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# Exposing Kubernetes Operators as-a-Service: Offering "Calculator as-a-Service"

Intended Audience: OpenSlice Service Designers

> To illustrate the powerful concept of Kubernetes operators and how they can be utilized to offer a service through OpenSlice, let's provide an example of a "Calculator as a Service."

> This example will demonstrate the flexibility and capabilities of Kubernetes operators in managing custom resources and automating operational tasks.

## Offering "Calculator as a Service" through OpenSlice

- We have a service that can accept two integers and an action (SUM, SUB, etc) and returns a result
- We would like to offer it as a Service through OpenSlice
- So when a user orders it with some initial parameters, OpenSlice will create it and return the result
- Also while the service is active, we can do further calculations, until we destroy it.

Assume the following simple CRD of a calculator model accepting two params (spec section) and an action and returning a result (status section)

The controller (the calculator code) is implemented in any language and is installed in a Kubernetes cluster

    apiVersion: apiextensions.k8s.io/v1
    kind: CustomResourceDefinition
    metadata:
      name: mycalculators.examples.osl.etsi.org
    spec:
      group: examples.osl.etsi.org
      names:
        kind: MyCalculator
        plural: mycalculators
        singular: mycalculator
      scope: Namespaced
      versions:
      - name: v1alpha1
        schema:
          openAPIV3Schema:
            properties:
              spec:
                properties:
                  parama:
                    type: integer
                  paramb:
                    type: integer
                  action:
                    type: string
                type: object
              status:
                properties:
                  result:
                    type: integer
                  status:
                    type: string
                type: object
            type: object
        served: true
        storage: true
        subresources:
          status: {}

Request to the cluster (through e.g. kubectl apply)

    apiVersion: examples.osl.etsi.org/v1alpha1
    kind: MyCalculator
    metadata:
      name: mycalculator.examples.osl.etsi.org
    spec:
      parama: 170
      paramb: 180
      action: 'SUM'

Response

    apiVersion: examples.osl.etsi.org/v1alpha1
    kind: MyCalculator
    metadata:
      creationTimestamp: '2023-12-05T12:26:07Z’

    <snip>

    status:
      result: 350
      status: CALCULATED
    spec:
      action: SUM
      parama: 170
      paramb: 180

To perform this through OpenSlice as a Service Specification ready to be ordered we need to do the following:

### CRD is saved automatically as Resource Specification

As soon as the CRD is deployed in the cluster (e.g. by your admin via kubectl or via any installation through the internet) it is automatically transformed and is available in OpenSlice catalogs as a Resource Specification.

- See also the fully qualified name of the resource specification.
  - MyCalculator@examples.osl.etsi.org/v1alpha1@docker-desktop@https://kubernetes.docker.internal:6443/
  - The resource specification name is quite unique, so you can install the CRD in many clusters around the internet. Each CRD on each cluster will appear here, for example:
    - MyCalculator@examples.osl.etsi.org/v1alpha1@default_cluster@https://10.10.10.8:6443/
    - MyCalculator@examples.osl.etsi.org/v1alpha1@edge1_cluster@https://172.16.10.10:6443/
  - Having this OpenSlice can manage resources in multiple clusters

> See also the detailed characteristics. See how OpenSlice makes all characteristics automatically flat and expanded with key-value style

## Expose to Users

### Create a ResourceFacingServiceSpecification

From the UI menu create a new Service Specification

#### Create CRD-related characteristics

- We need now to adjust some characteristics of this CRD as Resource Specification.
- OpenSlice translated automatically the CRD spec in a flat list of characteristics.So the "spec" section from the original yaml for example, is now unfold into: spec, spec.parama, spec.paramb, etc. the same for "status" object
- We need to make OpenSlice aware of when the service will be active.
  - So we go to characteristic _CR_CHECK_FIELD and we define that the field that shows the status of the service is the characteristic "status.status" (is a text field)
  - Then we go to _CR_CHECKVAL_AVAILABLE and we define the value CALCULATED, which signals the following: When the characteristic "status.status" has the value "CALCULATED" then OpenSlice will mark the underlying service as "ACTIVE"
  - We need also to define the yaml file that OpenSLice will use to create the new resource in the kubernetes cluster
  - We insert the YAML in the characteristic _CR_SPEC

The _CR_SPEC is:

    apiVersion: examples.osl.etsi.org/v1alpha1
    kind: MyCalculator
    metadata:
      name: mycalculator.examples.osl.etsi.org
    spec:
      parama: 170
      paramb: 180
      action: 'SUM'

> However the values are fixed. How do we allow a user to pass parameters through OpenSlice

### Expose in Catalog

Create a new CustomerFacingServiceSpecification

    - Go to the menu Service Specification>New Service Specification
    - Create a service My Calculator and mark it as a Bundle
    - Go to Service Specification Relationships and add MyCalculatorRFS
    - The service will be automatically transformed to a "CustomerFacingServiceSpecification"
    - Add the following characteristics as the image shows:

### Allow users to pass new values through OpenSlice

We need to Create LCM rules in CustomerFacingServiceSpecification:

    - The goal of the rules is to allow the user to pass parameters to the actual resource towards the cluster.
    - we will create one rule that will pass the parameters just before creating the service (PRE_PROVISION phase)
    - we will create one rule that will pass the parameters while the service is active (SUPERVISION phase)
    - The rules will be the same

If we see one rule it will look like the following:

- We need to change the _CR_SPEC characteristic of the referenced ResourceFacingServiceSpecification
- First bring a block from Service>Relationships>Service Refs and drop the "Service MyCalculatorRFS" block
- Then add a list block from Lists
- Then add the block that modifies a referenced characteristic from Service>Relationships>Service Refs the block "Set value to characteristic of a Referenced Service"
- Add a block for text _CR_SPEC
- We use a block that changes a String according to variables Text>"A formatted text replacing variables from List"
- See that we have as Input string the YAML string lines
  - See that parama, paramb has a %d (they accept integers), action is %s (accepts a string)
  - See that the variables tha will replace the %d, %d and %s are an list
    - the first %d will be replaced with the value from characteristic spec.parama
    - the second %d will be replaced with the value from characteristic spec.paramb
    - the %s will be replaced with the value from characteristic spec.action

If we see the SUPERVISION rule it will look like the following:

- It contains also the Result field, which takes the value from the referenced service
- Add a block for the Result field from Service>Number blocks
- Add a str to int block from Number blocks
- Add Service>Relationships>Service Refs and drop the input block [Service MyCalculatorRFS] "Get Service details from current context running service" and select from the drop down the "serviceCharacteristicValue"
- Add as name the "status.result"

Expose it then to a catalogue for orders through the Service Categories and Service Catalogs

## Order the Service

When a user orders the service, it will look like this:

- After the Service Order we have 2 services in service inventory on CFS and on RFS. Both have references to values
- OpenSlice (via CRIDGE service) updates the Resource in Resource Inventory and OSOM updates the Services in Service Inventory
- The Actual resources are running in the Kubernetes cluster managed by OpenSlice
- The result is in the characteristic status.result of the running service

## Modify the running service

The user can modify the service

- After a while the update is applied to the cluster, the controller will pick up the resource update and patch the resource
- OpenSlice (via CRIDGE service) updates the Resource in Resource Inventory and OSOM updates the Services in Service Inventory
- The result will be available to the respective characteristic "Result" after a few seconds, as need to go through various steps (OpenSlice orchestrator, down to kubernetes, to Calculator controller and back)