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Application-Level Policies and Customized Control-Logic Workflow Design

Background

The current model consists of mainly Components and Traits. While this enables the Application object to plug-in operational capabilities, it is still not flexible enough. Specifically, it has the following limitations:

  • The current control logic could not be customized. Once the Vela controller renders final k8s resources, it simply applies them without any extension points. In some scenarios, users want to do more complex operations like:
    • Blue-green style upgrade of the app.
    • User interaction like manual approval/rollback.
    • Distributing workloads across multiple clusters.
    • Actions to enforce policies and audit.
    • Pushing final k8s resources to other config store (e.g. Git repos).
  • There is only per-component config, but no application-level policies. In some scenarios, users want to define policies like:
    • Security: RBAC rules, audit settings, secret backend types.
    • Insights: app delivery lead time, frequence, MTTR.

Here is an overview of the features we want to expose and the capabilities we want to plug in:

alt

Proposal

To resolve the aforementioned problems, we propose to add app-level policies and customizable workflow to the Application CRD:

kind: Application
spec:
  components: ...

  # Policies are rendered after components are rendered but before workflow are started
  policies:
    - type: security
      name: my-rule
      properties:
        rbac: enabled
        audit: enabled
        secretBackend: vault

    - type: deployment-insights
      name: my-deploy-insight
      properties:
        leadTime: enabled
        frequency: enabled
        mttr: enabled

  # workflow is used to customize the control logic.
  # If workflow is specified, Vela won't apply any resource, but provide rendered resources in a ConfigMap, referenced via AppRevision.
  # workflow steps are executed in array order, and each step:
  # - will have a context in annotation.
  # - should mark "finish" phase in status.conditions.
  workflow:

    steps:

    # blue-green rollout
    - type: blue-green-rollout
      stage: post-render # stage could be pre/post-render. Default is post-render.
      properties:
        partition: "50%"

    # suspend can manually stop the workflow and resume. it will also allow suspend policy for workflow.
    - type: suspend

    # traffic shift
    - type: traffic-shift
      properties:
        partition: "50%"

    # promote/rollback
    - type: rollout-promotion
      properties:
        manualApproval: true
        rollbackIfNotApproved: true

This also implicates we will add two Definition CRDs -- PolicyDefinition and WorkflowStepDefinition.

PolicyDefinition looks like below:

apiVersion: core.oam.dev/v1beta1
kind: PolicyDefinition
spec:
  schematic:
    cue:
      template: |
        parameters: {
          frequency: *"enabled" | "disabled"
        }
        output: {
          apiVersion: app.oam.dev/v1
          kind: Insight
          spec:
            frequency: parameters.frequency
        }

CUE-Based Workflow Task

Outputing a CR object to complete a task in workflow requires users to implement an Operator which incurs heavy overhead. To simplify it, especially for users with simple use cases, we decide to provide lightweight CUE based workflow task.

apiVersion: core.oam.dev/v1beta1
kind: WorkflowStepDefinition
metadata:
  name: apply
spec:
  schematic:
    cue:
      template: |
        import "vela/op"

        parameters: {
          image: string
        }

        apply: op.#Apply & {
          resource: context.workload
        }

        wait: op.#ConditionalWait & {
          continue: apply.status.ready == true
        }

        export: op.#Export & {
          secret: apply.status.secret
        }

Stability mechanism

Backoff Time

Sometimes a workflow step can take a long time, so we need a backoff time for workflow reconciliation.

If the status of workflow step is waiting or failed, the workflow will be reconciled after a backoff time like below:

int(0.05 * 2^(n-1))

Based on the above formula, we will take 1s as the min time and 60s as the max time. You can change the max time by setting MaxWorkflowWaitBackoffTime.

For example, if the workflow is waiting, the first ten reconciliation will be like:

Times 2^(n-1) 0.05*2^(n-1) Requeue After(s)
1 1 0.05 1
2 2 0.1 1
3 4 0.2 1
4 8 0.4 1
5 16 0.8 1
6 32 1.6 1
7 64 3.2 3
8 128 6.4 6
9 256 12.8 12
10 512 25.6 25
... ... ... ...

Failed Workflow Steps

If the workflow step is failed, it means that there may be some error in the workflow step, like some cue errors.

Note that if the workflow step is unhealthy, the workflow step will be marked as wait but not failed and it will wait for healthy.

For this case, we will retry the workflow step 10 times by default, and if the workflow step is still failed, we will terminate this workflow, and it's message will be The workflow terminates automatically because the failed times of steps have reached the limit. You can change the retry times by setting MaxWorkflowStepErrorRetryTimes.

Implementation

In this section we will discuss the implementation details for supporting policies and workflow tasks.

Here's a diagram of how workflow internals work:

alt

1. Application Controller

Here are the steps in Application Controller:

  • On reconciling an Application event, Application Controller will render out all resources from components, traits, policies. It will also put rendered resources into a ConfigMap, and reference the ConfigMap name in AppRevision as below:

    kind: ApplicationRevision
spec:
  ...
  resourcesConfigMap:
    name: my-app-v1
---

kind: ConfigMap
metadata:
  name: my-app-v1
data:
  mysvc: |
    {
      "apiVersion": "apps/v1",
      "kind": "Deployment",
      "metadata": {
          "name": "mysvc"
      },
      "spec": {
          "replicas": 1
      }
    }
  ...more name:data pairs...

  • After render, Application Controller will execute spec.workflow. This will basically call Workflow Manager to execute workflow tasks starting from scratch or last-run step on retry.

2. Workflow Manager

Here are the steps in Workflow Manager:

  • The Workflow Manager will get the current workflow step via status.workflow.stepIndex.
  • If stepIndex is equal to the length of the all steps, it indicates that workflow is all done and return immediately.
  • If there are workflow tasks left, they will be run step by step. For each step, Workflow Manager will call Task Manager to handle it.
  • On return from calling Task Manager, Workflow Manager checks the return result:
    • If status = completed, Workflow Manager will increment status.workflow.stepIndex, and continue to run next step if any.
    • Otherwise, it will retry later.

3. Task Manager

Here are the steps in Task Manager:

  • A workflow task will be executed synchronously which requires that the steps of a task should be non-blocking.

  • A workflow task will be parsed with its properties first to retrieve the full CUE data.

  • Task manager will get all do-able steps from the CUE data. This is done by analyzing if the step has a #do field. Here is an example:

    apply: op.#Apply & {
  resource: ...
}

    The op.#Apply contains a hidden field #do:

    #Apply: {
  #do: "apply"
  ...
}

    This will inject the #do field to the apply step.

  • All do-able steps will be executed one by one by Task Manager.

4. CUE Step Execution

  • Task Manager will keep a map of actions. An action follows this interface:

    type TaskAction interface {
  // cueValue is the parsed CUE value for this action
  Run(cueValue interface{}) (TaskStatus, error)
}

  • Task Manager will use the #do field of the CUE step as the key to find an action to run.

  • An action returns a status indicating what to do next:

    • continue: continue to run the next action.
    • wait: makes the workflow manager to retry later.
    • break: makes the workflow manager to stop the entire workflow.
    • failedAfterRetries: if there are no other running steps, makes the workflow manager to suspend the workflow.

  • Task Manager will change status as needed based on the returned TaskStatus, e.g. change to wait.

Task Action

These are the task actions to be supported in vela/op CUE lib:

  • Load: loads the rendered component resources

    #Load: {
  #do: "load"
  component?: string
}

  • KubeRead: reads a k8s resource object

    #Read: {
  #do: "read"
  apiVersion: string
  kind: string
  namespace: string
  name: string
}

  • Apply: applies a k8s resource object

    #Apply: {
  #do: "apply"
  resource: string
}

  • Wait: waits until the continue condition is ready, otherwise makes the controller to reconcile later.

    #Wait: {
  #do: "wait"
  continue: bool
}

  • Break: breaks from the workflow, and reports reasoning message.

    #Break: {
  #do: "break"
  message: string
}

  • Export: exports the data into context for other workflow tasks to reuse

    #Export: {
  #do: "export"
  type: "patch" | *"var"
  if type == "patch" {
    component: string
  }
  value: _
}

Workflow Operation

These are the operations that users can use to control the workflow at global level.

1. Terminate Workflow

If the execution of the workflow does not meet expectations, it may be necessary to terminate the workflow

There are two ways to achieve that:

  1. Modify the workflow.terminated field in status
  kind: Application
  metadata:
    name: foo
  status:
    phase: runningWorkflow
    workflow:
      stepIndex: 1
      terminated: true
      steps:
      - name: ...
  1. Use op.#Break in workflowStep definition. When the task is executed, the op.#Break can be captured and then report terminated status
if job.status == "failed"{
  break: op.#Break & {
      message: "job failed: "+ job.status.message
  }
}

2. Pause Workflow

  1. Modify the value of the workflow.suspend field to true to pause the workflow
kind: Application
metadata:
  name: foo
status:
  phase: runningWorkflow
  workflow:
    stepIndex: 1
    suspend: true
    steps:
    - name: ...
  1. The built-in suspend task support pause workflow, the example as follow
kind: Application
spec:
  components: ...
workflow:
  steps:
  - name: manual-approve
    type: suspend

The workflow.suspend field will be set to true after the suspend-type task is started

3. Resume Workflow

Modify the value of the workflow.suspend field to false to resume the workflow

kind: Application
metadata:
  name: foo
status:
  phase: runningWorkflow
  workflow:
    stepIndex: 1
    suspend: false
    steps:
    - name: ...

4. Restart Workflow

The workflow will be restarted in the following two cases:

  1. Modify the value of the status.phase field to "runningWorkflow" and clear the status of the workflow
kind: Application
metadata:
  name: foo
status:
  phase: runningWorkflow
  workflow: {}
  1. The application spec changes

The spec change also means that the application needs to be re-executed, and the application controller will clear the status of application includes workflow status.

Operator Best Practice

Each workflow task has similar interactions with Task Manager as follows:

  • The Task Manager will apply the workflow object with annotation app.oam.dev/workflow-context. This annotation will pass in the context marshalled in json defined as the following:

      type WorkflowContext struct {
  	cli        client.Client
  	store      *corev1.ConfigMap
  	components map[string]*ComponentManifest
  	vars       *value.Value
  	modified   bool
  }

  • The workflow object's status condition should turn to be True status and Succeeded reason, and observedGeneration to match the resource's generation per se. This is to solve the issue of passing data from the old generation. We will provide CUE op library to check this condition to decide whether to wait.

    kind: SomeTask
metadata:
  generation: 2
status:
  observedGeneration: 2
  conditions:
    - type: workflow-progress
      status: 'True'
      reason: 'Succeeded'

Use Cases

In this section we will walk through how we implement workflow solutions for the following use cases.

Case 1: Multi-cluster

In this case, users want to distribute workflow to multiple clusters. The dispatcher implementation is flexible and could be based on open-cluster-management or other methods.

workflow:
  steps:
  - type: open-cluster-management
    properties:
      placement:
        - clusterSelector:
            region: east
          replicas: "70%"
        - clusterSelector:
            region: west
          replicas: "20%"

The process goes as:

  • During infra setup, the Cluster objects are applied and agents are setup in each cluster to manage lifecycle of k8s clusters.
  • Once the Application is applied, the OCM controller can retrieve all rendered resources from AppRevision. It will apply a ManifestWork object including all resources. Then the OCM agent will execute the workload creation in each cluster.

Case 2: Blue-green rollout

In this case, users want to rollout a new version of the application components in a blue-green rolling upgrade style.

workflow:
  steps:
  # blue-green rollout
  - type: blue-green-rollout
    properties:
      partition: "50%"

  # traffic shift
  - type: traffic-shift
    properties:
      partition: "50%"

  # promote/rollback
  - type: rollout-promotion
    properties:
      manualApproval: true
      rollbackIfNotApproved: true

The process goes as:

  • By default, each modification of the Application object will generate an AppRevision object. The rollout controller will get the current revision from the context and retrieve the previous revision via kube API.

  • Then the rollout controller will do the operation to rollings replicas between two revisions (the actual behavior depends on the workload type, e.g. Deployment or CloneSet).

  • Once the rollover is done, the rollout controller can shift partial traffic to the new revision too.

  • The rollout controller will wait for the manual approval. In this case, it is in the status of Rollout object:

    kind: Rollout
status:
  pause: true # change this to false

    The reference to the rollout object will be in the Application object:

    apiVersion: core.oam.dev/v1beta1
kind: Application
status:
  workflow:
    steps:
    - type: rollout-promotion
      resourceRef:
        kind: Rollout
        name: ...

Case 3: Data Passing

In this case, users want to deploy a database component first, wait the database to be up and ready, and then deploy the application with database connection secret.

components:
  - name: my-db
    type: mysql
    properties:

  - name: my-app
    type: webservice


workflow:
  steps:
  # Wait for the MySQL object's status.connSecret to have value.
  - type: apply-component
    outputs:
      - name: connSecret
        valueFrom: output.status.connSecret
    properties:
      name: my-db

  # Patch my-app Deployment object's field with the secret name
  # emitted from MySQL object. And then apply my-app component.
  - type: apply-component
    inputs:
      - from: connSecret
        parameterKey: patch.valueFrom.field
    properties:
      name: my-app
      patch:
        to:
          field: spec.containers[0].envFrom[0].secretRef.name
        valueFrom:
          apiVersion: database.example.org/v1alpha1
          kind: MySQLInstance
          name: my-db

Case 4: GitOps rollout

In this case, users just want Vela to provide final k8s resources and push them to Git, and then integrate with ArgoCD/Flux to do final rollout. Users will setup a GitOps workflow like below:

workflow:
  steps:
  - type: gitops # This part configures how to push resources to Git repo
    properties:
      gitRepo: git-repo-url
      branch: branch
      credentials: ...

The process goes as:

  • Everytime an Application event is triggered, the GitOps workflow controller will push the rendered resources to a Git repo. This will trigger ArgoCD/Flux to do continuous deployment.

Case 5: Template-based rollout

In this case, a template for Application object has already been defined. Instead of writing the spec.components, users will reference the template and provide parameters/patch to it.

workflow:
  steps:
  - type: helm-template
    stage: pre-render
    properties:
      source: git-repo-url
      path: chart/folder/path
      parameters:
        image: my-image
        replicas: 3
---
workflow:
  steps:
  - type: kustomize-patch
    stage: pre-render
    properties:
      source: git-repo-url
      path: base/folder/path
      patch:
        spec:
          components:
            - name: instance
              properties:
                image: prod-image

The process goes as:

  • On creating the application, app controller will apply the HelmTemplate/KustomizePatch objects, and wait for its status.
  • The HelmTemplate/KustomizePatch controller would read the template from specified source, render the final config. It will compare the config with the Application object -- if there is difference, it will write back to the Application object per se.
  • The update of Application will trigger another event, the app controller will apply the HelmTemplate/KustomizePatch objects with new context. But this time, the HelmTemplate/KustomizePatch controller will find no diff after the rendering. So it will skip this time.

Case 6: Conditional Check

In this case, users want to execute different steps based on the responseCode. When the if condition is not met, the step will be skipped.

workflow:
  steps:
    - name: request
      type: webhook
    - name: handle-200
      type: deploy
      if: request.output.responseCode == 200
    - name: handle-400
      type: notification
      if: request.output.responseCode == 400
    - name: handle-500
      type: rollback
      if: request.output.responseCode == 500

If users want to execute one step no matter what, they can use if: always in the step. In this way, whether the workflow is successful or not, the step will be executed`.

workflow:
steps:
   - type: deploy
     name: deploy-app
   - name: notificationA
     if: always
     type: notification

Case 7: step group

In this case, the user runs multiple workflow steps in the step-group workflow type. subSteps in a step group will be executed in dag mode.

workflow:
  steps:
  - type: step-group
    name: run-step-group1
    subSteps: 
    - name: sub-step1
      type: ...
      ...
    - name: sub-step2
      type: ...
      ...

The process is as follows:

  • When executing a step-group step, the subSteps in the step group are executed in dag mode. A step group will only complete when all subSteps have been executed to completion.

Considerations

Comparison with Argo Workflow/Tekton

The workflow defined here are k8s resource based and very simple one direction workflow. It's mainly used to customize Vela control logic to do more complex deployment operations.

While Argo Workflow/Tekton shares similar idea to provide workflow functionalities, they are container based and provide more complex features like parameters sharing (using volumes and sidecars). More importantly, these projects couldn't satisfy our needs. Otherwise we can just use them in our implementation.