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39
The purpose of podman quadlets? (lemmy.world)
submitted 2 months ago by [email protected] to c/[email protected]
16 comments fedilink hide all child comments
 

I'm afraid this is going to attract the "why use podman when docker exists"-folks, so let me put this under the supposition that you're already sold on (considering) using podman for whatever reason. (For me, it has been the existence of pods, to be used in situations where pods make sense, but in a non-redundant, single-node setup.)

Now, I was trying to understand the purpose of quadlets and, frankly, I don't get it. It seems to me that as soon as I want a pod with more than one container, what I'll be writing is effectively a kubernetes configuration plus some systemd unit-like file, whereas with podman compose I just have the (arguably) simpler compose file and a systemd file (which works for all pod setups).

I would get that it's sort of simpler, more streamlined and possibly more stable using quadlets to let systemd manage single containers instead of putting podman run commands in systemd service files. Is that all there is to it, or do people utilise quadlets as a kind of lightweight almost-kubernetes distro which leverages systemd in a supposedly reasonable way? (Why would you want to do that if lightweight, fully compliant kubernetes distros are a thing, nowadays?)

Am I missing or misunderstanding something?

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[–] [email protected] 11 points 2 months ago (2 children)

It's a systemd-style way to manage podman containers that aims to be as easy to manage as compose/swarm. Not quite an integration, but operates similarly, and about as easy to read. Less heavy than managing a local micro-k8s cluster. That's about it.

[–] [email protected] 3 points 2 months ago

Thank you, I think the "less heavy than managing a local micro-k8s cluster"-part was a great portion of what I was missing here.

[–] [email protected] 5 points 2 months ago* (last edited 2 months ago) (1 children)

Yup. I read it as "compose and manage containers with systemd."

Sure, there is a k8s layer abstracted into podman to do this, but you don't manage or interact with it. Everything is a systemd unit file, a simple text document with a well understood structure. Containers are started and logged like services.

Easy, direct, tidy.

[–] [email protected] 1 points 2 months ago (2 children)

Understood, thanks, but if I may ask, just to be sure: It seems to me that without interacting with the kubernetes layer, I'm not getting pods, only standalone containers, correct? (Not that I'm afraid of writing kube configuration, as others have inferred incorrectly. At this point, I'm mostly curious how this configuration would be looking, because I couldn't find any examples.)

[–] [email protected] 2 points 2 months ago (1 children)
[–] [email protected] 1 points 2 months ago* (last edited 2 months ago) (1 children)

Awesome, so, essentially, you create a name.pod file like so:

[Unit]
Description=Pod Description

[Pod]
# stuff like PublishPort or networking

and join every container into the pod through the following line in the .container files: Pod=name.pod

and I presume this all gets started via systemctl --user start name.service and systemd/podman figures out somehow which containers will have to be created and joined into the pod, or do they all have to be started individually?

(Either way, I find the documentation of this feature lacking. When I tested this stuff myself, I'll look into improving it.)

[–] [email protected] 2 points 2 months ago* (last edited 2 months ago) (1 children)

Awesome, so, essentially, you create a name.pod file like so: [...]and join every container into the pod through the following line in the .container files

Yep, that's the way!

and I presume this all gets started via systemctl --user start name.service and systemd/podman figures out somehow which containers will have to be created and joined into the pod, or do they all have to be started individually?

Systemd figures it out iff you have specified your service dependencies correctly, with things like After=, Upholds=, BindsTo=, etc. Have a look at systemd.unit manpage for details. For my paperless service, it goes something like this:

  1. The entrypoint is paperless.container, which I start with systemctl --user start paperless, which depends on:
    • paperless.pod
    • Three other services, which also depend on:
      • paperless.pod
  2. Systemd figures out that the paperless pod should be started first, and does that
  3. Systemd startes the three dependent containers
  4. Finally, systemd starts the paperless container itself

The point of quadlet was to lean as heavily as possible on systemd for the service and dependency bits and use podman only for translating the container bits into something systemd can handle. The one bit of dependency handling that quadlet does is to make sure that paperless.pod is started before all containers that have Pod=paperless.pod in their quadlet file.

Either way, I find the documentation of this feature lacking. When I tested this stuff myself, I’ll look into improving it.

That would be amazing, of course! :) I find that, if you're familiar with unit files, you're like 85% of the way there already. By the way, the unit files that quadlet generates are somewhere in $XDG_RUNTUME_DIR for you to inspect. I'm afraid I'm not at a computer right now andI don't know the exact path off the top of my head.

[–] [email protected] 2 points 2 months ago

Nice, thanks, again! I overlooked the dependency instructions in the container service file, which is why I wondered how the heck podman figures out the dependencies. It makes a lot of sense to do it like this, now that I think of it.

[–] [email protected] 3 points 2 months ago* (last edited 2 months ago)

I'm still new to this myself, but yes that's the gist of it. This isn't k8s or even k3s. It's an easy way to deploy a container via code on a single node system using the already present systemd for management. It let's you pretend that Linux handles containers natively like it does daemons.

This article from redhat has more information about the why and what.