Shopify
A merchant reported that the cart count in the header was wrong. Not stale by a few seconds. Wrong by a whole session. One customer added three items and the header said eleven. Then it said two. It settled on the correct number only after a hard reload. Eleven was
Performance
A service I worked on had a mean response time of 84 milliseconds. The dashboard was green for months. Then someone plotted the distribution and we found that one request in fifty took over two seconds, and those users were bouncing. The average was not lying. It was just describing
Bug Hunt
The ticket said search was broken. The user searched for a product they had bought before and got results for industrial fasteners. They do not sell industrial fasteners. Two hours later there were eleven tickets. Then a merchant called. The common thread was that every affected user was logged in,
Docker
One changed byte early in a Dockerfile throws away everything after it. Here is how the cache key is computed and how to order instructions so it survives.
PostgreSQL
Dead tuples that never get reclaimed, a table twice the size of its data, and queries getting slower every week. Here is what blocks vacuum and how to see it.
DevOps
Lag climbing on one partition while the rest keep up tells you something specific. Here is how to read the numbers before you add consumers that cannot help.
JavaScript
A single page app that was fine for an hour and unusable by the afternoon. Three wrong hypotheses, then a heap snapshot showed forty thousand detached DOM nodes.
MicroVMs
Query times doubling at unpredictable hours with no deploy and no traffic spike. When the cause is not in your process, it is usually not on your machine either.
Shopify
Everyone says apps slow your store down. Almost nobody says by how much. Here are measurements from real stores and a method for auditing your own.
Python
Your async service handles ten requests a second and pegs one core. Somewhere a synchronous call is blocking the event loop.
Shopify
Liquid has no query log, so N+1 problems are invisible until the page is slow. Here is how to find them with Server-Timing and how to restructure the loops.
Databases
Reading from cache is easy. Knowing when to remove something is where the bugs live, and most of them are races.