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docs/proposals/deletion-strategy-progressive-sync.md

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Deletion Strategy for Progressive Sync

This proposal is building upon the ideas presented in https://github.com/argoproj/argo-cd/pull/14892 to introduce deletion strategy for progressive sync. While the original proposal laid the groundwork, this proposal extends to address some unanswered sections and changes implementation details.

Introduce a new functionality of ArgoCD ProgressiveSync that will allow users to configure order of deletion for applicationSet's deployed applications. The deletion strategies can be:

  • AllAtOnce (current behaviour - where all applications are deleted in no particular order without waiting for an application to be deleted; can be the default value)
  • reverse ( delete applications in the reverse order of deployment, configured in progressiveSync. This expects the rollingSync field to have a specified order and implements deletion in the reverse order specified. Waits for one application to be fully deleted before moving onto the next application.)

Open Questions [optional]

The original proposal mentions another strategy - custom wherein the user can provide a specific order of deletion. Is such a usecase needed?

Summary

This feature can extend the application dependency from deployment to deletion as well. Ability to provide deletion order can complete the ProgressiveSync feature.

Motivation

Current deletion/removal strategy which ArgoCD use works fine if there aren't any dependencies between the different applications. However, it does not work when there are dependencies between the applications. This was noticed when some kubernetes core services were deployed in specific order and to be removed in reverse order.

Goals

Following goals should be achieved in order to conclude this proposal:

  1. Deletion strategy AllAtOnce as default value - deletes all applications at once as the current behavior of deletion.
  2. Deletion strategy Reverse lets applications be deleted in the reverse order of the steps configured in RollingSync strategy.

Non-Goals

custom deletion strategy - this will be a separate goal if there is enough demand for it.

Proposal

Ability to provide configuration related to the deletion/removal process when progressive sync is used. Implementation detail provides two options of introducing this field in ApplicationSet. The following use cases assumes Option 1 for the yaml file examples.

Use cases

Add a list of detailed use cases this enhancement intends to take care of.

AllAtOnce deletionStrategy:

yaml
apiVersion: argoproj.io/v1alpha1
kind: ApplicationSet
metadata:
  name: pricelist
  namespace: argocd
spec:
  generators:
  - list:
      elements:
      - srv: config
        path: applicationsets/rollingsync/apps/pricelist-config
      - srv: db
        path: applicationsets/rollingsync/apps/pricelist-db
      - srv: frontend
        path: applicationsets/rollingsync/apps/pricelist-frontend
  strategy:
    type: RollingSync
    rollingSync:
      steps:
        - matchExpressions:
            - key: pricelist-component # the "key" is based on the label (below as "pricelist-component: {{srv}}")
              operator: In
              values:
                - config
        - matchExpressions:
            - key: pricelist-component
              operator: In
              values:
                - db
        - matchExpressions:
            - key: pricelist-component
              operator: In
              values:
                - frontend
  ### Deletion configuration ###
    deletionOrder: AllAtOnce  # available options to be AllAtOnce/Reverse (maybe custom as well)
  ### Deletion configuration ###
  template:
    metadata:
      name: 'pricelist-{{srv}}'
      labels:
        pricelist-component: '{{srv}}'
    spec:
      project: default
      syncPolicy:
        automated:
          prune: true
          selfHeal: true
        retry:
          limit: 5
          backoff:
            duration: 5s
            maxDuration: 3m0s
            factor: 2
      source:
        repoURL: https://github.com/christianh814/gitops-examples
        targetRevision: main
        path: '{{path}}'
      destination:
        server: https://kubernetes.default.svc
        namespace: pricelist

Reverse deletionStrategy:

yaml
apiVersion: argoproj.io/v1alpha1
kind: ApplicationSet
metadata:
  name: pricelist
  namespace: argocd
spec:
  generators:
  - list:
      elements:
      - srv: config
        path: applicationsets/rollingsync/apps/pricelist-config
      - srv: db
        path: applicationsets/rollingsync/apps/pricelist-db
      - srv: frontend
        path: applicationsets/rollingsync/apps/pricelist-frontend
  strategy:
    type: RollingSync
    rollingSync:
      steps:
        - matchExpressions:
            - key: pricelist-component # the "key" is based on the label (below as "pricelist-component: {{srv}}")
              operator: In
              values:
                - config
        - matchExpressions:
            - key: pricelist-component
              operator: In
              values:
                - db
        - matchExpressions:
            - key: pricelist-component
              operator: In
              values:
                - frontend
    ### Deletion configuration ###
    deletionOrder: Reverse  # available options to be AllAtOnce/Reverse (maybe custom as well)
    ### Deletion configuration ###        
  template:
    metadata:
      name: 'pricelist-{{srv}}'
      labels:
        pricelist-component: '{{srv}}'
    spec:
      project: default
      syncPolicy:
        automated:
          prune: true
          selfHeal: true
        retry:
          limit: 5
          backoff:
            duration: 5s
            maxDuration: 3m0s
            factor: 2
      source:
        repoURL: https://github.com/christianh814/gitops-examples
        targetRevision: main
        path: '{{path}}'
      destination:
        server: https://kubernetes.default.svc
        namespace: pricelist

If custom deletionStrategy:

yaml
apiVersion: argoproj.io/v1alpha1
kind: ApplicationSet
metadata:
  name: pricelist
  namespace: argocd
spec:
  generators:
  - list:
      elements:
      - srv: config
        path: applicationsets/rollingsync/apps/pricelist-config
      - srv: db
        path: applicationsets/rollingsync/apps/pricelist-db
      - srv: frontend
        path: applicationsets/rollingsync/apps/pricelist-frontend
  strategy:
    type: RollingSync
    rollingSync:
      steps:
        - matchExpressions:
            - key: pricelist-component # the "key" is based on the label (below as "pricelist-component: {{srv}}")
              operator: In
              values:
                - config
        - matchExpressions:
            - key: pricelist-component
              operator: In
              values:
                - db
        - matchExpressions:
            - key: pricelist-component
              operator: In
              values:
                - frontend
    ### Deletion configuration ###
    deletionOrder: Custom # available options to be default/reverse/custom
    deletionSync:
      steps:
        - matchExpressions:
            - key: pricelist-component # the "key" is based on the label (below as "pricelist-component: {{srv}}")
              operator: In
              values:
                - config
        - matchExpressions:
            - key: pricelist-component
              operator: In
              values:
                - frontend
        - matchExpressions:
                - key: pricelist-component
                  operator: In
                  values:
                    - db
    ### Deletion configuration ###
  template:
    metadata:
      name: 'pricelist-{{srv}}'
      labels:
        pricelist-component: '{{srv}}'
    spec:
      project: default
      syncPolicy:
        automated:
          prune: true
          selfHeal: true
        retry:
          limit: 5
          backoff:
            duration: 5s
            maxDuration: 3m0s
            factor: 2
      source:
        repoURL: https://github.com/christianh814/gitops-examples
        targetRevision: main
        path: '{{path}}'
      destination:
        server: https://kubernetes.default.svc
        namespace: pricelist

Implementation Details/Notes/Constraints [optional]

There should be a check that correlates the deletionStrategy to ApplicationSet strategy. For example can only select reverse if rollingSync lists out an order of application deployment, otherwise should error out.

It was decided to have this field within strategy (which is a field associated with progressiveSync)

To be introduced in ApplicationSetStrategy as follows:

yaml
type ApplicationSetStrategy struct {
Type        string                         `json:"type,omitempty" protobuf:"bytes,1,opt,name=type"`
RollingSync *ApplicationSetRolloutStrategy `json:"rollingSync,omitempty" protobuf:"bytes,2,opt,name=rollingSync"`
// RollingUpdate *ApplicationSetRolloutStrategy `json:"rollingUpdate,omitempty" protobuf:"bytes,3,opt,name=rollingUpdate"`
// Add DeletionSync Strategy here
DeletionOrder string `json:"deletionOrder,omitempty" protobuf:"bytes,4,opt,name=deletionOrder"` // takes value AllAtOnce/Reverse
}

Looked at the following names for this field:

  1. DeletionSyncType
  2. DeletionSyncStrategy

But decided on having DeletionOrder for the following reasons:

  1. simpler to understand - Order is straightforward and thus sets expectation to user
  2. Since it's nested within strategy, suffix of strategy isn't needed.
  3. Leaving room for when/if it scales to have custom deletion strategy. i.e
yaml
type ApplicationSetStrategy struct {
Type        string                         `json:"type,omitempty" protobuf:"bytes,1,opt,name=type"`
RollingSync *ApplicationSetRolloutStrategy `json:"rollingSync,omitempty" protobuf:"bytes,2,opt,name=rollingSync"`
// Add DeletionSync Strategy here
DeletionOrder string `json:"deletionOrder,omitempty" protobuf:"bytes,3,opt,name=deletionOrder"` // takes value AllAtOnce/Reverse/Custom
DeletionSync *ApplicationSetRolloutStrategy `json:"deletionSync,omitempty" protobuf:"bytes,4,opt,name=deletionSync"`
}

Detailed examples

Already covered in Use cases

Security Considerations

Since no additional roles or privileges are needed to be able to delete deployed applications in a specific order, so no impact on the security aspects of Argo CD workloads.

Risks and Mitigations

No immediate Risks to consider

Upgrade / Downgrade Strategy

Introducing new fields to the ApplicationSet CRD, however, no existing fields are being changed. This means that a new ApplicationSet version is unnecessary, and upgrading to the new spec with added fields will be a clean operation.

Downgrading could risk users receiving K8s API errors if they continue to try to apply the deletionStrategy field to a downgraded version of the ApplicationSet resource. Downgrading the controller while keeping the upgraded version of the CRD should cleanly downgrade/revert the behavior of the controller to the previous version without requiring users to adjust their existing ApplicationSet specs.

Drawbacks

Slight increase in Argo CD code base complexity

Alternatives

TBD