# Discussing Trade-offs and Failure Modes — High-Level & Low-Level Design Interview Problems

Source: https://www.skillbyai.com/en/design-interviews/tradeoffs

> Show judgement, not just components.

## Every choice has a cost

Strong candidates state the alternative they rejected and why: SQL versus NoSQL, push versus pull, strong versus eventual consistency, precompute versus compute on read, monolith versus services. Then walk through **failure modes**: what happens when a node, a dependency, a region or the network fails? Mention timeouts, retries with backoff and jitter, idempotency, circuit breakers, replication and failover, and graceful degradation (serve stale data, disable a feature). For LLD, the equivalent is edge cases and concurrency: invalid input, races on shared state, and how the design extends to a new requirement.

## A failure-mode walk-through template

Apply to each critical component.

```text
component: Payment Service
if it is slow     -> timeout at Xs (assumed), retry with same idempotency key, backoff + jitter
if it is down     -> orders stay PENDING, queue retries, show "processing" to user
if DB primary dies -> failover to replica; writes paused for failover window
if duplicated msg -> idempotency key dedupes
if data mismatch  -> nightly reconciliation flags for manual review
what we sacrifice -> slower confirmation during incidents, never double charge
```

## A pilot pre-flight briefing

Pilots brief what they will do if an engine fails before it happens. Describing failure handling up front shows the same preparedness.

**Quiz:** Which statement best shows trade-off reasoning?

- [x] Choosing eventual consistency for the feed to gain availability, while keeping payments strongly consistent
- [ ] Using the newest database because it is popular
- [ ] Saying the system will never fail
- [ ] Adding a cache everywhere without discussion

*Answer:* Choosing eventual consistency for the feed to gain availability, while keeping payments strongly consistent. Tie choices to requirements and costs.