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Version: v1.1

X-Definition

In OAM model, Application used by end user consists many declarative moduels such as Component, Trait, Policy and Workflow etc. These types are actually shaped by many definitions behind them. The module definition (X-Definition) supported by the current OAM model includes ComponentDefinition, TraitDefinition, PolicyDefinition and WorkflowStepDefinition etc.

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, 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 description
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>

Here is a complete example to introduce how to use ComponentDefinition.

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: {
...
}

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>
schematic: # Abstract
cue: # There are many abstracts
template: <CUE format template>

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"
}

WorkloadDefinition#

WorkloadDefinition is a system-level feature. It's not a field that users should care about but as metadata checked, verified, and used by the OAM system itself.

The format is as follows:

apiVersion: core.oam.dev/v1beta1
kind: WorkloadDefinition
metadata:
name: <WorkloadDefinition name>
spec:
definitionRef:
name: <corresponding Kubernetes resource group>
version: <corresponding Kubernetes resource version>
podSpecPath: <path to the Pod field in the Workload>
childResourceKinds:
- apiVersion: <resource group>
kind: <resource type>

In addition, other Kubernetes resource type that need to be introduced into OAM model in the future will also be added as fields to the workload definition.

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:

LabelDescription
workload.oam.dev/typeCorresponds to the name of ComponentDefinition
trait.oam.dev/typeCorresponds to the name of TraitDefinition
app.oam.dev/nameApplication name
app.oam.dev/componentComponent name
trait.oam.dev/resourceoutputs.\<resource type\>in Trait
app.oam.dev/appRevisionApplication Revision Name

X-Definition Runtime Context#

In the X-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 VariableDescriptionScope
context.appRevisionThe app version name corresponding to the current instance of the applicationComponentDefinition, TraitDefinition
context.appRevisionNumThe app version number corresponding to the current instance of the application.ComponentDefinition, TraitDefinition
context.appNameThe app name corresponding to the current instance of the application.ComponentDefinition, TraitDefinition
context.namecomponent name in ComponentDefinition and TraitDefinition,policy in PolicyDefinitionComponentDefinition, TraitDefinition, PolicyDefinition
context.namespaceThe namespace of the current instance of the applicationComponentDefinition, TraitDefinition
context.revisionThe version name of the current component instanceComponentDefinition, TraitDefinition
context.parameterThe parameters of the current component instance, it can be obtained in the traitTraitDefinition
context.outputObject structure after instantiation of current componentComponentDefinition, TraitDefinition
context.outputs.<resourceName>Structure after instantiation of current component and traitComponentDefinition, 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 VariableDescriptionScope
context.nameThe name of the current instance of the applicationWorkflowStepDefinition
context.namespaceThe namespace of the current instance of the applicationWorkflowStepDefinition
context.labelsThe labels of the current instance of the applicationWorkflowStepDefinition
context.annotationsThe annotations of the current instance of the applicationWorkflowStepDefinition

Please note that all the X-Definition concepts introduced in this section only need to be understood by the platform administrator when they want to expand the functions of KubeVela, and end users do not need to have any perception of these concepts.

Last updated on by yangsoon