The image awaiter works in conjunction with a daemonset that will schedule pods that pull images to all nodes. It works by repeatedly checking the daemonset's pods are ready, and exits when they are.
Because it can delay the hub to be upgraded before the relevant images are made available, and that can for large images cut down startup time from almost ten minutes to a few seconds.
You need to know about Kubernetes Jobs and Kubernetes DaemonSets, about Helm and helm hooks, and about the programming language Go.
The size of the image needed to run this image-awaiter needs to be as small as possible in order
to improve helm upgrade and helm install performance. If the image was large,
all helm upgrades might have to wait for the big image to be pulled, even if no
image pulling needs to happen. This makes for a slow user experience.
We <3 python, but the smallest image with the kubernetes python library installed
is about 116MB, and the smallest Python image is about 36MB. This Go image is only
about 4MB, which is almost an order of magnitude smaller. This is the primary reason
Go is used.
It is currently hard to depend on in small Go projects. Once that situation changes, we'll simplify our code by switching to it.