Problems / Databases, medium

One viral key

A URL shortener where one link went viral: its partition melts while the rest are idle.

Solve it in the labTake it as a mock interview

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Your goals

Success ≥ 99%, p99 ≤ 300 ms.

Test cases

Like a coding problem, a solution has to pass every case, not just the one it was built for. Submit in the lab runs all 6; 2 of them are hidden until then.

  1. As given. The problem’s own traffic, scheduled failures and seed.
  2. Peak hour. 25% more traffic than the problem states, with the same failures.
  3. A slow dependency. The main data store or model answers twice as slowly for part of the run.
  4. A server dies. One replica of the service with the most replicas is killed for part of the run.

The traffic

4,000 requests a second for 30 seconds: 95% redirect (reads), 5% shorten (writes).

The system

What each component does in this design, and what calls it. The simulator mimics each one from these settings.

ComponentIts job hereSettings
Load balancerLoad balancerTraffic enters here.Calls API.Layer 4 balancers route by IP and port; layer 7 balancers read HTTP and can route by path or header.–
APIApp serviceLoad balancer forwards to it.Calls Redis cache (checked first), Key-value store.Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s.4 replicas × 200 slots · timeout 300 ms
Redis cacheCacheAPI checks it first for reads.An in-memory cache like Redis usually answers in well under a millisecond inside one region.60% hit rate
Key-value storeKey-value storeAPI reads it when Redis cache misses. API calls it for writes.Key-value stores spread keys over partitions; one very popular key can overload a single partition.4 partitions × 400 ops/s

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