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+# ADR-010: Operator App-Config Validation
+
+## Context
+
+**Problem**: App-config errors are only discovered at Backstage runtime, after deployment, when plugins fail to load or behave incorrectly.
+
+**Current state:**
+- Backstage validates configs at startup, but errors only appear in pod logs
+- No validation occurs during CR reconciliation — operator blindly passes config to Backstage
+- Configuration errors require pod restart to diagnose
+- ADR-003 introduced "early validation" concept but without specific validation mechanisms
+
+**Types of errors only discovered at runtime:**
+1. **Schema violations**: Wrong types, missing required fields, invalid values
+2. **Missing references**: Env vars that don't exist, files that aren't mounted
+
+**Two categories of plugins require validation:**
+
+1. **Core plugins** (hardcoded/non-dynamic): Plugins built into RHDH image
+   - Discovered from RHDH's `packages/backend/package.json` and `packages/app/package.json` dependencies
+
+2. **Dynamic plugins**: Plugins loaded at runtime from catalog
+   - Discovered from catalog `index.json`
+
+Both configure via **app-config**. Schemas are fetched from respective plugin source repositories.
+
+**Who is impacted:**
+- **Operators/SREs**: Invalid configs cause runtime failures, not reconciliation failures
+- **Users**: No feedback on config errors until pod starts and plugin fails
+- **Debugging**: Must read Backstage pod logs to understand issues
+
+**Constraints:**
+- Schemas defined in upstream plugin source repos, not in RHDH
+- TypeScript schemas require conversion to JSON Schema
+- Not all plugins have schemas — schema validation should be best-effort
+- Schema validation only for plugins in a catalog (schemas extracted at catalog build time)
+- Must not block deployment for plugins without schemas
+
+## Decision
+
+Implement two complementary validation mechanisms during Backstage CR reconciliation:
+
+1. **Existence validation**: Verify that referenced env vars and files exist
+2. **Schema validation**: Validate app-config against plugin JSON schemas
+
+### 1. Existence Validation
+
+Verify that config references resolve to actual data:
+
+| Pattern | Validation |
+|---------|------------|
+| `${VAR}` | Env var exists (in ConfigMap, Pod env, etc.) |
+| `$env: VAR` | Env var exists |
+| `$file: path` | File/mount exists |
+| `$include: path` | File exists |
+
+**Behavior:**
+- Non-secret references: fail if not found
+- Secret references: warn only (may not have access)
+- Report missing references in CR status
+
+### 2. Schema Validation
+
+Generate JSON Schema files during plugin catalog creation for both core and dynamic plugins.
+
+**Schema extraction during catalog build** (in `rhdh-plugin-export-overlays`):
+
+**For core plugins:**
+- Core plugin schemas are included only in the [Default Catalog](https://github.com/redhat-developer/rhdh-adr/blob/main/decisions/002-devhub-plugin-catalog-crd.md) only
+- Fetch `packages/backend/package.json` and `packages/app/package.json` from RHDH repo
+- Extract plugin dependencies
+- For each plugin, resolve source repository and fetch `configSchema`
+- Convert TypeScript schemas to JSON Schema using `ts-json-schema-generator`
+
+**For dynamic plugins:**
+- For each plugin in catalog `index.json`, extract repository info from `io.backstage.dynamic-packages`
+- Fetch `configSchema` from plugin source repo
+- Convert TypeScript schemas to JSON Schema
+
+**Catalog index image contents** (extended):
+```
+catalog-index/
+├── index.json
+├── dynamic-plugins.default.yaml
+├── schemas/                                    # NEW
+│   ├── manifest.json                           # Plugin inventory with schema status
+│   ├── backstage-plugin-catalog-backend.json   # Core plugin
+│   ├── backstage-plugin-kubernetes-backend.json
+│   ├── backstage-community-plugin-acr.json     # Dynamic plugin
+│   └── ...
+└── ...
+```
+
+**Schema manifest format**:
+```json
+{
+  "generatedAt": "2026-06-29T10:00:00Z",
+  "corePlugins": ["backstage-plugin-catalog-backend", ...],
+  "dynamicPlugins": ["backstage-community-plugin-acr", ...],
+  "pluginsWithSchema": ["backstage-plugin-kubernetes-backend", ...],
+  "pluginsWithoutSchema": ["backstage-plugin-home", ...],
+  "stats": {
+    "totalPlugins": 150,
+    "corePlugins": 30,
+    "dynamicPlugins": 120,
+    "withSchema": 55,
+    "withoutSchema": 95
+  }
+}
+```
+
+**Environment variable handling in schema validation:**
+
+Config values may contain env var references (`${VAR}`, `$env: VAR`) substituted at Backstage runtime.
+
+- **Stage 1**: Treat env var patterns as wildcard — accept any type, warn
+- **Stage 2**: Resolve non-secret env vars and validate; for Secrets — warn only
+
+**Schema validation behavior:**
+- Schemas set `additionalProperties: true` (allow unknown fields for forward compatibility)
+- Validation is non-blocking for plugins without schemas
+- Strict JSON Schema draft-07 validation
+
+### Operator Validation Flow
+
+During reconciliation (extends ADR-003):
+1. Load schemas from default-config (populated by DHPC from catalog)
+2. **Existence validation**: Check all `${VAR}`, `$env:`, `$file:`, `$include:` references
+3. **Schema validation**: Validate app-config against plugin schemas
+4. Report validation errors/warnings in CR status
+5. Fail reconciliation on errors (fail fast)
+
+### CR Status Reporting
+
+```yaml
+status:
+  conditions:
+    - type: AppConfigValid
+      status: "False"
+      reason: ValidationFailed
+      message: |
+        Existence errors:
+        - ${DATABASE_URL}: env var not found
+        - $file: /etc/secrets/api-key: file not mounted
+
+        Schema errors (backstage-plugin-kubernetes-backend):
+        - $.kubernetes.clusterLocatorMethods[0].type: must be one of [config, gke, ...]
+        - $.kubernetes.serviceLocatorMethod: required property missing
+```
+
+## Alternatives Considered
+
+### Alternative 1: Schema extraction at operator runtime
+- **Approach**: Operator fetches schemas from plugin source repos during reconciliation
+- **Rejected because**: Requires network access during reconciliation; slow; unreliable; not air-gap compatible
+
+### Alternative 2: Manual schema curation in operator
+- **Approach**: Maintain curated JSON schemas in operator repo for known plugins
+- **Rejected because**: High maintenance burden; schemas drift from upstream; doesn't scale
+
+### Alternative 3: Schema extraction during plugin OCI build
+- **Approach**: Include schema in each plugin's OCI image, extract at init container time
+- **Rejected because**: Schemas scattered across images; harder to aggregate; doesn't cover core plugins
+
+## Consequences
+
+### Positive
+
+✅ **Fail fast**: Invalid configs caught during reconciliation, not at runtime
+✅ **Clear error messages**: Validation errors in CR status with specific field paths
+✅ **Two-layer validation**: Catches both missing references and schema violations
+✅ **CI/CD friendly**: Validate configs without deploying pods
+✅ **Consistent with ADR-003**: Extends early validation pattern
+✅ **Complete coverage**: Validates both core and dynamic plugins
+✅ **Best-effort**: Plugins without schemas still get existence validation
+✅ **Air-gap compatible**: Schemas bundled in catalog image, no runtime fetching
+
+### Negative
+
+❌ **Build complexity**: Catalog build must handle TypeScript-to-JSON-Schema conversion
+❌ **Larger catalog image**: Schema files add ~2-5MB to catalog index image
+
+### Neutral
+
+⚖️ **Requires ts-json-schema-generator**: Build dependency for TypeScript schema conversion
+⚖️ **Best-effort schema validation**: Silent pass for schema-less plugins
+⚖️ **additionalProperties: true**: Allows unknown fields (forward compatible but less strict)
+⚖️ **Env var limitation**: `${VAR}` values treated as wildcards in schema validation (Stage 1)
+
+## Notes
+
+**Related ADRs:**
+- ADR-003 (Operator-Based Plugin Configuration Processing) — this extends early validation
+- ADR-002 (DevHubPluginCatalog CRD) — catalog provides schema files
+
+**Implementation phases:**
+1. Add existence validation to operator reconciliation loop
+2. Add schema extraction to catalog build scripts (`rhdh-plugin-export-overlays`)
+3. Include schemas in catalog index OCI image
+4. Add schema validation to operator reconciliation loop
+5. Report validation results in CR status
+6. (Future) Resolve non-secret env vars for full schema validation
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