Problems / Real world, medium
A news article goes viral: traffic ramps to 4× over the run. Pages are cacheable at the edge, but the CDN is tiny.
Free to open. Runs in your browser, no sign-up.
Success ≥ 99%, p99 ≤ 500 ms.
1,000 requests a second, ramping to 4× for 30 seconds: 98% reads, 2% writes.
What each component does in this design, and what calls it. The simulator mimics each one from these settings.
| Component | Its job here | Settings |
|---|---|---|
| CDNCDN | Traffic enters here.Calls Load balancer.A CDN keeps copies at edge locations near users, so a cache hit skips the trip to your origin. | – |
| Load balancerLoad balancer | CDN forwards to it.Calls API.Layer 4 balancers route by IP and port; layer 7 balancers read HTTP and can route by path or header. | – |
| APIApp service | Load balancer forwards to it.Calls Redis cache (checked first), Postgres.Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s. | 2 replicas × 150 slots · timeout 1.0 s |
| Redis cacheCache | API checks it first for reads.An in-memory cache like Redis usually answers in well under a millisecond inside one region. | 90% hit rate |
| PostgresSQL database | API reads it when Redis cache misses. API calls it for writes.Postgres runs one process per connection, which is why poolers like PgBouncer sit in front of busy databases. | 8 vCPU · 60 connections |
Try it first. Open one hint at a time if you get stuck.
How much traffic reaches the origin?
Edge caching is the cheapest capacity there is.
Give the CDN more storage (higher hit rate), and/or scale the API.