manual_scaler_trait.md

Manual Scaler

This trait definition allows operators to manually scale the number of replicas for workload types that allow scaling operations.

Applicable Workloads

• core.oam.dev/v1alpha2.ContainerizedWorkload

Schematic

Attribute	Type	Required	Default Value	Description
replicaCount	integer	Y	0	The target number of replicas to create for a given component instance.

Spec

As a core trait, the Manual Scaler trait should be part of OAM implementation, so we don't need trait definition for it.

But assuming manual scaler trait is not a core trait, we could define as the following workflow.

Firstly, we need register trait definition like below:

apiVersion: core.oam.dev/v1alpha2
kind: TraitDefinition
metadata:
  name: manualscalertrait.core.oam.dev
spec:
  appliesToWorkloads:
    - core.oam.dev/v1alpha2.ContainerizedWorkload
  definitionRef:
    name: manualscalertrait.core.oam.dev

Then OAM runtime should have a version of this schema, the schema SHOULD be able to validate. For example using OpenAPIv3 schema like this:

openAPIV3Schema:
  type: object
  description: Open Application Model ManualScaler Trait OpenAPIv3 schema.
  required:
    - apiVersion
    - kind
    - spec
  properties:
    apiVersion:
      description: APIVersion defines the versioned schema of this representation of an object.
      type: string
      enmu:
        - core.oam.dev/v1alpha2
    kind:
      description: Must be ManualScalerTrait.
      type: string
      enum:
        - ManualScalerTrait
    spec:
      description: Defines the desired state of the ManualScaler trait.
      type: object
      required:
        - replicaCount
      properties:
        replicaCount:
          description: the target number of replicas to scale a component to.
          type: integer

Or using JSON Schema:

{
    "$schema": "http://json-schema.org/draft-07/schema#",
    "title": "Open Application Model ManualScaler Trait JSON Schema",
    "type": "object",
    "properties": {
        "apiVersion": {
            "type": "string",
            "description": "APIVersion defines the versioned schema of this representation of an object.",
            "enum": ["core.oam.dev/v1alpha2"]
        },
        "kind": {
            "type": "string",
            "description": "must be ManualScalerTrait.",
            "enum": ["ManualScalerTrait"]
        },
        "spec": {
            "type": "object",
            "description": "Defines the desired state of the ManualScaler trait.",
            "required": ["replicaCount"],
            "properties": {
              "replicaCount": {
                "type": "integer",
                "description": "the target number of replicas to scale a component to.",
                "minimum": 0
              }
            }
        }
    },
    "required": ["apiVersion", "kind", "spec"]
}

Usage examples

The following snippet from an application configuration shows how the manual scaler trait is applied and configured for a component. In this example, it is assumed component frontend following workload of core.oam.dev/v1alpha2.ContainerizedWorkload.

apiVersion: core.oam.dev/v1alpha2
kind: ApplicationConfiguration
metadata:
  name: custom-single-app
  annotations:
    version: v1.0.0
    description: "Customized version of single-app"
spec:
  components:
    - componentName: frontend
      traits:
        - trait:
            apiVersion: core.oam.dev/v1alpha2
            kind: ManualScalerTrait
            spec:
              replicaCount: 5

This example illustrates using the manual scaler trait to manually scale a component by specifying the number of replicas the runtime should create. This trait has only one attribute in its properties: replicaCount. This takes an integer, and a value is required for this trait to be successfully applied. If, for example, an application configuration used this trait but did not provide the replicaCount, the system would reject the application configuration.