docs: adding expose depoyed chart hip

hips/hip-0027.md

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+---
+hip: 0027
+title: "Expose Previously Installed Chart Metadata During Template Rendering"
+authors: ["Andrew Shoell <mrlunchbox777@gmail.com>"]
+created: "2025-11-12"
+type: "feature"
+status: "draft"
+---
+
+## Abstract
+
+This HIP proposes exposing metadata from currently deployed chart versions during template rendering. Currently, Helm templates have access to `.Chart` for the chart being installed but no equivalent access to deployed releases. This forces chart authors to use complex workarounds like post-renderers, pre-upgrade hooks, or manual values conventions to implement version-aware upgrade logic.
+
+The proposal introduces `.DeployedChart` (singular, latest deployed) and `.DeployedCharts` (array, multiple versions) objects available in all template contexts, populated during `helm upgrade` and `helm rollback` operations. The `--deployed-depth` flag controls how many deployed versions to retrieve (default: 1), with `--deployed-depth 0` disabling the feature.
+
+## Motivation
+
+### Current Limitations
+
+Helm provides comprehensive chart metadata through `.Chart` but offers no native way to access deployed release metadata during template evaluation. Chart developers must resort to problematic workarounds:
+
+**Post-Renderers:** External tools that query the cluster, parse manifests, and make version-aware modifications. This moves upgrade logic outside the chart, requires additional tooling, and breaks Helm's self-contained design.
+
+**Pre-Upgrade Hooks:** Store version metadata in ConfigMaps via hooks, creating ordering dependencies and potential failure points.
+
+**Manual Values:** Require users to specify previous versions in values files—error-prone and defeats Helm's release tracking.
+
+### Real-World Impact
+
+This limitation prevents or complicates legitimate use cases:
+
+- **Breaking Changes:** No clean migration path for renamed resources or changed structures
+- **Conditional Resources:** Cannot create migration Jobs based on version deltas
+- **Smart Defaults:** Cannot distinguish fresh installs from upgrades for intelligent defaults
+- **Advanced Deployments:** Blue-green and similar strategies require external orchestration
+
+Post-rendering solutions violate Helm's design philosophy that template rendering should be deterministic and self-contained. Making deployed chart metadata available at template time keeps upgrade logic in the chart itself, maintaining Helm's portability, testability, and transparency.
+
+## Rationale
+
+### Naming: `.DeployedChart` and `.DeployedCharts`
+
+`.DeployedChart` (singular) provides ergonomic access to the most recent deployed version. `.DeployedCharts` (array) provides access to historical deployed versions in reverse chronological order (index 0 is most recent). `.DeployedChart` is syntactic sugar for `.DeployedCharts[0]`.
+
+Alternatives considered and rejected:
+
+- `.PreviousChart` - Ambiguous during rollbacks
+- `.InstalledChart` - Confusing with current installation
+- `.CurrentChart` - Ambiguous which is "current"
+- `.Release.Deployed.Chart` - Unnecessarily nested
+
+### Always Available as Template Objects
+
+`.DeployedChart` (nil or chart metadata) and `.DeployedCharts` (empty array or populated) are always present to ensure consistent template behavior, prevent undefined variable errors, and enable testing with `helm template`.
+
+### Populated Only During Upgrades/Rollbacks
+
+`.DeployedChart`/`.DeployedCharts` contain chart metadata only during `helm upgrade` and `helm rollback` when deployed releases exist. During rollback, they reflect the currently deployed version (being rolled back _from_). They're nil/empty for:
+
+- `helm install` - No deployed release
+- `helm template` / dry-runs - No cluster context
+- When `--deployed-depth 0` is used
+
+### Chart Metadata Only
+
+This proposal exposes only Chart.yaml metadata (same structure as `.Chart`), not values, manifests, or release metadata. This maintains security (values may contain secrets), performance (manifests can be large), and simplicity while solving 90% of use cases. Future HIPs could extend this if needed.
+
+### Depth Control Flag
+
+The `--deployed-depth` flag controls how many deployed chart versions to retrieve (default: 1). Setting `--deployed-depth 0` disables the feature for security or determinism requirements. Higher depths may impact performance and should only be used for specific multi-version migration scenarios.
+
+### Design Decisions
+
+- **Different Chart Names:** Still populates `.DeployedChart` even if chart names differ—templates can detect and handle this
+- **Helm's Record:** Reflects Helm's stored release record, not actual cluster state (use `lookup()` for that)
+- **Dry-Run/Template:** Always nil to maintain cluster-agnostic, deterministic behavior
+
+## Specification
+
+### New Template Objects
+
+**`.DeployedChart`**: Singular object containing the most recent deployed chart metadata (nil if none exists). Syntactic sugar for `.DeployedCharts[0]`.
+
+**`.DeployedCharts`**: Array of deployed chart metadata objects in reverse chronological order (most recent first). Empty array if no deployed releases exist.
+
+**Metadata Structure:** Each chart object is identical to `.Chart`, including `Name`, `Version`, `AppVersion`, and other [Chart.yaml fields](https://helm.sh/docs/topics/charts/#the-chartyaml-file).
+
+**Usage Examples:**
+
+```yaml
+# Simple case: check latest deployed version
+{{- if .DeployedChart }}
+{{- if and (semverCompare ">=2.0.0" .Chart.Version) (semverCompare "<2.0.0" .DeployedChart.Version) }}
+apiVersion: batch/v1
+kind: Job
+metadata:
+  name: {{ include "mychart.fullname" . }}-migration
+  annotations:
+    "helm.sh/hook": pre-upgrade
+spec:
+  template:
+    spec:
+      containers:
+      - name: migrate
+        image: myapp/migrator:{{ .Chart.AppVersion }}
+        command: ["migrate", "v1-to-v2"]
+{{- end }}
+{{- end }}
+
+# Multi-version migration: handle complex upgrade paths
+{{- if gt (len .DeployedCharts) 0 }}
+{{- range .DeployedCharts }}
+{{- if semverCompare "<1.5.0" .Version }}
+# Run migration for versions before 1.5.0
+{{- end }}
+{{- end }}
+{{- end }}
+```
+
+### Command-Line Flag
+
+```bash
+# Default: retrieve latest deployed chart
+helm upgrade myrelease mychart
+
+# Retrieve last 3 deployed versions
+helm upgrade myrelease mychart --deployed-depth 3
+
+# Disable feature
+helm upgrade myrelease mychart --deployed-depth 0
+```
+
+### Behavior Matrix
+
+| Operation                  | `.DeployedChart` / `.DeployedCharts`                            |
+| -------------------------- | --------------------------------------------------------------- |
+| `helm install`             | `nil` / `[]` (no deployed release)                              |
+| `helm upgrade` (new)       | `nil` / `[]` (no deployed release)                              |
+| `helm upgrade` (existing)  | Populated with deployed metadata (most recent first)            |
+| `helm rollback`            | Populated with currently deployed version (rolling back _from_) |
+| `helm template` / dry-runs | `nil` / `[]` (no cluster context)                               |
+| `--deployed-depth 0`       | `nil` / `[]` (explicitly disabled)                              |
+| `--deployed-depth N`       | Up to N most recent deployed versions                           |
+
+## Backwards Compatibility
+
+Fully backwards compatible. The `.DeployedChart` and `.DeployedCharts` objects are purely additive—existing charts work unchanged. Go templates handle nil and empty arrays safely; the recommended `{{ if .DeployedChart }}` pattern works in all scenarios.
+
+## Security Implications
+
+**Not Exposed:** Previous values (may contain secrets) or previous manifest (may contain sensitive data). Only Chart.yaml metadata is exposed.
+
+**Considerations:** Chart authors should not store sensitive data in Chart.yaml. The `--deployed-depth 0` flag provides opt-out for security-sensitive environments. Higher depth values increase data exposure; use the minimum required.
+
+## How to Teach This
+
+### Documentation Additions
+
+1. **Template Objects Reference:** Add `.DeployedChart` and `.DeployedCharts` to built-in objects with availability details
+2. **Upgrade Guide:** "Implementing Version-Aware Upgrades" covering nil/empty checks, version comparisons, and best practices
+3. **Migration Examples:** Show replacement of post-renderers and pre-upgrade hooks
+4. **Performance Note:** Document that `--deployed-depth` should be kept minimal; default of 1 is recommended
+5. **Chart Linting:** Update `helm lint` to warn on usage without nil/empty checks
+
+### Key Example Pattern
+
+```yaml
+{{- if and .DeployedChart (semverCompare "<3.0.0" .DeployedChart.Version) }}
+# Handle breaking change from versions < 3.0.0
+{{- end }}
+```
+
+## Reference Implementation
+
+A future pull request will:
+
+1. Extend template rendering context to include `.DeployedChart` and `.DeployedCharts`
+2. Populate from release records during upgrade/rollback (reverse chronological order)
+3. Add `--deployed-depth` flag (default: 1)
+4. Implement `.DeployedChart` as alias to `.DeployedCharts[0]`
+5. Include comprehensive unit and integration tests covering depth behavior
+
+## Rejected Ideas
+
+- **Full Release Object:** Security/performance concerns; chart metadata sufficient
+- **Only Version Strings:** Inconsistent with `.Chart`; prevents access to other metadata
+- **Environment Variable Control:** Less explicit than CLI flag
+- **Cluster Query During `helm template`:** Violates cluster-agnostic design principle
+- **Mutable Objects:** Violates read-only template model; no clear use case
+- **Separate `--disable-deployed-chart` Flag:** Unified `--deployed-depth` with 0 value is cleaner
+- **Unlimited History:** Performance implications; requiring explicit depth prevents accidental overhead
+
+## References
+
+- [Helm Built-in Objects](https://helm.sh/docs/chart_template_guide/builtin_objects/)
+- [Helm Chart.yaml](https://helm.sh/docs/topics/charts/#the-chartyaml-file)
+- [Go Templates](https://pkg.go.dev/text/template)
+- [Semantic Versioning](https://semver.org/)
+- [Example of current workaround](https://github.com/helm/community/pull/421#issuecomment-3662769874)