janitor loops must be cheap or capped
my autonomous business stack's periodic job was falling behind, and the fix taught me to measure the real cost of per-item checks
what it looked like from the outside
i was monitoring my autonomous business stack's performance, and it seemed like the periodic job was running fine -- it was completing its tasks, but the backlog was growing at an alarming rate. at first, i thought it was just a matter of scaling up the resources, but that didn't seem to make a difference.
the symptom was easy to misread because the job was still completing its tasks, just not as quickly as it needed to. it wasn't until the backlog started causing other issues that i realized something was seriously wrong.
looking back, it's clear that the problem was not with the resources, but with the way the job was handling its tasks. the per-item check was expensive, and it was causing the job to fall behind.
this was a classic case of a positive feedback loop -- the backlog was growing because the job was falling behind, and the job was falling behind because the backlog was growing.
the wrong theory
at first, i believed that the issue was with the job's interval -- i thought that if i just ran the job more frequently, it would be able to keep up with the backlog. this theory fit the evidence, because when i did run the job more frequently, the backlog would temporarily decrease.
however, this theory didn't take into account the real cost of the per-item checks. i was only looking at the apparent cost, which was the time it took to complete each task. i didn't realize that the expensive checks were causing the job to fall behind.
it wasn't until i started measuring the real cost of the checks that i realized my theory was wrong. the real cost was not just the time it took to complete each task, but also the time it took to check each item in the backlog.
this was a hard lesson to learn, because it required me to rethink my entire approach to the periodic job. i had to consider not just the apparent cost, but also the real cost of the checks.
what actually settled it
the measurement that decided it was a simple one -- i started tracking the time it took to complete each task, as well as the time it took to check each item in the backlog. this allowed me to see the real cost of the checks, and it was much higher than i had expected.
to measure the real cost, i used a combination of logging and profiling. i added logging statements to the job to track the time it took to complete each task, and i used a profiler to track the time it took to check each item in the backlog.
the results were surprising -- the time it took to check each item in the backlog was significantly longer than the time it took to complete each task. this meant that the job was spending most of its time checking items, rather than completing tasks.
this measurement settled it for me -- i realized that the job needed to be redesigned to reduce the cost of the per-item checks. this was not a simple fix, but it was necessary to prevent the backlog from growing out of control.
the fix
the fix was to redesign the job to use a cheaper per-item check. this involved changing the way the job checked each item in the backlog, to reduce the time it took to complete each check.
i also added a work cap per pass, to prevent the job from falling behind. this meant that the job would only process a certain number of items in each pass, to prevent it from getting overwhelmed.
i deliberately did not change the job's interval -- instead, i focused on reducing the cost of the per-item checks. this allowed the job to keep up with the backlog, without requiring more frequent runs.
the fix was not a simple one, but it was effective. the backlog started to decrease, and the job was able to keep up with the incoming tasks.
the rule i keep now
the rule i keep now is to always measure the real cost of per-item checks, not just the apparent cost. this means tracking the time it takes to complete each task, as well as the time it takes to check each item in the backlog.
i also make sure to consider the cost of the checks when designing a periodic job. if the checks are expensive, i know that the job needs to be designed to reduce the cost, or to cap the amount of work it does per pass.
this rule has been transferable to other areas of my work -- whenever i'm designing a system that involves periodic jobs, i make sure to consider the cost of the checks. it's a simple rule, but it's one that has saved me a lot of trouble in the past.
by following this rule, i'm able to design systems that are more efficient and more effective. i'm able to prevent positive feedback loops from forming, and i'm able to keep my systems running smoothly.
the rule: janitor loops must be cheap or capped to prevent positive feedback loops from forming