggers.
  • Q: What assumptions should you state first? A: Structure the answer as constraints then tradeoffs: SLOs, capacity assumptions, bottlenecks, failure modes, and mitigation plans with clear triggers.
  • Q: Which tradeoffs matter most for Android clients? A: State load and SLO assumptions first, identify the first bottleneck, choose scaling and consistency strategy, and explain fallback behavior for partial failures.

Production Considerations

  • Define SLIs/SLOs before choosing technology.
  • Plan observability from day one.
  • Keep migration strategy for schema/API changes.

Tradeoffs

  • Simplicity vs flexibility.
  • Consistency vs availability.
  • Latency vs cost.

Senior-Level Insights

  • Strong candidates make uncertainty explicit.
  • Staff-level answers include rollout and failure modes.