QABattle

Performance / Scenario

The Black Friday Slowdown

Checkout latency spiked to 8 seconds at peak traffic. Read the metrics, the DB stats, and the deploy timeline, find the bottleneck.

Difficulty
Boss
Format
Scenario
Points
300
Estimate
20 min

// MISSION BRIEF

Your Mission

Black Friday, 2 PM peak. Checkout latency (p95) climbed from 400ms to 8 seconds over 20 minutes. Cart and browse stayed fast. Revenue is bleeding.

You have the latency graph description, the database metrics, and the recent change log. Find the bottleneck and recommend the fix, under pressure, like it's really happening.

// FIRST CONTACT

Battle teaser

First artifact

Metrics snapshot (2:15 PM peak)

What is the bottleneck?

  1. AApp servers are out of CPU
  2. BNot enough app-server memory
  3. CThe checkout query's correlated subqueries scan a 50M-row unindexed orders table per checkout, saturating the DB primary
  4. DThe CDN change slowed checkout
Answers, scoring, hints, and the full battle stay sealed.

// SKILL TAGS

performancedatabaseincidentn+1indexing