Bottlenecks

Bottlenecks are components that saturate first under growth: hot database keys, fan-out writes, single leader nodes, unindexed queries, queue backlogs, lock contention, CPU on image processing, or third-party rate limits. Proactively name them after the first design pass.

For each bottleneck propose mitigation: shard the hot key, cache reads, async fan-out, read replicas, batching, or rate limiting ingress. Interviewers reward seeing problems before they ask "what breaks at 100× scale?"

On interviews: list three likely saturation points for your design and one mitigation each; prioritize by estimated traffic.

Common pitfalls: only mentioning database; ignoring celebrity or hot-key skew; no plan for dependency outages.

The trade-off is flexibility versus complexity—know when the simpler path is enough.

Checklist:

• Name the first components to saturate.
• Tie bottlenecks to traffic estimates.
• Propose concrete mitigations.
• Consider third-party and operational limits.