Two weeks with alertmanager-en: what feels right and what drives me up the wall
It took me two weeks of on-and-off digging and plenty of wrong turns. Writing the process down as it happened so the next person spends less time.
# Load test: do not jump straight to the max concurrency. # Ramp it up, otherwise you miss the knee. for c in 50 100 200 400 800; do wrk -t8 -c$c -d60s --latency http://127.0.0.1:8080/api/feed sleep 20 done
We also fixed monitoring along the way: replaced average-based alerts with percentiles and split them per endpoint. False alerts dropped by about seventy percent and the on-call rotation visibly cheered up.
Worth noting: the official docs do cover this, just in a very inconspicuous spot. I only found it reading the source comments, where the author explains the reasoning — roughly "so that it degrades into predictable behaviour in extreme cases".
The first thing was to collapse the variables. We were changing config and upgrading the version at the same time, and afterwards nobody could say which change caused what. We rolled back to moving one variable at a time, re-ran three times, and only then did the curve settle. Tedious, but not skippable.
What genuinely surprised me was the tail. The average looked great while P99 jumped by an order of magnitude past some threshold. The cause was not alertmanager-en itself but our upstream connection reuse — the load test traffic was too clean and hid the long-tail requests.