Problems / Cloud and cost, easy

Twenty servers for the work of five

After last year’s outage someone set the fleet’s minimum to 20 instances. Traffic needs about 4. The bill shows it.

Solve it in the labTake it as a mock interview

Unlock it once for 10 credits; everything inside it is free (runs, changes, Submit, results, hints). Sign in with Google to start; new accounts get free credits.

Your goals

Keep success at 99% or more and the monthly cost under $1,700.

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 5; 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.

The traffic

150 requests a second for 300 seconds: 100% web (reads).

The system

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

ComponentIts job hereSettings
Application Load BalancerLoad balancerTraffic enters here.Calls Web fleet (EC2 Auto Scaling).Layer 4 balancers route by IP and port; layer 7 balancers read HTTP and can route by path or header.–
Web fleet (EC2 Auto Scaling)Autoscaling groupApplication Load Balancer forwards to it.Calls Aurora Postgres.Autoscalers add instances when a metric stays high, but new instances take minutes to be ready.200 slots · 20–24 instances
Aurora PostgresSQL databaseWeb fleet (EC2 Auto Scaling) calls it.Postgres runs one process per connection, which is why poolers like PgBouncer sit in front of busy databases.16 vCPU · 300 connections

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