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Version: v1.0

How-to

In this section, it will introduce how to use CUE to declare app components via ComponentDefinition.

Before reading this part, please make sure you've learned the Definition CRD in KubeVela.

Declare ComponentDefinition#

Here is a CUE based ComponentDefinition example which provides a abstraction for stateless workload type:

apiVersion: core.oam.dev/v1beta1
kind: ComponentDefinition
metadata:
name: stateless
spec:
workload:
definition:
apiVersion: apps/v1
kind: Deployment
schematic:
cue:
template: |
parameter: {
name: string
image: string
}
output: {
apiVersion: "apps/v1"
kind: "Deployment"
spec: {
selector: matchLabels: {
"app.oam.dev/component": parameter.name
}
template: {
metadata: labels: {
"app.oam.dev/component": parameter.name
}
spec: {
containers: [{
name: parameter.name
image: parameter.image
}]
}
}
}
}

In detail:

  • .spec.workload is required to indicate the workload type of this component.
  • .spec.schematic.cue.template is a CUE template, specifically:
    • The output filed defines the template for the abstraction.
    • The parameter filed defines the template parameters, i.e. the configurable properties exposed in the Applicationabstraction (and JSON schema will be automatically generated based on them).

Let's declare another component named task, i.e. an abstraction for run-to-completion workload.

apiVersion: core.oam.dev/v1beta1
kind: ComponentDefinition
metadata:
name: task
annotations:
definition.oam.dev/description: "Describes jobs that run code or a script to completion."
spec:
workload:
definition:
apiVersion: batch/v1
kind: Job
schematic:
cue:
template: |
output: {
apiVersion: "batch/v1"
kind: "Job"
spec: {
parallelism: parameter.count
completions: parameter.count
template: spec: {
restartPolicy: parameter.restart
containers: [{
image: parameter.image
if parameter["cmd"] != _|_ {
command: parameter.cmd
}
}]
}
}
}
parameter: {
count: *1 | int
image: string
restart: *"Never" | string
cmd?: [...string]
}

Save above ComponentDefinition objects to files and install them to your Kubernetes cluster by $ kubectl apply -f stateless-def.yaml task-def.yaml

Declare an Application#

The ComponentDefinition can be instantiated in Application abstraction as below:

apiVersion: core.oam.dev/v1alpha2
kind: Application
metadata:
name: website
spec:
components:
- name: hello
type: stateless
properties:
image: crccheck/hello-world
name: mysvc
- name: countdown
type: task
properties:
image: centos:7
cmd:
- "bin/bash"
- "-c"
- "for i in 9 8 7 6 5 4 3 2 1 ; do echo $i ; done"

Under The Hood#

Above application resource will generate and manage following Kubernetes resources in your target cluster based on the output in CUE template and user input in Application properties.

apiVersion: apps/v1
kind: Deployment
metadata:
name: backend
... # skip tons of metadata info
spec:
template:
spec:
containers:
- name: mysvc
image: crccheck/hello-world
metadata:
labels:
app.oam.dev/component: mysvc
selector:
matchLabels:
app.oam.dev/component: mysvc
---
apiVersion: batch/v1
kind: Job
metadata:
name: countdown
... # skip tons of metadata info
spec:
parallelism: 1
completions: 1
template:
metadata:
name: countdown
spec:
containers:
- name: countdown
image: 'centos:7'
command:
- bin/bash
- '-c'
- for i in 9 8 7 6 5 4 3 2 1 ; do echo $i ; done
restartPolicy: Never

CUE Context#

KubeVela allows you to reference the runtime information of your application via context keyword.

The most widely used context is application name(context.appName) component name(context.name).

context: {
appName: string
name: string
}

For example, let's say you want to use the component name filled in by users as the container name in the workload instance:

parameter: {
image: string
}
output: {
...
spec: {
containers: [{
name: context.name
image: parameter.image
}]
}
...
}

Note that context information are auto-injected before resources are applied to target cluster.

Full available information in CUE context#

Context VariableDescription
context.appRevisionThe revision of the application
context.appNameThe name of the application
context.nameThe name of the component of the application
context.namespaceThe namespace of the application
context.outputThe rendered workload API resource of the component, this usually used in trait
context.outputs.<resourceName>The rendered trait API resource of the component, this usually used in trait

Composition#

It's common that a component definition is composed by multiple API resources, for example, a webserver component that is composed by a Deployment and a Service. CUE is a great solution to achieve this in simplified primitives.

Another approach to do composition in KubeVela of course is using Helm.

How-to#

KubeVela requires you to define the template of workload type in output section, and leave all the other resource templates in outputs section with format as below:

outputs: <unique-name>:
<full template data>

The reason for this requirement is KubeVela needs to know it is currently rendering a workload so it could do some "magic" like patching annotations/labels or other data during it.

Below is the example for webserver definition:

apiVersion: core.oam.dev/v1beta1
kind: ComponentDefinition
metadata:
name: webserver
annotations:
definition.oam.dev/description: "webserver is a combo of Deployment + Service"
spec:
workload:
definition:
apiVersion: apps/v1
kind: Deployment
schematic:
cue:
template: |
output: {
apiVersion: "apps/v1"
kind: "Deployment"
spec: {
selector: matchLabels: {
"app.oam.dev/component": context.name
}
template: {
metadata: labels: {
"app.oam.dev/component": context.name
}
spec: {
containers: [{
name: context.name
image: parameter.image
if parameter["cmd"] != _|_ {
command: parameter.cmd
}
if parameter["env"] != _|_ {
env: parameter.env
}
if context["config"] != _|_ {
env: context.config
}
ports: [{
containerPort: parameter.port
}]
if parameter["cpu"] != _|_ {
resources: {
limits:
cpu: parameter.cpu
requests:
cpu: parameter.cpu
}
}
}]
}
}
}
}
// an extra template
outputs: service: {
apiVersion: "v1"
kind: "Service"
spec: {
selector: {
"app.oam.dev/component": context.name
}
ports: [
{
port: parameter.port
targetPort: parameter.port
},
]
}
}
parameter: {
image: string
cmd?: [...string]
port: *80 | int
env?: [...{
name: string
value?: string
valueFrom?: {
secretKeyRef: {
name: string
key: string
}
}
}]
cpu?: string
}

The user could now declare an Application with it:

apiVersion: core.oam.dev/v1beta1
kind: Application
metadata:
name: webserver-demo
namespace: default
spec:
components:
- name: hello-world
type: webserver
properties:
image: crccheck/hello-world
port: 8000
env:
- name: "foo"
value: "bar"
cpu: "100m"

It will generate and manage below API resources in target cluster:

$ kubectl get deployment
NAME READY UP-TO-DATE AVAILABLE AGE
hello-world-v1 1/1 1 1 15s
$ kubectl get svc
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
hello-world-trait-7bdcff98f7 ClusterIP <your ip> <none> 8000/TCP 32s

What's Next#

Please check the Learning CUE documentation about why we support CUE as first-class templating solution and more details about using CUE efficiently.

Last updated on by FogDong