VEP 100: Introduce metaData and MetaDataSecretRef feild for adding custom medata to cloud-init

veps/sig-compute/introduce-custom-metadata-to-cloudinit.md

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+# VEP #100: Introduce Custom Metadata to Cloud-init
+
+## Release Signoff Checklist
+Items marked with (R) are required *prior to targeting to a milestone / release*.
+- [X] (R) Enhancement issue created, which links to VEP dir in [kubevirt/enhancements] (not the initial VEP PR)
+- [ ] (R) Target version is explicitly mentioned and approved
+- [ ] (R) Graduation criteria filled
+
+## Overview
+This VEP proposes enhancing KubeVirt's cloud-init metadata support by enabling users to add custom metadata to cloud-init. 
+This addresses missing functionality for users to be able to add custom values to the cloud-init metadata.
+
+## Motivation
+KubeVirt's cloud-init support previously did not allow users to add custom values to the metadata. 
+This prevented users from passing application-specific information to their VMs through cloud-init.
+Issue https://github.com/kubevirt/kubevirt/issues/15836 the motivation behind this VEP.
+
+## Goals
+- Enable users to add custom metadata fields to cloud-init through the metaData field in both `NoCloud` and `ConfigDrive` configurations
+- Implement  `MetaData` in the `CloudInitNoCloudSource` and `CloudInitConfigDriveSource` struct to allow users add custom metadata to the cloud-init
+- Implement `MetaDataSecretRef` in the `CloudInitNoCloudSource` and `CloudInitConfigDriveSource` struct to allow users to reference Kubernetes secrets containing custom metadata
+- Ensure custom metadata is properly merged with standard metadata fields (instance-id, local-hostname, instance-type, etc.) that KubeVirt automatically generates
+
+## Non Goals
+
+
+## Definition of Users
+VM owners and workload developers who need to pass custom metadata to their virtual machines 
+through cloud-init for application configuration, environment-specific settings, or runtime parameters. 
+This includes developers deploying applications that require custom configuration data accessible via cloud-init metadata,
+such as environment variables, application settings, or deployment-specific parameters.
+Cluster administrators may be indirectly impacted as they need to understand the new
+metadata capabilities and potentially create secrets for users, 
+but they are not the primary users of this functionality.
+
+## User Stories
+- As a VM owner, I want to add custom metadata fields to my cloud-init configuration 
+so that my applications can access application-specific configuration data.
+- As a VM owner, I want to reference Kubernetes secrets containing
+custom metadata so that I can securely pass sensitive configuration data to my VMs.
+
+## Repos
+- kubevirt/kubevirt (core API)
+
+## Design
+Enhance `CloudInitNoCloudSource` and `CloudInitConfigDriveSource` with `MetaData` field as `map[string]string `and `MetaDataSecretRef` for secret references. 
+Secret resolution reads custom metadata and merges with standard fields.
+
+## API Examples
+```yaml
+volumes:
+- name: cloudinitdisk
+  cloudInitNoCloud:
+    userData: |
+      #cloud-config
+      package_update: true
+    metaData:
+      app_name: "my-application"
+      environment: "production"
+      cost_center: "12345"
+```
+
+```yaml
+volumes:
+- name: cloudinitdisk
+  cloudInitNoCloud:
+    userData: |
+      #cloud-config
+      package_update: true
+    metaDataSecretRef:
+      name: my-metadata-secret
+---
+apiVersion: v1
+kind: Secret
+metadata:
+  name: my-metadata-secret
+type: Opaque
+stringData:
+  app_name: "my-application"
+  environment: "production"
+  cost_center: "12345"      
+```
+
+## Alternatives
+
+## Scalability
+
+## Update/Rollback Compatibility
+- New field is optional; existing VMIs remain unaffected.
+- On downgrade, projected volumes fall back to errors or no-op.
+## Functional Testing Approach
+- Unit tests: API validation, struct marshaling, metadata merging.
+- Integration tests: Deploy VM/VMI with custom metadata, verify metadata is accessible in guest.
+## Implementation History
+- October 17, 2024: Implementation completed and PR submitted.
+- October 19, 2024: VEP drafted (after reviewer feedback indicated VEP was required for API changes).
+## Graduation Requirements
+### Alpha (v1.8)
+- Custom metadata support implemented.
+- MetaData functionality complete.
+- MetaDataSecretRef functionality complete.
+### Beta
+- Full IRSA support.
+### GA
+