Problems / Resilience, medium

Active-active: one region goes down

Two regions each serve half the traffic at about 60% load. At 10 s the US region fails for 15 seconds and the load balancer sends everything to Europe.

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Your goals

Success ≥ 99% through the failover.

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

2,000 requests a second for 30 seconds: 90% reads, 10% 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 (US region), API (EU region).Layer 4 balancers route by IP and port; layer 7 balancers read HTTP and can route by path or header.–
API (US region)App serviceLoad 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 cacheCacheAPI (US region) checks it first for reads. API (EU region) checks it first for reads.An in-memory cache like Redis usually answers in well under a millisecond inside one region.95% hit rate
PostgresSQL databaseAPI (US region) reads it when Redis cache misses. API (US region) calls it for writes. API (EU region) reads it when Redis cache misses. API (EU region) 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
API (EU region)App serviceLoad 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

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