Fix KServe minReplicas=0 Support for Scale-to-Zero

[Leoyzen]

Problem

This PR addresses two critical issues with the minReplicas configuration in KServe:

Issue 1: Helm Template Condition Fails for minReplicas=0

The Helm template used {{- if .Values.minReplicas }} to conditionally render the minReplicas field. In Helm, the value 0 is falsy, so when users set minReplicas: 0 to enable scale-to-zero, the configuration was silently ignored and not rendered in the generated manifest.

Issue 2: Update Command Resets minReplicas to 1

When updating an InferenceService with any field (not --min-replicas), the minReplicas value was always reset to 1. This occurred because:

• The default value for --min-replicas flag was 1 in the update command
• The update logic checked if args.MinReplicas >= 0, which was always true (default 1 >= 0)
• This overwrote existing minReplicas settings, breaking scale-to-zero deployments

Solution

1. Fixed Helm Template Condition

Changed the condition from:

{{- if .Values.minReplicas }}

To:

{{- if ge (int .Values.minReplicas) 0 }}

This ensures that minReplicas=0 is correctly rendered since 0 >= 0 evaluates to true.

2. Fixed Update Command Default Value

Changed the default value of --min-replicas flag in update_serving_kserve.go from 1 to -1:

command.Flags().IntVar(&s.args.MinReplicas, "min-replicas", -1, "...")

Now:

• When --min-replicas is not specified, args.MinReplicas = -1, condition >= 0 is false, no update occurs
• When --min-replicas 0 is explicitly set, args.MinReplicas = 0, condition >= 0 is true, updates to 0
• When --min-replicas <N> (N>0) is explicitly set, updates to the specified value

Files Changed

• charts/kserve/templates/inferenceservice.yaml - Fixed Helm template condition
• charts/kserve/values.yaml - Added default values for minReplicas: 0 and maxReplicas: 10
• pkg/argsbuilder/update_serving_kserve.go - Changed default --min-replicas to -1

Behavior

Create New InferenceService:

• Default minReplicas remains 1 (no breaking change)
• Users can explicitly set --min-replicas 0 for scale-to-zero

Update InferenceService:

• Updating other fields no longer resets minReplicas
• Users can explicitly set --min-replicas 0 to enable scale-to-zero
• Users can explicitly set --min-replicas <N> to change the value

Testing Recommendations

1. Deploy a new InferenceService with --min-replicas 0 and verify scale-to-zero works
2. Update an existing InferenceService (changing non-minReplicas fields) and verify minReplicas is preserved
3. Update an existing InferenceService with --min-replicas 0 and verify it enables scale-to-zero
4. Verify existing deployments without explicit minReplicas continue to work as before

Related

This fix enables true scale-to-zero functionality for KServe inference services, which is essential for cost optimization in development/test environments and low-traffic production workloads.

[google-oss-prow]

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[ChenYi015]

@Leoyzen Thanks for reporting and fixing the issue! Could you sign off your commits to pass the DCO check? See Contributing | Kubeflow.