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Load balancing the rest

The situation

A results portal is fine all year and collapses for two hours every results day. The traffic on that day is roughly 200 times normal.

A page load consists of:

  • the HTML page, different for each student
  • a 2 MB logo and stylesheet bundle, identical for everyone
  • a 40 KB result PDF, different for each student

All three are served by one origin server in the college data centre.

Measured on results day: 94% of bytes leaving the origin are the logo and stylesheet bundle, requested afresh by every student on every page load.

Two origin servers now handle the per-student traffic behind a load balancer. Students report being logged out at random. What is the likely cause?