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+# ADR-0021: Notification Architecture
+
+## Status
+
+Proposed
+
+## Context
+
+The homelab infrastructure generates notifications from multiple sources:
+
+1. **CI/CD pipelines** (Gitea Actions) - build success/failure
+2. **Alertmanager** - Prometheus alerts for critical/warning conditions
+3. **Gatus** - Service health monitoring
+4. **Flux** - GitOps reconciliation events
+
+Currently, ntfy serves as the primary notification hub, but there are several issues:
+
+- **Topic inconsistency**: CI workflows were posting to `builds` while documentation (ADR-0015) specified `gitea-ci`
+- **No Alertmanager integration**: Critical Prometheus alerts had no delivery mechanism
+- **Discord integration desire**: Team wants notifications forwarded to Discord for visibility
+
+## Decision
+
+### 1. ntfy as the Notification Hub
+
+ntfy will serve as the central notification aggregation point. All internal services publish to ntfy topics via the internal Kubernetes service URL:
+
+```
+http://ntfy-svc.observability.svc.cluster.local/<topic>
+```
+
+This keeps ntfy auth-protected externally while allowing internal services to publish freely.
+
+### 2. Standardized Topics
+
+| Topic | Source | Description |
+|-------|--------|-------------|
+| `gitea-ci` | Gitea Actions | CI/CD build notifications |
+| `alertmanager-alerts` | Alertmanager | Prometheus critical/warning alerts |
+| `gatus` | Gatus | Service health status changes |
+| `flux` | Flux | GitOps reconciliation events |
+
+### 3. Alertmanager Integration
+
+Alertmanager is configured to forward alerts to ntfy using the built-in `tpl=alertmanager` template:
+
+```yaml
+receivers:
+  - name: ntfy-critical
+    webhookConfigs:
+      - url: "http://ntfy-svc.observability.svc.cluster.local/alertmanager-alerts?tpl=alertmanager&priority=urgent&tags=rotating_light"
+        sendResolved: true
+  - name: ntfy-warning
+    webhookConfigs:
+      - url: "http://ntfy-svc.observability.svc.cluster.local/alertmanager-alerts?tpl=alertmanager&priority=high&tags=warning"
+        sendResolved: true
+```
+
+Routes direct alerts based on severity:
+- `severity=critical` → `ntfy-critical` receiver
+- `severity=warning` → `ntfy-warning` receiver
+
+### 4. Discord Integration (Future)
+
+Discord integration will be implemented as a dedicated bridge service that:
+
+1. **Subscribes** to ntfy topics via SSE/WebSocket
+2. **Transforms** ntfy message format to Discord embed format
+3. **Forwards** to Discord webhook URL (stored in Vault at `kv/data/discord`)
+
+#### Design Options
+
+**Option A: Sidecar Container (Simple)**
+- Alpine container with curl/jq
+- Subscribes to ntfy JSON stream
+- Transforms and POSTs to Discord
+- Pros: Simple, no custom code
+- Cons: Shell script fragility, limited error handling
+
+**Option B: Dedicated Python Service (Recommended)**
+- Small Python service using `httpx` or `aiohttp`
+- Proper reconnection logic and error handling
+- Configurable topic-to-channel mapping
+- Health endpoint for monitoring
+- Pros: Robust, testable, maintainable
+- Cons: Requires building/publishing container image
+
+**Option C: ntfy Actions (Limited)**
+- Configure ntfy server with `upstream-base-url` or actions
+- Pros: Built into ntfy
+- Cons: ntfy doesn't natively support Discord webhook format
+
+#### Recommended Architecture (Option B)
+
+```
+┌─────────────────┐     ┌──────────────────┐     ┌─────────────┐
+│ CI/Alertmanager │────▶│      ntfy        │────▶│  ntfy App   │
+│ Gatus/Flux      │     │  (notification   │     │  (mobile)   │
+└─────────────────┘     │      hub)        │     └─────────────┘
+                        └────────┬─────────┘
+                                 │ SSE subscribe
+                                 ▼
+                        ┌──────────────────┐     ┌─────────────┐
+                        │ ntfy-discord-    │────▶│  Discord    │
+                        │ bridge           │     │  (webhook)  │
+                        └──────────────────┘     └─────────────┘
+```
+
+The bridge service would be a new repo (`ntfy-discord-bridge`) following the same patterns as other Python services in the homelab.
+
+## Consequences
+
+### Positive
+
+- **Single source of truth**: All notifications flow through ntfy
+- **Auth protection maintained**: External ntfy access requires Authentik auth
+- **Flexible routing**: Can subscribe to specific topics per destination
+- **Separation of concerns**: Discord bridge is independent, can be disabled without affecting ntfy
+
+### Negative
+
+- **Additional service**: Discord bridge adds operational overhead
+- **Latency**: Two-hop delivery (source → ntfy → Discord) adds minimal latency
+
+### Neutral
+
+- Topic naming must be documented and followed consistently
+- Discord webhook URL must be maintained in Vault
+
+## Implementation Checklist
+
+- [x] Standardize CI notifications to `gitea-ci` topic
+- [x] Configure Alertmanager → ntfy for critical/warning alerts
+- [ ] Create `ntfy-discord-bridge` repository
+- [ ] Implement bridge service with proper error handling
+- [ ] Add ExternalSecret for Discord webhook from Vault
+- [ ] Deploy bridge to observability namespace
+- [ ] Document topic-to-Discord-channel mapping
+
+## Related
+
+- ADR-0015: CI Notifications and Semantic Versioning
+- ADR-0020: Internal Registry for CI/CD