feat: Update CLAUDE.md files, add docs

2026-02-08 08:36:29 +01:00
parent 9542d704e4
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+# Real-Time Notifications Architecture Decision
+
+**Status:** POSTPONED
+**Created:** 2026-02-08
+**Last Updated:** 2026-02-08
+
+---
+
+## Context
+
+The Legal Consent Hub requires real-time notification delivery to users. When events occur (e.g., form submissions, approvals, comments), affected users should receive notifications immediately without refreshing the page.
+
+### Constraint: Stateless Backend
+
+The backend **must be stateless** for horizontal scaling. Multiple backend instances run behind a load balancer, and any instance must be able to handle any request.
+
+---
+
+## Current Implementation
+
+**Location:** `src/main/kotlin/com/betriebsratkanzlei/legalconsenthub/notification/`
+
+### Files
+
+| File | Purpose |
+|------|---------|
+| `SseNotificationEmitterRegistry.kt` | In-memory registry of SSE connections |
+| `NotificationEventListener.kt` | Listens for `NotificationCreatedEvent` and triggers SSE delivery |
+| `NotificationCreatedEvent.kt` | Spring application event for new notifications |
+| `NotificationService.kt` | Creates notifications and publishes events |
+| `NotificationController.kt` | REST endpoints including SSE stream endpoint |
+
+### How It Works
+
+1. Frontend opens SSE connection to `/api/notifications/stream`
+2. Backend stores `SseEmitter` in `SseNotificationEmitterRegistry` (in-memory `CopyOnWriteArrayList`)
+3. When notification created, `NotificationService` publishes `NotificationCreatedEvent`
+4. `NotificationEventListener` receives event, uses registry to send SSE to matching users
+5. Registry filters by `userId`, `organizationId`, and `roles` to route notifications
+
+### Current Code (SseNotificationEmitterRegistry.kt)
+
+```kotlin
+@Service
+class SseNotificationEmitterRegistry {
+    // IN-MEMORY STORAGE - THIS IS THE PROBLEM
+    private val connections = CopyOnWriteArrayList<SseConnectionContext>()
+
+    fun registerEmitter(userId: String, organizationId: String, userRoles: List<String>): SseEmitter {
+        val emitter = SseEmitter(TIMEOUT_MS)
+        val context = SseConnectionContext(emitter, userId, organizationId, userRoles)
+        connections.add(context)
+        // ... cleanup handlers
+        return emitter
+    }
+
+    fun sendToUser(userId: String, organizationId: String, event: SseEmitter.SseEventBuilder) {
+        connections.filter { it.userId == userId && it.organizationId == organizationId }
+            .forEach { it.emitter.send(event) }
+    }
+
+    // Also: sendToRoles(), sendToOrganization()
+}
+```
+
+---
+
+## The Problem
+
+**In-memory storage breaks with multiple instances.**
+
+```
+┌─────────────────────────────────────────────────────────────┐
+│                      Load Balancer                          │
+└─────────────────────────────────────────────────────────────┘
+              │                              │
+              ▼                              ▼
+┌─────────────────────────┐    ┌─────────────────────────┐
+│      Instance A         │    │      Instance B         │
+│                         │    │                         │
+│  connections = [        │    │  connections = [        │
+│    User1-Emitter,       │    │    User2-Emitter,       │
+│    User3-Emitter        │    │    User4-Emitter        │
+│  ]                      │    │  ]                      │
+└─────────────────────────┘    └─────────────────────────┘
+```
+
+**Scenario:**
+1. User1 connects via SSE → routed to Instance A → emitter stored in A's memory
+2. User2 connects via SSE → routed to Instance B → emitter stored in B's memory
+3. User1 creates notification for User2
+4. Instance A handles the request, tries to send SSE to User2
+5. **FAILURE:** User2's emitter is in Instance B, not Instance A
+6. User2 never receives the notification
+
+---
+
+## Options
+
+### Option 1: Redis Pub/Sub
+
+**How it works:**
+- When notification created, publish event to Redis channel
+- All backend instances subscribe to this channel
+- Each instance forwards matching events to its local SSE connections
+
+```
+┌──────────────┐     publish      ┌─────────────┐
+│  Instance A  │ ───────────────► │    Redis    │
+│  (creates    │                  │   Pub/Sub   │
+│  notification)                  └─────────────┘
+└──────────────┘                        │
+                                        │ subscribe (all instances)
+                    ┌───────────────────┼───────────────────┐
+                    ▼                   ▼                   ▼
+             ┌──────────┐        ┌──────────┐        ┌──────────┐
+             │Instance A│        │Instance B│        │Instance C│
+             │(send to  │        │(send to  │        │(send to  │
+             │local SSE)│        │local SSE)│        │local SSE)│
+             └──────────┘        └──────────┘        └──────────┘
+```
+
+| Pros | Cons |
+|------|------|
+| Low latency (~1-5ms) | Requires Redis infrastructure |
+| Battle-tested pattern | Additional operational complexity |
+| Spring has good support (`spring-boot-starter-data-redis`) | Another service to monitor/maintain |
+| Can reuse Redis for caching later | |
+
+**Implementation effort:** ~2-4 hours
+
+**Dependencies to add:**
+```kotlin
+implementation("org.springframework.boot:spring-boot-starter-data-redis")
+```
+
+---
+
+### Option 2: PostgreSQL LISTEN/NOTIFY
+
+**How it works:**
+- Use Postgres native pub/sub mechanism
+- On notification insert, trigger sends NOTIFY
+- All backend instances LISTEN on the channel
+- Each instance forwards to its local SSE connections
+
+```sql
+-- Trigger function
+CREATE FUNCTION notify_new_notification() RETURNS trigger AS $$
+BEGIN
+  PERFORM pg_notify('new_notification', row_to_json(NEW)::text);
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+-- Trigger on insert
+CREATE TRIGGER notification_insert_trigger
+AFTER INSERT ON notifications
+FOR EACH ROW EXECUTE FUNCTION notify_new_notification();
+```
+
+| Pros | Cons |
+|------|------|
+| No new infrastructure (already have Postgres) | Slightly higher latency (~5-20ms) |
+| Simple conceptually | Requires manual JDBC listener setup |
+| Database is source of truth | Less common pattern, fewer examples |
+| Works even if notification created by SQL directly | Connection pool considerations |
+
+**Implementation effort:** ~3-5 hours
+
+**No additional dependencies** (uses existing JDBC driver)
+
+---
+
+### Option 3: Message Queue (RabbitMQ / Kafka)
+
+**How it works:**
+- Publish notification events to queue/topic
+- All instances consume from the queue
+- Each instance forwards to local SSE connections
+
+| Pros | Cons |
+|------|------|
+| Very reliable, durable messages | Significant new infrastructure |
+| Can handle high throughput | Overkill for notification use case |
+| Good for future event-driven architecture | Higher operational complexity |
+| Supports complex routing | Longer setup time |
+
+**Implementation effort:** ~4-8 hours
+
+**Dependencies:**
+```kotlin
+// RabbitMQ
+implementation("org.springframework.boot:spring-boot-starter-amqp")
+
+// OR Kafka
+implementation("org.springframework.kafka:spring-kafka")
+```
+
+---
+
+### Option 4: Client-Side Polling (Remove SSE)
+
+**How it works:**
+- Remove SSE entirely
+- Frontend polls `/api/notifications?since={timestamp}` every N seconds
+- Completely stateless, no server-side connection tracking
+
+```typescript
+// Frontend polling
+setInterval(async () => {
+  const notifications = await fetch(`/api/notifications?since=${lastCheck}`)
+  lastCheck = Date.now()
+  // Update UI with new notifications
+}, 10000) // Poll every 10 seconds
+```
+
+| Pros | Cons |
+|------|------|
+| Simplest solution | Not real-time (10-30s delay) |
+| Truly stateless | Higher database load |
+| No new infrastructure | More API requests |
+| Easy to implement | Worse UX for time-sensitive notifications |
+| Works through all proxies/firewalls | |
+
+**Implementation effort:** ~1-2 hours (mostly removing SSE code)
+
+---
+
+### Option 5: WebSocket with External STOMP Broker
+
+**How it works:**
+- Use Spring WebSocket with STOMP protocol
+- External message broker (RabbitMQ/ActiveMQ) handles message routing
+- Clients subscribe to user-specific topics
+
+| Pros | Cons |
+|------|------|
+| Full-duplex communication | Requires message broker |
+| Rich protocol (STOMP) | More complex than SSE |
+| Spring has excellent support | WebSocket connection management |
+| Can do request-response patterns | Some proxies don't support WebSocket well |
+
+**Implementation effort:** ~4-6 hours
+
+---
+
+## Comparison Matrix
+
+| Criteria | Redis Pub/Sub | Postgres NOTIFY | Message Queue | Polling | WebSocket+STOMP |
+|----------|---------------|-----------------|---------------|---------|-----------------|
+| Latency | ~1-5ms | ~5-20ms | ~5-50ms | 10-30s | ~1-5ms |
+| New Infrastructure | Redis | None | RabbitMQ/Kafka | None | RabbitMQ/ActiveMQ |
+| Implementation Effort | Low | Medium | High | Very Low | Medium |
+| Operational Complexity | Low | Low | High | None | Medium |
+| Scalability | Excellent | Good | Excellent | Limited | Excellent |
+| Spring Support | Excellent | Manual | Excellent | N/A | Excellent |
+
+---
+
+## Recommendation
+
+**For this project, the top candidates are:**
+
+1. **PostgreSQL LISTEN/NOTIFY** - Already have Postgres, no new infra, acceptable latency
+2. **Redis Pub/Sub** - If Redis is added for caching anyway, use it for this too
+3. **Polling** - If real-time isn't critical, simplest solution
+
+**Avoid:** Message Queue (overkill), WebSocket+STOMP (unnecessary complexity)
+
+---
+
+## Why Postponed
+
+1. **Current state works for single instance** - Development and testing can proceed
+2. **No immediate production multi-instance deployment** - Decision can wait
+3. **Infrastructure decisions pending** - Unknown if Redis will be used for other purposes
+4. **Need to evaluate real-time requirements** - How critical is sub-second notification delivery?
+
+---
+
+## Questions to Answer Before Deciding
+
+1. Will Redis be used for caching or session storage?
+2. How critical is real-time delivery? (Is 10-30s polling acceptable?)
+3. What is the expected notification volume?
+4. What is the deployment target? (Kubernetes, Docker Swarm, VMs?)
+
+---
+
+## How to Resume This Work
+
+When ready to implement multi-instance support:
+
+1. Answer the questions above
+2. Choose an option based on infrastructure and requirements
+3. Implementation steps:
+   - Keep `SseNotificationEmitterRegistry` for local emitter management
+   - Add cross-instance event distribution layer
+   - Modify `NotificationEventListener` to publish to chosen channel
+   - Add subscriber that receives events and calls registry's send methods
+
+**Key insight:** The local SSE connection management stays the same. Only the event distribution mechanism changes.
+
+---
+
+## Related Files
+
+- `SseNotificationEmitterRegistry.kt` - Current in-memory implementation
+- `NotificationEventListener.kt` - Event handling logic
+- `NotificationCreatedEvent.kt` - Event payload
+- `NotificationService.kt` - Notification creation
+- `NotificationController.kt` - SSE endpoint
+
+---
+
+## References
+
+- [Spring SSE Documentation](https://docs.spring.io/spring-framework/reference/web/webmvc/mvc-ann-async.html#mvc-ann-async-sse)
+- [Redis Pub/Sub with Spring](https://docs.spring.io/spring-data/redis/reference/redis/pubsub.html)
+- [PostgreSQL LISTEN/NOTIFY](https://www.postgresql.org/docs/current/sql-notify.html)