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Version: v1.6

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.

Generate a Trait scaffold

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.

Define a trait to compose resources

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.

Render Multiple Resources With a Loop

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

Execute HTTP Request in Trait Definition

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.

CUE Context for runtime information

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.

Patch Trait

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.

Define Health for Definition

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

Health Check

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.

Custom Status

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.

Full available context in Trait

Context VariableDescriptionType
context.appNameThe app name corresponding to the current instance of the application.string
context.namespaceThe target namespace of the current resource is going to be deployed, it can be different with the namespace of application if overridden by some policies.string
context.clusterThe target cluster of the current resource is going to be deployed, it can be different with the namespace of application if overridden by some policies.string
context.appRevisionThe app version name corresponding to the current instance of the application.string
context.appRevisionNumThe app version number corresponding to the current instance of the application.int
context.nameThe component name corresponding to the current instance of the application.string
context.revisionThe version name of the current component instance.string
context.outputThe object structure after instantiation of current component.Object Map
context.outputs.<resourceName>Structure after instantiation of current component auxiliary resources.Object Map
context.workflowNameThe workflow name specified in annotation.string
context.publishVersionThe version of application instance specified in annotation.string
context.componentsThe object structure of components spec in this application.Object Map
context.appLabelsThe labels of the current application instance.Object Map
context.appAnnotationsThe annotations of the current application instance.Object Map

Cluster Version

Context VariableDescriptionType
context.clusterVersion.majorThe major version of the runtime Kubernetes cluster.string
context.clusterVersion.gitVersionThe gitVersion of the runtime Kubernetes cluster.string
context.clusterVersion.platformThe platform information of the runtime Kubernetes cluster.string
context.clusterVersion.minorThe minor version of the runtime Kubernetes cluster.int

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"
}

Trait definition in Kubernetes

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.

More examples to learn

You can check the following resources for more examples: