I learned about fan-out, and just how the average size of groups in messaging servers affects their load. It is pretty basic stuff I guess but not something you think about unless that is your jam.

I was stress testing a server I made and noticed a discrepancy between two tests. In on test 15 virtual users could send 450msg/sec (not realistic I know) for about 6750 incoming messages per second. In another, 1000 users sending a message about every two seconds was the limit, about 500msg/sec incoming.

Took me a little bit to figure out why, the outgoing messages between the tests were wildly different. In the smaller user tests the groups were about 3 on average, in the larger one they were 30. Group size is effectively a multiplier on incoming msg requests so when looking at the total traffic it was much closer between the two tests 15k and 20k.