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Comment by I_dev_outdoors | Hacker News Reader

I probably wouldn't do this, but what problem does this cause?

The declarative approach is a more sustainable way to run Kubernetes. If you define some desired state in manifests and apply them to a cluster, they can be applied again to new clusters or the same one and Kubernetes will attempt to maintain the desired state.

This state can be version controlled, written in stone, whatever you prefer and it can always be attained.

When administrators start issuing imperative commands to a cluster, state starts being changed and there is no record[0] of the state Kubernetes is being asked to maintain.

[0] Not entirely true, the state can always be retrieved from the cluster so long as it hasn't failed.

I find this to be a common misconception, stemming from a misunderstanding of what "declarative" means (especially common when people are discussing tools like Terraform).

Firstly as you point out, there is a record of the state Kubernetes is being asked to maintain: it's in the API server as the spec of each resource.

Secondly, using `kubectl` "patch" in the manner described is not making changes to the cluster state directly, it's making changes to the specification of what should be maintained, and the various controllers effect the state changes.

Fundamentally, the argument seems to come down to "you don't have a record of what you once asked the API server to do", and that's fair enough - you don't. But that has nothing to do with imperative or declarative models.

I'm not advocating actually doing this on a day-to-day basis, but the arguments against it are not ones of imperative vs declarative.

Given that Kubernetes' docs[0] discuss using imperative commands, I think it's a fairly reasonable way to describe it.

[0] https://kubernetes.io/docs/tasks/manage-kubernetes-objects/i...

Why is applying a full manifest “declarative” but applying a patch is “imperative”? That’s clearly an error.

Then open a PR.

This is a strange way to concede an argument, but I’ll take it.

Applying a full manifest is "this is your state now".

Applying a patch is "make these changes to your existing state".

That dependency on existing state is a difference, and it seems to map reasonably well to what declarative/imperative seem to usually be used to mean in this context.

1. A strategic merge patch says "this is your state now" (the only difference is the scope of the state in question, with a full manifest including a bunch of extraneous stuff)

2. "make these changes to your existing state" is still declarative

> Applying a full manifest is "this is your state now".

It isn’t though. It’s “please make the state look like this eventually”. You do not patch state, just spec. Controllers effect the changes.

You're both correct, but you're talking about different states. The parent's "state" is the state of the etcd database and your state is the actual state of the resources that the controllers are managing.

That said, the parent is wrong that a full manifest is declaring the (etcd) state in a way that a (strategic merge) patch isn't--both are declaring etcd state, but a strategic merge patch is doing so in finer-grained increments. A strategic merge patch can declare zero state or many full manifests, while applying full manifests can only work in increments of complete resource manifests. But both are telling Kubernetes "this is your (etcd) state now".

> Fundamentally, the argument seems to come down to "you don't have a record of what you once asked the API server to do", and that's fair enough - you don't. But that has nothing to do with imperative or declarative models.

You do actually have this record. First of all, because k8s has an audit log, and secondly because deployments maintain a revision history (so you can always rollback--kubectl even supports this via `kubectl rollback undo`).

You're correct, but, when approaching this from the human perspective, it's less about the technology and the reality and more about "do I have a record of the last state I asked the cluster to be in before I fucked it up?". Ultimately that's what matters. Auditability is great, but in practice, when the shit hits the fan, can I get the application that brings in our company's money, and our customers rely on to breath, back to the state it was after 5 years and 73 updates?

Updating the manifests, pushing them to version control and having CD deploy them encourages humans to "do the right thing".

It would be quite easy though to just tweak that one environment variable though while I patch in the new image version to update that one service; until the entire cluster dies and I can't retrieve the last definition and need an equivalent cluster up within the hour.

This is really more about the practice of writing down the state you want the cluster to be in (the spec) and showing it to the cluster, than just ordering it to do one thing without context.

In this sense, Declarative Vs Imperative is just a proxy term for "do I have a record of the state I asked the cluster to keep?"

People are incorrectly assuming that using kubectl implies invoking it from an administrator's laptop. Of course, you can and should invoke kubectl from your CD pipeline. The CD pipeline maintains its own record of runs, and Kubernetes deployments have a revision history.

Moreover, people in this thread also don't know what "declarative" means. The patch is declarative, and "declarative" doesn't provide the claimed benefits. For example, as with the patch command, I can create, apply, and delete a Kubernetes manifest file (indeed, I can apply directly from stdin!) and there is no additional record of the change beyond that which would exist if I had just applied the patch.

A patch is declarative in that it's an idempotent command which requires you to fully define the patch. It's still something that is being manually defined rather than contained in a file to be applied. Of course, if that file is just being locally held and modified on someone's laptop instead of committed to version control, it's a moot point.

I will grant you that `kubectl delete -f` breaks my argument, since it's imperatively modifying objects given a declarative manifest. Re: changes via stdin, I mean sure; you can also pipe a deployment manifest into `ex` or `sed` to make changes on the fly and then echo it back to `kubectl apply -` but I wouldn't recommend it.

Yeah, my point is that whether you're changing a manifest and doing an apply versus a patch isn't relevant, the relevant bit is whether or not you have a version history and in both cases you do via your CD pipeline and via k8s deployment revision history. You can also commit to git, but I don't think there's much value in committing every image version change to git.

Also, working in raw manifests isn't a panacea; if you delete a resource's manifest, Kubernetes won't delete its corresponding resource (maybe this is what you were referring to with your bit about `kubectl delete -f`). You need something that diffs the manifests against the current state of the world and applies the requisite patches (something like Terraform) but this isn't related to how you change image versions.

I agree with your points, but as I mentioned in another comment, if we take "imperative" and "declarative" as proxy terms for "I do or do not have a reference to the state I requested of the cluster OUTSIDE of the cluster"; my point is that updating the state, in full, by modifying the manifests and committing them to be deployed by your CD pipeline or otherwise is a better approach to ensuring you or someone else can rebuild your "empire" were it to unceremoniously burn to the ground.

I think you’re assuming that just because we’re updating the image via kubectl (which itself is invoked via CD pipeline) that the infrastructure isn’t codified and persisted, which isn’t the case. You can (and should) still have your infrastructure manifests saved in your infra repo/directory and version controlled; that’s orthogonal to how you update the current image version.

The same problems any imperative management within declarative config causes -- drift. If the tool you're using supports declarative configuration, all changes should be made exclusively via the declarative interface to prevent that drift. In this example, the new image should be added to the original manifest itself, not via a CLI update.

It depends on how you manage your changes. A lot of people don't have their infra-as-code manage the deployment's image field--rather, that's updated by the application's CD pipeline. There's no drift to worry about.

So upon deploy the CD pipeline calls kubectl with the proper deployment image and that's ok?

Yes.

Drift, as the other child comment mentions, but also loss of version control. I do not want to have to trawl through someone's shell history to figure out what they changed, nor do I want to have to redirect `kubectl get foo -o yaml` output into diff.

If everything is in code, and you have a reasonable branching strategy, it's much easier to control change, to rollback bad merges, to run pre-hooks like security checks and configuration validation tools, etc.

You know kubectl has a built in diff subcommand?

I do now! Thank you.

It's not drift, because the infra-as-code doesn't manage the image field (the application's CD pipeline does). You don't trawl through someone's shell history', you look at the CD pipeline history. Rollbacks are easy--you just deploy the prior version via your CD tool.

I think you're assuming that invoking kubectl means invoking it directly from a user's command line, but kubectl can also be called in a CD script.

If you're using ArgoCD or something then sure, but bear in mind the original statement you made was directed at someone who is new to K8s, and given a command that can be executed from their shell, they would likely assume that's what you meant.

It doesn’t have to be Argo, it can be Jenkins. Whether or not you use a CD pipeline is orthogonal to whether or not you use k8s. The best practice is to use a pipeline whether you’re targeting k8s or bare VMs or a higher level PaaS abstraction.

I'm not new to Kubernetes and I've been using containers since Solaris zones were introduced.

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