Version: v1.3

CUE Components

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.

First, generate ComponentDefinition scaffolds via vela def init with existed YAML file.

The YAML file:

apiVersion: "apps/v1"
kind: "Deployment"
spec:
  selector:
    matchLabels:
      "app.oam.dev/component": "name"
  template:
    metadata:
      labels:
        "app.oam.dev/component": "name"
    spec:
      containers: 
      - name: "name"
        image: "image"

Generate ComponentDefinition based on the YAML file:

vela def init stateless -t component --template-yaml ./stateless.yaml -o stateless.cue

It generates a file:

$ cat stateless.cue
stateless: {
    annotations: {}
    attributes: workload: definition: {
        apiVersion: "<change me> apps/v1"
        kind:       "<change me> Deployment"
    }
    description: ""
    labels: {}
    type: "component"
}
template: {
    output: {
        spec: {
            selector: matchLabels: "app.oam.dev/component": "name"
            template: {
                metadata: labels: "app.oam.dev/component": "name"
                spec: containers: [{
                    name:  "name"
                    image: "image"
                }]
            }
        }
        apiVersion: "apps/v1"
        kind:       "Deployment"
    }
    outputs: {}
    parameters: {}
}

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).

Add parameters in this auto-generated custom component file :

stateless: {
    annotations: {}
    attributes: workload: definition: {
        apiVersion: "<change me> apps/v1"
        kind:       "<change me> Deployment"
    }
    description: ""
    labels: {}
    type: "component"
}
template: {
    output: {
        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
                }]
            }
        }
        apiVersion: "apps/v1"
        kind:       "Deployment"
    }
    outputs: {}
    parameters: {
    name: string
    image: string
  }
}

You can use vela def vet to validate the format:

$ vela def vet stateless.cue
Validation succeed.

Declare another component named task which is an abstraction for run-to-completion workload.

vela def init task -t component -o task.cue

It generates a file:

$ cat task.cue
task: {
    annotations: {}
    attributes: workload: definition: {
        apiVersion: "<change me> apps/v1"
        kind:       "<change me> Deployment"
    }
    description: ""
    labels: {}
    type: "component"
}
template: {
    output: {}
    parameter: {}
}

Edit the generated component file:

task: {
    annotations: {}
    attributes: workload: definition: {
        apiVersion: "batch/v1"
        kind:       "Job"
    }
    description: ""
    labels: {}
    type: "component"
}
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]
  }
}

Apply above ComponentDefinition files to your Kubernetes cluster:

$ vela def apply stateless.cue
ComponentDefinition stateless created in namespace vela-system.
$ vela def apply task.cue
ComponentDefinition task created in namespace vela-system.

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: oamdev/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"

Details

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: oamdev/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

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.

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.

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:

webserver: {
    annotations: {}
    attributes: workload: definition: {
        apiVersion: "apps/v1"
        kind:       "Deployment"
    }
    description: ""
    labels: {}
    type: "component"
}
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
  }
}

Apply to your Kubernetes cluster:

$ vela def apply webserver.cue
ComponentDefinition webserver created in namespace vela-system.

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: oamdev/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

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 Nov 1, 2022 by Tianxin Dong