Local sessions vs shared sessions
Local sessions live in the memory of one server, so a user who lands on another server loses the session. Shared sessions live in a store that every server reads. When one of two hosts fails at 0:30, local sessions give 74.5 percent availability. Shared sessions give 99.9 percent for 32 USD/mo.
Lasers in orbit, New York to London · 7,151 km · 24 ms one way
5 servers · 72% busy · p99 66 ms
Case 1
Two API servers share 1,000 requests/s. One host fails at 0:30. Sessions must still work on the survivor. Availability must stay above 95 percent, at 40 USD/mo or less.
| Measure | Goal | Sessions in server memory | Sessions in a shared store |
|---|---|---|---|
| Availability | at least 95% | 74.5% Fail | 99.924% Pass |
| Error rate | under 5% | 25.5% Fail | 0.076% Pass |
| Monthly cost | 32 USD/mo | 32 USD/mo | |
| p99 latency | 68 ms | 69 ms |