Why this monorepo-frontend-qa pattern gets slow in production: a source-level explanation
Some background first. Our setup is monorepo-frontend-qa plus three downstream services, seven figures of daily requests, peaking around nine in the evening.
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 monorepo-frontend-qa itself but our upstream connection reuse — the load test traffic was too clean and hid the long-tail requests.
One last trap: in container environments remember to adjust the memory-related parameters in step. Otherwise the host limit and the process expectation disagree, and the symptom is intermittent, unreproducible failure.
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.