Version: v1.8

Trait Definition

In this section we will introduce how to define a custom trait with CUE. Make sure you’ve learned the basic knowledge about Definition Concept and how to manage definition.

A trait can be something similar to component, while they’re attached operational resources.

• Kubernetes API resources like ingress, service, rollout.
• The composition of these operational resources.
• A patch of data, for example, patch sidecar to workload.

Let’s use vela def init to create a basic trait scaffold:

vela def init my-route -t trait --desc "My ingress route trait." > myroute.cue

The content of the scaffold expected to be:

// $ cat myroute.cue
"my-route": {
    annotations: {}
    attributes: {
        appliesToWorkloads: []
        conflictsWith: []
        podDisruptive:   false
        workloadRefPath: ""
    }
    description: "My ingress route trait."
    labels: {}
    type: "trait"
}
template: {
    patch: {}
    parameter: {}
}

caution

There’s a bug in vela CLI(<=1.4.2), the vela def init command will generate definitionRef: "" in attributes which is wrong, please remove that line.

Unlike component definition, KubeVela requires objects in traits MUST be defined in outputs section (not output) in CUE template with format as below:

outputs: {
  <unique-name>: {
    <template of trait resource structural data>
  }
}

tip

Actually the CUE template of trait here will be evaluated with component CUE template in the same context, so the name can’t be conflict. That also explain why the output can’t be defined in trait.

Below is an example that we combine ingress and service of Kubernetes into our my-route trait.

"my-route": {
    annotations: {}
    attributes: {
        appliesToWorkloads: []
        conflictsWith: []
        podDisruptive:   false
        workloadRefPath: ""
    }
    description: "My ingress route trait."
    labels: {}
    type: "trait"
}
template: {
    parameter: {
        domain: string
        http: [string]: int
    }
    // trait template can have multiple outputs in one trait
    outputs: service: {
        apiVersion: "v1"
        kind:       "Service"
        spec: {
            selector:
                app: context.name
            ports: [
                for k, v in parameter.http {
                    port:       v
                    targetPort: v
                },
            ]
        }
    }
    outputs: ingress: {
        apiVersion: "networking.k8s.io/v1beta1"
        kind:       "Ingress"
        metadata:
            name: context.name
        spec: {
            rules: [{
                host: parameter.domain
                http: {
                    paths: [
                        for k, v in parameter.http {
                            path: k
                            backend: {
                                serviceName: context.name
                                servicePort: v
                            }
                        },
                    ]
                }
            }]
        }
    }
}

Apply to our control plane to make this trait work:

vela def apply myroute.cue

Then our end users can discover it immediately and attach this trait to any component instance in Application.

After using vela up by the following command:

cat <<EOF | vela up -f -
apiVersion: core.oam.dev/v1beta1
kind: Application
metadata:
  name: testapp
spec:
  components:
    - name: express-server
      type: webservice
      properties:
        cmd:
          - node
          - server.js
        image: oamdev/testapp:v1
        port: 8080
      traits:
        - type: my-route
          properties:
            domain: test.my.domain
            http:
              "/api": 8080
EOF

It will generate Kubernetes resources by KubeVela.

With the help of CUE, we can achieve many advanced features in trait.

You can define the for-loop inside the outputs.

note

In this case the type of parameter field used in the for-loop must be a map.

Below is an example that will render multiple Kubernetes Services in one trait:

"expose": {
    type: "trait"
}
template: {
    parameter: {
        http: [string]: int
    }
    outputs: {
        for k, v in parameter.http {
            "\(k)": {
                apiVersion: "v1"
                kind:       "Service"
                spec: {
                    selector:
                        app: context.name
                    ports: [{
                        port:       v
                        targetPort: v
                    }]
                }
            }
        }
    }
}

The usage of this trait could be:

apiVersion: core.oam.dev/v1beta1
kind: Application
metadata:
  name: testapp
spec:
  components:
    - name: express-server
      type: webservice
      properties:
        ...
      traits:
        - type: expose
          properties:
            http:
              myservice1: 8080
              myservice2: 8081

The trait definition can send a HTTP request and capture the response to help you rendering the resource with keyword processing.

You can define HTTP request method, url, body, header and trailer in the processing.http section, and the returned data will be stored in processing.output.

tip

Please ensure the target HTTP server returns a JSON data as output.

Then you can reference the returned data from processing.output in patch or output/outputs.

Below is an example:

"auth-service": {
    type: "trait"
}
template: {
    parameter: {
        serviceURL: string
    }
    processing: {
        output: {
            token?: string
        }
        // The target server will return a JSON data with `token` as key.
        http: {
            method: *"GET" | string
            url:    parameter.serviceURL
            request: {
                body?: bytes
                header: {}
                trailer: {}
            }
        }
    }
    patch: {
        data: token: processing.output.token
    }
}

In above example, this trait definition will send request to get the token data, and then patch the data to given component instance.

A trait definition can read the generated resources (rendered from output and outputs) of given component definition.

caution

Generally, KubeVela will ensure the component definitions are evaluated before its traits. But there’re a stage mechanism that allow trait be deployed before component.

Specifically, the context.output contains the rendered workload API resource (whose GVK is indicated by spec.workloadin component definition), and use context.outputs.<xx> to contain all the other rendered API resources.

Let’s use an example to see this feature:

1. Let’s define a component definition myworker like below:

"myworker": {
    attributes: workload: definition: {
        apiVersion: "apps/v1"
        kind:       "Deployment"
    }
    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
                        ports: [{containerPort: parameter.port}]
                        envFrom: [{
                            configMapRef: name: context.name + "game-config"
                        }]
                        if parameter["cmd"] != _|_ {
                            command: parameter.cmd
                        }
                    }]
                }
            }
        }
    }
    outputs: gameconfig: {
        apiVersion: "v1"
        kind:       "ConfigMap"
        metadata: {
            name: context.name + "game-config"
        }
        data: {
            enemies: parameter.enemies
            lives:   parameter.lives
        }
    }
    parameter: {
        // +usage=Which image would you like to use for your service
        // +short=i
        image: string
        // +usage=Commands to run in the container
        cmd?: [...string]
        lives:   string
        enemies: string
        port:    int
    }
}

1. Define a new myingress trait that read the port.

"myingress": {
    type: "trait"
  attributes: {
        appliesToWorkloads: ["myworker"]
  }
}
template: {
    parameter: {
        domain:     string
        path:       string
        exposePort: int
    }
    // trait template can have multiple outputs in one trait
    outputs: service: {
        apiVersion: "v1"
        kind:       "Service"
        spec: {
            selector:
                app: context.name
            ports: [{
                port:       parameter.exposePort
                targetPort: context.output.spec.template.spec.containers[0].ports[0].containerPort
            }]
        }
    }
    outputs: ingress: {
        apiVersion: "networking.k8s.io/v1beta1"
        kind:       "Ingress"
        metadata:
                name: context.name
        labels: config: context.outputs.gameconfig.data.enemies
        spec: {
            rules: [{
                host: parameter.domain
                http: {
                    paths: [{
                        path: parameter.path
                        backend: {
                            serviceName: context.name
                            servicePort: parameter.exposePort
                        }
                    }]
                }
            }]
        }
    }
}

In detail, The trait myingress can only apply to the specific ComponentDefinition myworker:

1. the rendered Kubernetes Deployment resource will be stored in the context.output,
2. all other rendered resources will be stored in context.outputs.<xx>, with <xx> is the unique name in every template.outputs.

Thus, in TraitDefinition myingress, it can read the rendered API resources (e.g. context.outputs.gameconfig.data.enemies) from the context and get the targetPort and labels.config.

Besides generate objects from trait, one more important thing we can do in trait is to patch or override the configuration of Component. But why do we need to patch or override?

There’re several reasons:

1. The component could be defined by another person, for separation of concern, the operator can attach an operational trait to change that data.
2. The component could be defined by third party which is not controlled by the one who use it.

So KubeVela allow patch or override in this case, please refer to patch trait for more details. As trait and workflow step can both patch, so we write them together.

You can also define health check policy and status message when a trait deployed and tell the real status to end users.

The spec of health check is <trait-type-name>.attributes.status.healthPolicy, it’s similar to component definition.

If not defined, the health result will always be true, which means it will be marked as healthy immediately after resources applied to Kubernetes. You can define a CUE expression in it to notify if the trait is healthy or not.

The keyword in CUE is isHealth, the result of CUE expression must be bool type.

KubeVela runtime will evaluate the CUE expression periodically until it becomes healthy. Every time the controller will get all the Kubernetes resources and fill them into the context variables.

So the context will contain following information:

context:{
  name: <component name>
  appName: <app name>
  outputs: {
    <resource1>: <K8s trait resource1>
    <resource2>: <K8s trait resource2>
  }
}

The example of health check likes below:

my-ingress: {
    type: "trait"
    ...
    attributes: {
        status: {
            healthPolicy: #"""
              isHealth: len(context.outputs.service.spec.clusterIP) > 0
              """#
        }
    }
}

You can also use the parameter defined in the template like:

mytrait: {
    type: "trait"
    ...
    attributes: {
        status: {
            healthPolicy: #"""
                isHealth: context.outputs."mytrait-\(parameter.name)".status.state == "Available"
              """#
    }
  }
template: {
  parameter: {
    name: string
  } 
  ...
}

The health check result will be recorded into the corresponding trait in .status.services of Application resource.

apiVersion: core.oam.dev/v1beta1
kind: Application
status:
  ...
  services:
  - healthy: true
    ...
    name: myweb
    traits:
    - healthy: true
      type: my-ingress
  status: running

Please refer to this doc for more complete example.

The spec of custom status is <trait-type-name>.attributes.status.customStatus, it shares the same mechanism with the health check.

The keyword in CUE is message, the result of CUE expression must be string type.

Application CRD controller will evaluate the CUE expression after the health check succeed.

The example of custom status likes below:

my-service: {
    type: "trait"
    ...
    attributes: {
        status: {
            customStatus: #"""
                if context.outputs.ingress.status.loadBalancer.ingress != _|_ {
                    let igs = context.outputs.ingress.status.loadBalancer.ingress
                  if igs[0].ip != _|_ {
                    if igs[0].host != _|_ {
                        message: "Visiting URL: " + context.outputs.ingress.spec.rules[0].host + ", IP: " + igs[0].ip
                    }
                    if igs[0].host == _|_ {
                        message: "Host not specified, visit the cluster or load balancer in front of the cluster with IP: " + igs[0].ip
                    }
                  }
                }
                """#
        }
    }
}

The message will be recorded into the corresponding trait in .status.services of Application resource.

apiVersion: core.oam.dev/v1beta1
kind: Application
status:
  ...
  services:
  - healthy: true
    ...
    name: myweb
    traits:
    - healthy: true
      message: 'Visiting URL: www.example.com, IP: 47.111.233.220'
      type: my-ingress
  status: running

Please refer to this doc for more complete example.

The cluster version context info can be used for graceful upgrade of definition. For example, you can define different API according to the cluster version.

 outputs: ingress: {
    if context.clusterVersion.minor < 19 {
        apiVersion: "networking.k8s.io/v1beta1"
    }
    if context.clusterVersion.minor >= 19 {
        apiVersion: "networking.k8s.io/v1"
    }
    kind: "Ingress"
}

Or use string contain pattern for this usage:

import "strings"
if strings.Contains(context.clusterVersion.gitVersion, "k3s") {
     provider: "k3s"
}
if strings.Contains(context.clusterVersion.gitVersion, "aliyun") {
     provider: "aliyun"
}

KubeVela is fully programmable via CUE, while it leverage Kubernetes as control plane and align with the API in yaml. As a result, the CUE definition will be converted as Kubernetes API when applied into cluster.

The trait definition will be in the following API format:

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>

You can check the detail of this format here.

You can check the following resources for more examples:

• Builtin trait definitions in the KubeVela github repo.
• Definitions defined in addons in the catalog repo.

Last updated on May 6, 2023 by Tianxin Dong