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Version: v1.4

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 Resources by Trait

A trait can be something similar to component that you can define operational Kubernetes API resources such as ingress, service as trait.

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

Note: 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.

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: {}
}

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>: 
<full template data>

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 -f 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 that 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.

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

Get context data of Component parameters

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

KubeVela will ensure the component definitions are always evaluated before its traits.

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.

What's Next