Should set the default value of `.spec.completions`

[zhouying7780]

What happened:
If a job not define the .spec.replicatedJobs[0].template.spec.completions and .spec.replicatedJobs[0].template.spec.completionMode field , the job can't be distributed by jobset and no error .

What you expected to happen:
If the default value for completionMode is Indexed , should also define the default value same as parallelism.

How to reproduce it (as minimally and precisely as possible):
Deploy the following JobSet spec:

apiVersion: jobset.x-k8s.io/v1alpha2
kind: JobSet
metadata:
  name: success-policy
spec:
# We want to declare our JobSet successful if workers finish.
# If workers finish we should clean up the remaining replicatedJobs.
  successPolicy:
    operator: All
    targetReplicatedJobs:
    - workers
  replicatedJobs:
  - name: leader
    replicas: 1
    template:
      spec:
        # Set backoff limit to 0 so job will immediately fail if any pod fails.
        backoffLimit: 0 
        completions: 1
        parallelism: 1
        template:
          spec:
            containers:
            - name: leader
              image: quay.io/openshifttest/hello-openshift:1.2.0
              command:
              - bash
              - -xc
              - |
                sleep 100
  - name: workers
    replicas: 1
    template:
      spec:
        backoffLimit: 0 
        parallelism: 5
        template:
          spec:
            containers:
            - name: worker
              image: quay.io/openshifttest/hello-openshift:1.2.0
              command:
              - bash
              - -xc
              - |
                if [[ "$JOB_COMPLETION_INDEX" == "0" ]]; then
                  for i in $(seq 10 -1 1)
                  do
                    echo "Sleeping in $i"
                    sleep 1
                  done
                  exit $(rand 0 1)
                fi

The worker job never by deployed , and see logs from the jobset-manager :
2025-09-05T11:26:59Z ERROR Reconciler error {"controller": "jobset", "controllerGroup": "jobset.x-k8s.io", "controllerKind": "JobSet", "JobSet": {"name":"success-policy","namespace":"testzhouy"}, "namespace": "testzhouy", "name": "success-policy", "reconcileID": "1a43fe16-be7d-4075-8fd7-3e674adb1505", "error": "job \"success-policy-workers-0\" creation failed with error: Job.batch \"success-policy-workers-0\" is invalid: spec.completions: Required value: when completion mode is Indexed"} sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).reconcileHandler /workspace/vendor/sigs.k8s.io/controller-runtime/pkg/internal/controller/controller.go:353 sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).processNextWorkItem /workspace/vendor/sigs.k8s.io/controller-runtime/pkg/internal/controller/controller.go:300 sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).Start.func2.1 /workspace/vendor/sigs.k8s.io/controller-runtime/pkg/internal/controller/controller.go:202

Anything else we need to know?:

Environment:

• Kubernetes version (use kubectl version): v1.33.3
• JobSet version (use git describe --tags --dirty --always):
• Cloud provider or hardware configuration:
• Install tools:
• Others:

[kannon92]

We have #422 and the goal was to validate Job template fields at creation of the JobSet.

This is not that easy to do.

I don't think we should default Job settings.

cc @andreyvelich @mimowo @tenzen-y @GiuseppeTT

[GiuseppeTT]

I agree with Kevin. Despite completions = parallelism being a common pattern, I don't think that introducing a new default value is the right solution, specially now that the JobSet API is stable. The surprise of successfully creating a JobSet object with a mistakenly missing field is worse than the convenience of not setting a field.

+1 for solving this issue with validation.