InterviewCrafted

Viral link · cache miss 95% · redirect p99 8s · DB read overload

Read-path Infra · Incident brief

The URL Shortener That Redirected Users to Nowhere

Viral link · cache miss 95% · redirect p99 8s · DB read overload

Problem statement

A viral marketing link drove 50× normal redirect traffic to one short code. Cache miss rate hit 95%. Redirect p99 reached 8s; PostgreSQL read replicas saturated. Sync click analytics on the hot path made it worse.

Diagram shows Redis cache-aside with no CDN, no hot-key tier, and analytics coupled to redirect.

  • Viral link caused 95% cache miss rate for one short code.
  • Redirect p99 latency 8s; users saw timeouts.
  • PostgreSQL read replicas saturated.
  • No CDN edge cache for redirect path.
  • Click analytics on sync path worsened hot key.

Live evidence

  • MarketingT+0

    Viral campaign link posted — traffic 100× on single short code

  • SyntheticsT+3m

    Redirect 404 rate 22% — cache cold · DB read QPS exceeded capacity

  • Status pageT+8m

    Public incident: shortened links redirecting to error page

Architecture

Team whiteboard — incomplete. Missing paths implied by the incident.

The sketch on your whiteboard is the team's incomplete draft from a design review — not a correct or complete architecture. It omits major runtime paths and components implied by the incident.

Impacted services

  • Redirect APIcritical

    p99 8s; error rate 18%

  • Redisdegraded

    Hot key on single shard

  • PostgreSQL replicascritical

    Read IOPS maxed

  • End userscritical

    Timeouts and broken redirects