# Trade-offs and Bottlenecks — System Design Interview Prep

Source: https://www.skillbyai.com/en/system-design-interview/v-tradeoffs

> Every choice has a cost.

## Name the alternative you rejected

For each major decision, state the alternative and why you chose differently: SQL versus NoSQL, push versus pull, strong versus eventual consistency, cache-aside versus write-through, synchronous versus queued processing. Then walk through bottlenecks: the hottest path, hot keys, single points of failure, and what happens at ten times the load. Close with observability: metrics, logs, traces and alerts on the key service levels.

## Trade-offs, communication, practice

Strong candidates explain trade-offs, communicate clearly and practise deliberately.

![Three ideas: trade-offs and bottlenecks, communication, practice plan.](assets/figures/system-design-interview/section-8-map.svg) — Figure 8.1 — Trade-offs, communication and practice.

## Trade-off phrases that work

Patterns for explaining decisions.

```text
"We could do X, which gives A but costs B. Given requirement R, I would choose Y."
"This is eventually consistent; a like count being a few seconds stale is acceptable."
"The bottleneck at 10x is the database write path; I would shard by user_id."
"If the cache cluster fails, we fall back to the DB with rate limiting to protect it."
"To verify it works in production: p99 latency, error rate, queue age, cache hit ratio."
```

## Volunteer weaknesses

Pointing out the limits of your own design before being asked is a strong senior signal.

**Quiz:** What makes a design decision well justified in an interview?

- [x] Linking it to a requirement and naming the alternative's trade-off
- [ ] Using the newest technology
- [ ] Saying it is industry standard without detail
- [ ] Avoiding any mention of downsides

*Answer:* Linking it to a requirement and naming the alternative's trade-off. Requirements drive trade-offs.