KEP-2839: Add KEP for in-use protection

[mkimuram]

• One-line PR description: A generic feature to protect objects from deletion while it is in use

• Issue link: In-use protection #2839

• Other comments:

[bgrant0607]

Cross-linking an old issue: kubernetes/kubernetes#10179

[mkimuram]

@lavalamp @bgrant0607

Sharing just a rough idea.

Just for preventing deletion, the below design will be enough (We still need to consider how to protect deletion of LienClaim itself). Also, update may be implemented in the same way, like adding "spec-update" to LienClaim.spec.prevents and appending uuid to *.metadata.spec-update-liens. However, it would be difficult to define preventing update for specific fields as we can specify through "field manager", by this way.

1. User creates LienClaim in their namespace with specifying targets and prevents.

apiVersion: v1beta1
kind: LienClaim
metadata:
  namespace: ns1
  name: my-lien
spec:
  targets:
  - apiVersion: v1
    kind: secret
    namespace: ns2
    name: important-secret
  prevents:
  - delete

2. Lien controller watches LienClaim and create cluster-scoped Lien and append its name to *-liens, or delete-liens in the below example, to the target if a new LienClaim is created. Note that a name for Lien is generated as UUID.

apiVersion: v1beta1
kind: Lien
metadata:
  name: 12345678-1234-1234-1234-1234567890ab
spec:
  targets:
  - apiVersion: v1
    kind: secret
    namespace: ns2
    name: important-secret
  source:
    namespace: ns1
    name: my-lien
  prevents:
  - delete

apiVersion: v1
kind: Secret
metadata:
  namespace: ns2
  name: important-secret
  delete-liens:
  - 12345678-1234-1234-1234-1234567890ab
type: Opaque
data:

3. Lien controller may also update LienClaim status if 2 succeeds

apiVersion: v1beta1
kind: LienClaim
metadata:
  namespace: ns1
  name: my-lien
spec:
  targets:
  - apiVersion: v1
    kind: secret
    namespace: ns2
    name: important-secret
  prevents:
  - delete
status:
  lien: 12345678-1234-1234-1234-1234567890ab
  phase: preventing

4. On deletion of resources, lien admission controller checks if metadata.delete-liens is empty. If not, return error to block deletion.

5. Lien controller watches LienClaim and delete the lien from *-liens on the target and delete Lien if the LienClaim is deleted.

[mkimuram]

From the viewpoint of consuming lien from secret protection, which is my biggest concern. How to create LienClaim before creating referencing objects and delete LienClaim after deleting referencing objects is need to be considered.
(I'm not sure if mutation hook or ownerReference can achieve it. Although, we can at least leave it for users.)

[lavalamp]

Why do you want to have lien objects? Why are text strings not sufficient (as used for finalizers). Adding additional object types adds more places to have races, I'm against it unless you can convince me it's absolutely necessary.

[mkimuram]

Why do you want to have lien objects?

It's to make it easier to manage who has control over the specific lien in the delete-liens field.

If multiple liens are set to a single resource, we will need to ensure that the specific lien is managed by the client/controller when deleting the lien. We may add the LienClaim's identity information to delete-liens but it will tend to be long and difficult to much.

Instead, we might just add LienClaim's uid. However, just adding uid, it is difficult for the owner of the resource to find from the uid. Also, I'm not sure if object uid can be regarded as unique. Creating an object with the name should ensure it.

Adding additional object types adds more places to have races, I'm against it unless you can convince me it's absolutely necessary.

Agree. If above concern can be solved by the other way, I agree to remove cluster-scoped Lien object.

[lavalamp]

You want to ACL liens? I don't see a need to enforce this any more than we do for finalizers. And I would not solve that problem by adding Lien or LienClaim objects. I don't think a solution here should require any additional object types.

[mkimuram]

If only talking about the use case for secret protection, the feature needed is like block deleting "Secret A" that can be used by "Pod B" and "PersistentVolume C" while the secret is used. So, something like below will be enough.

apiVersion: v1
kind: Secret
metadata:
  namespace: ns2
  name: A
  delete-liens: 
    - "ID to show that Pod B is using"
    - "ID to show that PersistentVolume C is using"
type: Opaque
data:

And to generalize it, I was thinking about a way to manage such ID for each reason to avoid conflict in deleting liens by lien system's side. However, we can leave it to consumers.

[mkimuram]

And also thinking about using a lien per controller, not per reason, like below.

apiVersion: v1
kind: Secret
metadata:
  namespace: ns2
  name: A
  delete-liens: 
    - "k8s.io/secret-protection-controller"
type: Opaque
data:

Then, I start to think about which will be easier "implementing own admission hook to block deletion" or "implementing controller to add lien"?

(Obviously, for users who would like to add such protection manually, lien systems is useful as this shows, but for controllers, it might not.)

[lavalamp]

There is no need for a lien controller. We would add code to kube-apiserver (reject deletion if there are liens, don't permit new liens if deletion timestamp is set).

We might want slightly more structure than raw text.

You can prototype today with e.g. specially formatted annotations and a webhook admission controller.

[lavalamp]

Sorry, I misread :) A lien per controller or per reason doesn't matter from the perspective of the lien mechanism, it's up to controller authors how they want to use the mechanism.

[mkimuram]

@lavalamp

You can prototype today with e.g. specially formatted annotations and a webhook admission controller.

I've implemented a prototype of lien as this.
It is separated to 4 commits, but only 2 commits, the first commit and the last commit, will be the points of interest.

I will update the KEP based on this prototype.

Note that I've tested below way and work as shown below:

• Deploy:

ENABLE_ADMISSION_PLUGINS=Lien hack/local-up-cluster.sh 

• Test:

1. Without liens, deletion is not blocked

kubectl create secret generic test-secret --from-literal='username=my-app' --from-literal='password=39528$vdg7Jb'
kubectl get secret test-secret -o jsonpath='{.metadata.liens}{"\n"}'

kubectl delete secret test-secret
secret "test-secret" deleted

2. With liens, deletion is blocked, and once all liens are removed, deletion is not blocked

kubectl create secret generic test-secret --from-literal='username=my-app' --from-literal='password=39528$vdg7Jb'
kubectl patch secret test-secret -p '{"metadata":{"liens":["never delete me"]}}' --type=merge
secret/test-secret patched

kubectl get secret test-secret -o jsonpath='{.metadata.liens}{"\n"}'
[never delete me]

kubectl delete secret test-secret
Error from server (Forbidden): secrets "test-secret" is forbidden: deletion not allowed by liens

kubectl patch secret test-secret -p '{"metadata":{"liens":[]}}' --type=merge
secret/test-secret patched

kubectl delete secret test-secret
secret "test-secret" deleted

[mkimuram]

@lavalamp

I've updated the KEP. PTAL

Also, I will upate the prototype of secret-protection to rely on this in-use protection mechanism to check the feasibility and also update the KEP for secret-protection.

[lavalamp]

I would recommend removing this whole section, honestly. I think it's a separate discussion for each controller and I don't think this KEP should recommend a general solution; I'm not convinced there's a problem at all.

[mkimuram]

Actually, I would like to confirm that this situation is allowed by design from community. If it is allowed, I agree to delete such a mechanism to avoid it completely.

[sftim]

Here's how a docs framing can help work out a viable way forward: write (or sketch) two things.

1. The documentation page for liens, with security advice and including any relevant cautionary messages for cluster operators and controller authors.
2. The blog announcement for when these reach on-by-default (assume there wasn't any previous detailed blog article), again including any message of caution.

Sometimes - rarely - I've tried documenting a thing, the required words of caution have seemed faintly ridiculous, and the implementers have felt the same on seeing a draft. There's a redesign or something to avoid the worst of the snags.
I'm proposing these docs / blog article sketches as a way to get the same feedback earlier.

[thockin]

@mkimuram is this something you want to push for in the current cycle?

[negz]

This puts the onus on clients to keep retrying if they want something to be deleted, right?

I prefer the declarative, eventually consistent nature of the current PVC protection implementation where a client can declare that it wants a thing to be deleted, and once it's safe to delete the thing, it is. We've built similar machinery to this in Crossplane and I believe it works pretty well.

For our use case "eventually consistent deletes" has nice symmetry with other operations. Most Crossplane resources correspond to some external system like a cloud provider. Let's say I want to create a GCP Subnet with Crossplane, but doing so requires the VPC it lives in to exist first. We don't reject the creation of the Subnet object in the API server because the VPC doesn't exist. Instead we accept the desired state (that a Subnet should exist) and our controller keeps trying to create it. Presumably it will eventually succeed when when the VPC exists.

Is there some way to address the requirement that deletes "don't start" (in the controller-reconciling-the-delete sense) without out-right rejecting the desire to delete a resource by an entity that has permission to do so?

[thockin]

where a client can declare that it wants a thing to be deleted, and once it's safe to delete the thing, it is

This is a very different semantic than "I want to delete this" -> "no". The only way I know to achieve what you describe is with a finalizer, but this is not the right semantic. At that point, the object in question is VISIBLY being deleted and async controllers are doing their own cleanup. This is way to signal "begin the end-of-life process", not a way to delay it it.

Is there a different mechanism you are thinking of?

[negz]

This is a very different semantic than "I want to delete this" -> "no".

Right. This KEP appears to propose that all forms of "in-use protection" use "I want to delete this" -> "no" semantics. Those semantics appear to be different from how the contemporary PV and PVC implementations of in-use protection work, which are more like "I want to delete this" -> "okay, eventually". For Crossplane's use-case, I prefer the semantics of the contemporary implementations to what is proposed by this KEP.

My understanding is that PV and PVC in-use protection are more like "I want to delete this" -> "okay, eventually, when not in-use". The deleted objects transition into a Terminating state and go away once they're no longer in-use. We have the same "okay, eventually" semantics for Crossplane's ProviderConfig in-use protection, and are looking to generalize the pattern.

Is there a different mechanism you are thinking of?

I'm not sure what you're asking. Can you elaborate?

To be clear - I think I understand the issues with finalizers and agree that they're not a good solution. I also don't have a good solution in mind. 🙂 What I would like is that somehow:

• If a client asks for an object be deleted
• The object can't be deleted yet because doing so would break some other, dependent object
• The delete (i.e. desired state) be accepted and reconciled when possible

I don't think my preferred semantics necessarily make sense for all forms of deletion protection. The "Motivation" section of this KEP states that it's attempting to solve for two use cases:

1. Guaranteeing proper order of deletion (i.e. the PV and PVC use case, and Crossplane's)
2. Avoiding accidental deletion of important resources

I feel that "I want to delete this" -> "no" semantics makes sense for the second use case, but not the first.

[fabiand]

In KubeVirt we have a use-case with same or similar requirements:

1. We are running our processes in the original pod
2. There is an eviction request for this pod
3. Upon this request we want to transfer some state of the processes to a new pod
4. Upon the completed state transfer, the original pod is now free to be terminated

Important aspects are
a. Application can observe eviction request
b. Pod stays running until the state is transfered
c. caller of eviction is aware that the pod is preparing a shut-down

finalizers do not work, because the pod will be stuck in terminating state, even if the state transfer fails, and we need ot keep the original pod
pdbs protect the pods, but have no way to tell the caller that the eviction was observed and application is startng the process (we use this today, but it's leading to trouble, i.e. with descheduler).

[sftim]

a. Application can observe eviction request
b. Pod stays running until the state is transfered

That sounds like grounds for a separate enhancement. Right now evictions are, AIUI, either accepted immediately or denied.

[thockin]

@fabiand We could model this a few ways, but evictions are generally "urgent", right? It's not clear to me how that should intersect with any sort of deletion delay.

[negz]

@thockin bringing your related comment back into this thread: 😄

To do what you want, we need a new lifecycle which includes some indefinite pre-delete state.

That may be the case, but what we want does seem in-scope for the goals defined by this design - i.e. guaranteeing proper order of deletion.

Take this manifest:

pod-with-volume.yaml

---
apiVersion: v1
kind: Pod
metadata:
  name: task-pv-pod
spec:
  volumes:
    - name: task-pv-storage
      persistentVolumeClaim:
        claimName: task-pv-claim
  containers:
    - name: task-pv-container
      image: nginx
      ports:
        - containerPort: 80
          name: "http-server"
      volumeMounts:
        - mountPath: "/usr/share/nginx/html"
          name: task-pv-storage
---
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: task-pv-claim
spec:
  storageClassName: manual
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 3Gi
---
apiVersion: v1
kind: PersistentVolume
metadata:
  name: task-pv-volume
  labels:
    type: local
spec:
  storageClassName: manual
  capacity:
    storage: 10Gi
  accessModes:
    - ReadWriteOnce
  hostPath:
    path: "/mnt/data"

Today I can kubectl apply -f pod-with-volume.yaml and despite there being dependencies between the resources, the create just works and the controllers eventually arrive at my desired state - that they're created. I can kubectl delete -f pod-with-volume.yaml too. My desired state (that all this stuff be deleted) is immediately accepted, and eventually it's all gone thanks to (ab)use of finalizers.

I believe under this proposal the kubectl delete -f pod-with-volume.yaml UX would change. The Pod would delete immediately but the PersistentVolume and PersistentVolumeClaim would fail with a 409 Conflict, and I'd basically have to retry a couple of times for everything to actually get deleted.

This does technically achieve the goal of guaranteeing proper order of deletion - it just seems like a pretty poor UX for doing so.

[fabiand]

@thockin I was not ware that evictions had the notion of urgency.
Evictions respect PDBs, evictions are less hard than DELETE. Thus isn't eviction rather the less urgent approach and delete the real urgent way of getting rid of workloads?
fwiw kubectl drain --force --grace-period=0 is urgent.

Thus to me a delay to an eviction is similar to systemd inhibitors - a way to allow workloads to shut down gracefully. or even fail. And that's the difference: Fail gracefully, give admin/owner time to eeact to this. Before an admin jumps in with --force --grace-period=0

and to me this relates to kubernetes/kubernetes#66811 - making eviction declarative.

[thockin]

Is there a different mechanism you are thinking of?

I'm not sure what you're asking. Can you elaborate?

I meant to ask if there is some other mechanism you know of that I don't :) It's a big project nowadays.

To do what you want, we need a new lifecycle which includes some indefinite pre-delete state.

[fabiand]

I've came across https://github.com/adobe/k8s-shredder#how-it-works

K8s-shredder will periodically run eviction loops, based on configured EvictionLoopInterval, trying to clean up all the pods from the parked nodes. Once all the pods are cleaned up, cluster-autoscaler should chime in and recycle the parked node.

To me this connects with the desire to make applications aware of interruption requests.
Without further effort, PDBs will reject evictions, apps don't know about it.

If we had an i.e. taint based draining, apps could watch this life-cycling - terminate themselfes properly, and free the node eventually.

k8s-shredder would not need to do (imperative) eviction loops, instead: Taint (declare) a node to be drained, watch for node to be free.

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[turkenh]

We have implemented a new API named Usage in Crossplane which is following a similar approach as proposed here, e.g. block deletion of objects in use with an admission webhook by returning a 409.

This works fine in most cases, except the degraded experience due to huge delays caused by the backoff in Kubernetes Garbage collector. We discussed some possible solutions during the design but couldn't find any elegant solution. Basically, we need a way to reset garbage collectors backoff on a given resources. Is there were a way to achieve this? Just like most controllers, if making a change on a given resource (e.g. add an annotation/label) could requeue the resource immediately to the garbage collector work queue, we could just do that. Apparently this is not possible.

Is there a way to reset garbage collectors backoff period by patching the resource somehow? Otherwise, the proposal here is in subject to the same problem I believe.

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[thockin]

I repeat myself: #2840 (comment)

@deads2k any softening of position?

[sftim]

You know what, I'd like to repeat #2840 (comment) as well. I still recommend: write viable docs for the thing, then build it if we like what we see.

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