quest

Sign in to start training

quest keeps your progress, streaks, and scores tied to your account — so you can pick up on any device.

Synced everywhere. Progress follows you across laptop and phone.
Streaks & stats. See your daily run and every score over time.
Your profile. A name and avatar for the work you do here.

We only use Google to sign you in and sync progress. No posting, no contacts — ever.

← All subjects · System Design

System Design Intermediate

02  Caching

The fastest query is the one you never run. A cache buys you latency and shields the database -- and it charges you back in staleness, invalidation bugs, and brand-new failure modes.

Step 2 / 2

Flash sale: the cached entry for one hot product expires at peak traffic, and 20,000 concurrent requests all miss together and hit the database, which melts. Name this failure, explain why plain TTL expiry causes it, and give two distinct mitigations, saying briefly how each works.

Hints
  • This failure mode has a well-known name. Why do ALL the misses reach the database at the same instant, rather than spread out over time?
  • Mitigation family one: arrange for only ONE request to rebuild the value while the rest wait or are served something else.
  • Mitigation family two: never let a hot key hard-expire -- serve stale while refreshing in the background, or spread expiries so they cannot align.
Your design is reviewed against a hidden rubric.