Choosing a Data Centre Before Choosing a Server

Subtitle: Two layers, two jobs, one request.

• Left column - Global: which site:
• Answered by DNS or anycast
• Resolver returns nearest region
• Chosen before any packet lands
• Optimises distance and blast radius
• Holds no connection state

• Right column - Local: which machine:
• Sits inside the request path
• Sees real connection state
• Least-connections, consistent hash
• Retries, per-request health checks
• Optimises utilisation

• Simple difference:
• Global = pick the site, then step aside
• Local = pick the machine, every request

• Name the real stack:
• Geographic DNS steering on top
• Layer 4 or layer 7 balancer below
• You need both, not either

• Honest test - three steps:
• Blackhole one whole region
• Count real clients still arriving
• Stop only when arrivals hit zero

Sticky note - the global layer lies, and it lies through caching: A resolver honours the TTL, so during failover clients keep reaching the dead site until it expires. A 60 second TTL still means minutes of errors, and some resolvers ignore short values. Anycast skips the cache but waits on route convergence instead.