Definition Protocol

Definition Protocol

KubeVela is fully programmable via CUE, while it leverage Kubernetes as control plane and align with the API in yaml.

You can manage the definition in CUE and the vela def command will render it into Kubernetes API with the following protocol.

Overview

Essentially, a definition object in KubeVela is a programmable building block. A definition object normally includes several information to model a certain platform capability that would used in further application deployment:

Capability Indicator
- ComponentDefinition uses spec.workload to indicate the workload type of this component.
- TraitDefinition uses spec.definition to indicate the provider of this trait.
- These indicators can be auto detected, so they’re not necessary.
Interoperability Fields
- they are for the platform to ensure a trait can work with given workload type. Hence only TraitDefinition has these fields.
Capability Encapsulation and Abstraction defined by spec.schematic
- this defines the templating and parametering (i.e. encapsulation) of this capability.

Hence, the basic structure of definition object is as below:

apiVersion: core.oam.dev/v1beta1
kind: XxxDefinition
metadata:
  name: <definition name>
spec:
  ...
  schematic:
    cue:
      # cue template ...
    helm:
      # Helm chart ...
  # ... interoperability fields

Let’s explain them one by one.

ComponentDefinition

The design goal of ComponentDefinition is to allow platform administrators to encapsulate any type of deployable products into “components” to be delivered. Once defined, this type of component can be referenced, instantiated and delivered by users in the Application.

Common component types include Helm Chart, Kustomize directory, a set of Kubernetes YAML files, container images, cloud resource IaC files, or CUE configuration file modules, etc. The component supplier corresponds to the real-world role, which is generally a third-party software distributor (ISV), a DevOps team engineer, or a code package and image generated by the CI system you built.

ComponentDefinition can be shared and reused. For example, a Helm chart, a CUE module, or a Terraform module. Another example is, for an Alibaba Cloud RDS component type, end users can select the same Alibaba Cloud RDS component type in different applications and instantiate them into cloud database instances with different specifications and different parameter configurations.

Let’s take a look at the frame format of ComponentDefinition:

apiVersion: core.oam.dev/v1beta1
kind: ComponentDefinition
metadata:
  name: <ComponentDefinition name>
  annotations:
    definition.oam.dev/description: <Function description>
spec:
  workload: # Workload Capability Indicator
    definition:
      apiVersion: <Kubernetes Workload resource group>
      kind: <Kubernetes Workload types>
  schematic:  # Component description
    cue: # Details of components defined by CUE language
      template: <CUE format template>

The indicator of workload type is declared as spec.workload.

Below is a definition for Web Service in KubeVela:

apiVersion: core.oam.dev/v1beta1
kind: ComponentDefinition
metadata:
  name: webservice
  namespace: default
  annotations:
    definition.oam.dev/description: "Describes long-running, scalable, containerized services that have a stable network endpoint to receive external network traffic from customers."
spec:
  workload:
    definition:
      apiVersion: apps/v1
      kind: Deployment
    ...

In above example, it claims to leverage Kubernetes Deployment (apiVersion: apps/v1, kind: Deployment) as the workload type for component.

The programmable template of given capability are defined in spec.schematic field. For example, below is the full definition of Helm type in KubeVela:

Details

apiVersion: core.oam.dev/v1beta1
kind: ComponentDefinition
metadata:
  name: helm
  namespace: vela-system
  annotations:
    definition.oam.dev/description: "helm release is a group of K8s resources from either git repository or helm repo"
spec:
  workload:
    type: autodetects.core.oam.dev
  schematic:
    cue:
      template: |
        output: {
          apiVersion: "source.toolkit.fluxcd.io/v1beta1"
          metadata: {
            name: context.name
          }
          if parameter.repoType == "git" {
            kind: "GitRepository"
            spec: {
              url: parameter.repoUrl
              ref:
                branch: parameter.branch
              interval: parameter.pullInterval
            }
          }
          if parameter.repoType == "helm" {
            kind: "HelmRepository"
            spec: {
              interval: parameter.pullInterval
              url:      parameter.repoUrl
              if parameter.secretRef != _|_ {
                secretRef: {
                  name: parameter.secretRef
                }
              }
            }
          }
        }
        outputs: release: {
          apiVersion: "helm.toolkit.fluxcd.io/v2beta1"
          kind:       "HelmRelease"
          metadata: {
            name: context.name
          }
          spec: {
            interval: parameter.pullInterval
            chart: {
              spec: {
                chart:   parameter.chart
                version: parameter.version
                sourceRef: {
                  if parameter.repoType == "git" {
                    kind: "GitRepository"
                  }
                  if parameter.repoType == "helm" {
                    kind: "HelmRepository"
                  }
                  name:      context.name
                  namespace: context.namespace
                }
                interval: parameter.pullInterval
              }
            }
            if parameter.targetNamespace != _|_ {
              targetNamespace: parameter.targetNamespace
            }
            if parameter.values != _|_ {
              values: parameter.values
            }
          }
        }
        parameter: {
          repoType: "git" | "helm"
          // +usage=The Git or Helm repository URL, accept HTTP/S or SSH address as git url.
          repoUrl: string
          // +usage=The interval at which to check for repository and relese updates.
          pullInterval: *"5m" | string
          // +usage=1.The relative path to helm chart for git source. 2. chart name for helm resource
          chart: string
          // +usage=Chart version
          version?: string
          // +usage=The Git reference to checkout and monitor for changes, defaults to master branch.
          branch: *"master" | string
          // +usage=The name of the secret containing authentication credentials for the Helm repository.
          secretRef?: string
          // +usage=The namespace for helm chart
          targetNamespace?: string
          // +usage=Chart version
          value?: #nestedmap
        }
        #nestedmap: {
          ...
        }

The specification of schematic is explained in the CUE with KubeVela documentations.

TraitDefinition

TraitDefinition provides a series of DevOps actions for the component that can be bound on demand. These operation and maintenance actions are usually provided by the platform administrator, such as adding a load balancing strategy, routing strategy, or performing scaler, gray release strategy, etc.

The format and field functions of the TraitDefinition are as follows:

apiVersion: core.oam.dev/v1beta1
kind: TraitDefinition
metadata:
  name: <TraitDefinition name>
  annotations:
    definition.oam.dev/description: <function description>
spec:
  definition:
    apiVersion: <corresponding Kubernetes resource group>
    kind: <corresponding Kubernetes resource type>
  workloadRefPath: <The path to the reference field of the Workload object in the Trait>
  podDisruptive: <whether the parameter update of Trait cause the underlying resource (pod) to restart>
  manageWorkload: <Whether the workload is managed by this Trait>
  skipRevisionAffect: <Whether this Trait is not included in the calculation of version changes>
  appliesToWorkloads:
  - <Workload that TraitDefinition can adapt to>
  conflictsWith:
  - <other Traits that conflict with this><>
  revisionEnabled: <whether Trait is aware of changes in component version>
  controlPlaneOnly: <Whether resources generated by trait are dispatched to the hubcluster (local)>
  schematic:  # Abstract
    cue: # There are many abstracts
      template: <CUE format template>

The interoperability fields are trait only. Let’s explain them in detail.

`.spec.appliesToWorkloads`

This field defines the constraints that what kinds of workloads this trait is allowed to apply to.

It accepts an array of string as value.
Each item in the array refers to one or a group of workload types to which this trait is allowed to apply.

There are three approaches to denote one or a group of workload types.

ComponentDefinition definition reference (CRD name), e.g., deployments.apps
Resource group of ComponentDefinition definition reference prefixed with *., e.g., *.apps, *.oam.dev. This means the trait is allowed to apply to any workloads in this group.
* means this trait is allowed to apply to any workloads

If this field is omitted, it means this trait is allowed to apply to any workload types.

For now, this field is not checked when applying application. Strict checking is planed to be implemented in the KubeVela 1.6.

`.spec.conflictsWith`

This field defines that constraints that what kinds of traits are conflicting with this trait, if they are applied to the same workload.

It accepts an array of string as value.
Each item in the array refers to one or a group of traits.

There are four approaches to denote one or a group of workload types.

TraitDefinition name, e.g., ingress
Resource group of TraitDefinition definition reference prefixed with *., e.g., *.networking.k8s.io. This means the trait is conflicting with any traits in this group.
* means this trait is conflicting with any other trait.

If this field is omitted, it means this trait is NOT conflicting with any traits.

`.spec.workloadRefPath`

This field defines the field path of the trait which is used to store the reference of the workload to which the trait is applied.

It accepts a string as value, e.g., spec.workloadRef.

If this field is set, KubeVela core will automatically fill the workload reference into target field of the trait. Then the trait controller can get the workload reference from the trait latter. So this field usually accompanies with the traits whose controllers relying on the workload reference at runtime.

Please check scaler trait as a demonstration of how to set this field.

`.spec.podDisruptive`

This field defines that adding/updating the trait will disruptive the pod or not. In this example, the answer is not, so the field is false, it will not affect the pod when the trait is added or updated. If the field is true, then it will cause the pod to disruptive and restart when the trait is added or updated. By default, the value is false which means this trait will not affect. Please take care of this field, it’s really important and useful for serious large scale production usage scenarios.

Let’s look at a practical example:

apiVersion: core.oam.dev/v1beta1
kind: TraitDefinition
metadata:
  annotations:
    definition.oam.dev/description: "configure k8s Horizontal Pod Autoscaler for Component which using Deployment as worklaod"
  name: hpa
spec:
  appliesToWorkloads:
    - deployments.apps
  workloadRefPath: spec.scaleTargetRef
  schematic:
    cue:
      template: |
        outputs: hpa: {
          apiVersion: "autoscaling/v2beta2"
          kind:       "HorizontalPodAutoscaler"
          spec: {
            minReplicas: parameter.min
            maxReplicas: parameter.max
            metrics: [{
              type: "Resource"
              resource: {
                name: "cpu"
                target: {
                  type:               "Utilization"
                  averageUtilization: parameter.cpuUtil
                }
              }
            }]
          }
        }
        parameter: {
          min:     *1 | int
          max:     *10 | int
          cpuUtil: *50 | int
        }

PolicyDefinition

PolicyDefinition is simimarly to TraitDefinition, the difference is that TraitDefinition acts on a single component but PolicyDefinition is to act on the entire application as a whole (multiple components).

It can provide global policy for applications, commonly including global security policies (such as RBAC permissions, auditing, and key management), application insights (such as application SLO management, etc.).

The format is as follows:

apiVersion: core.oam.dev/v1beta1
kind: PolicyDefinition
metadata:
  name: <PolicyDefinition name>
  annotations:
    definition.oam.dev/description: <function description>
spec:
  schematic:  # strategy description
    cue: 
      template: <CUE format template>

A specific example is shown below:

apiVersion: core.oam.dev/v1beta1
kind: PolicyDefinition
metadata:
  name: env-binding
  annotations:
    definition.oam.dev/description: <Provide differentiated configuration and environmental scheduling strategies for applications>
spec:
  schematic:
    cue:
      template: |
        output: {
          apiVersion: "core.oam.dev/v1alpha1"
          kind:       "EnvBinding"
          spec: {
            engine: parameter.engine
            appTemplate: {
              apiVersion: "core.oam.dev/v1beta1"
              kind:       "Application"
              metadata: {
                name:      context.appName
                namespace: context.namespace
              }
              spec: {
                components: context.components
              }
            }
            envs: parameter.envs
          }
        }
        #Env: {
          name: string
          patch: components: [...{
            name: string
            type: string
            properties: {...}
          }]
          placement: clusterSelector: {
            labels?: [string]: string
            name?: string
          }
        }
        parameter: {
          engine: *"ocm" | string
          envs: [...#Env]
        }

WorkflowStepDefinition

WorkflowStepDefinition is used to describe a series of steps that can be declared in the workflow, such as the deployment of execution resources, status check, data output, dependent input, external script call, etc.

apiVersion: core.oam.dev/v1beta1
kind: WorkflowStepDefinition
metadata:
  name: <WorkflowStepDefinition name>
  annotations:
    definition.oam.dev/description: <function description>
spec:
  schematic:  # node description
    cue: 
      template: <CUE format template>

An actual WorkflowStepDefinition is as follows:

apiVersion: core.oam.dev/v1beta1
kind: WorkflowStepDefinition
metadata:
  name: apply-component
spec:
  schematic:
    cue:
      template: |
        import ("vela/op")
        parameter: {
           component: string
        }
        // load component from application
        component: op.#Load & {
           component: parameter.component
        }
        // apply workload to kubernetes cluster
        apply: op.#ApplyComponent & {
           component: parameter.name
        }
        // wait until workload.status equal "Running"
        wait: op.#ConditionalWait & {
           continue: apply.status.phase =="Running"
        }

The Standard Protocol Behind Abstraction

Once the application is created, KubeVela will tag the created resources with a series of tags, which include the version, name, type, etc. of the application. Through these standard protocols, application components, traits and policies can be coordinated. The specific metadata list is as follows:

Label	Description
`workload.oam.dev/type`	Corresponds to the name of `ComponentDefinition`
`trait.oam.dev/type`	Corresponds to the name of `TraitDefinition`
`app.oam.dev/name`	Application name
`app.oam.dev/component`	Component name
`trait.oam.dev/resource`	`outputs.\<resource type\>`in Trait
`app.oam.dev/appRevision`	Application Revision Name

Definition Runtime Context

In the Definition, some runtime context information can be obtained through the context variable. The specific list is as follows, where the scope indicates which module definitions the Context variable can be used in:

Context Variable	Description	Scope
`context.appRevision`	The app version name corresponding to the current instance of the application	ComponentDefinition, TraitDefinition
`context.appRevisionNum`	The app version number corresponding to the current instance of the application.	ComponentDefinition, TraitDefinition
`context.appName`	The app name corresponding to the current instance of the application.	ComponentDefinition, TraitDefinition
`context.name`	component name in ComponentDefinition and TraitDefinition，policy in PolicyDefinition	ComponentDefinition, TraitDefinition, PolicyDefinition
`context.namespace`	The namespace of the current instance of the application	ComponentDefinition, TraitDefinition
`context.revision`	The version name of the current component instance	ComponentDefinition, TraitDefinition
`context.parameter`	The parameters of the current component instance, it can be obtained in the trait	TraitDefinition
`context.output`	Object structure after instantiation of current component	ComponentDefinition, TraitDefinition
`context.outputs.<resourceName>`	Structure after instantiation of current component and trait	ComponentDefinition, TraitDefinition

At the same time, in the Workflow system, because the context has to act on the application level, it is very different from the above usage. We introduce it separately:

Context Variable	Description	Scope
`context.name`	The name of the current instance of the application	WorkflowStepDefinition
`context.namespace`	The namespace of the current instance of the application	WorkflowStepDefinition
`context.labels`	The labels of the current instance of the application	WorkflowStepDefinition
`context.annotations`	The annotations of the current instance of the application	WorkflowStepDefinition

Please note that all the Definition concepts introduced in this section only need to be understood by the platform administrator when they want to expand the functions of KubeVela. The end users will learn the schema of above definitions with visualized forms (or the JSON schema of parameters if they prefer) and reference them in application deployment plan. Please check the Generate Forms from Definitions section about how this is achieved.