Dynamic values in ConfigMap

[themanw...@gmail.com]

Hello All,

I have a rather specific usecase and was looking for some suggestions on how to tackle it.

We have a java application that reads a configuration file on start up. We can run multiple copies of this application for horizontal scaling however it requires we give each a unique ID between 1 and 1023. This ID needs to be preset at startup and cannot be set after.

For the moment we hacked a awful bash script to inject a random number into the file on pod start. Ideally we want to move the entire config file into a ConfigMap so we don't have to version the container image for every change. Ideally we need a way to have the ConfigMap generate the ID or call something external that does. Eventually we'll need to ensure this ID is actually unique, right now we're relying on random chance.

Is this something possible with kubernetes? We've been reading all the documentation we can find but so far have not come up with a real solution.

Thanks,
Andy

[Tim Hockin]

Publish the base configmap into a volume. Run a sidecar container
that consumes that and merges it with the unique ID (which you get
from StatefulSet or somewhere else - different problem) and publish
THAT to a shared emptyDir volume. When the base configmap changes,
the sidecar wakes up (e.g. inotify or just compare checksum) and
repeats the process.

There isn't a generic "template this data on the way to becoming a file" logic.

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[Rodrigo Campos]

Do you need the id to be on the same file? Can't that be in another file? If it can be in another file, and not in the configmap, I think it will be way more easy (and easier to share a configmap, etc.)

Because, maybe if you use a stateful set and a init container, you can just place it in an emptyDir at init. Not beautiful, but might work for a first step.

Or maybe just an env variable or file with the index of the stateful set is possible with the downward api? Don't remember.

But wouldn't that do the trick? Or am I missing something?