# TDD Wave 8: WebSocket Event Broadcasting
**Status**: ✅ Complete
**Date**: 2026-02-19
**Focus**: Real-time event streaming from CheckpointService to WebSocket clients
---
## Overview
Wave 8 implements the missing piece of WebSocket functionality: **event broadcasting**. When checkpoints are created, restored, deleted, or compacted via the CheckpointService, connected WebSocket clients receive real-time notifications.
---
## Test Results
| 53 | Server broadcasts checkpoint events to subscribers | ✅ PASS |
**Previous Status**: This test was `#[ignore]` with a TODO comment.
**Current Status**: Fully implemented and passing.
---
## Implementation Details
### Architecture
```
┌─────────────────────┐
│ CheckpointService │
│ (emits events) │
└──────────┬──────────┘
│
│ subscribe(session_id)
│
▼
┌─────────────────────┐
│ Event Forwarder │
│ (per connection) │
│ │
│ ┌───────────────┐ │
│ │ Session A RX │──┼──► WebSocket Client 1
│ └───────────────┘ │
│ ┌───────────────┐ │
│ │ Session B RX │──┼──► WebSocket Client 2
│ └───────────────┘ │
└─────────────────────┘
```
### Key Changes
#### 1. Service Event Channel (service.rs)
Changed from `std::sync::mpsc` to `tokio::sync::mpsc` for async compatibility:
```rust
// Before
pub fn subscribe(&self, session_id: &SessionId) -> Result<mpsc::Receiver<CheckpointEvent>>
// After
pub fn subscribe(&self, session_id: &SessionId) -> Result<tokio::sync::mpsc::Receiver<CheckpointEvent>>
```
#### 2. Event Conversion (websocket.rs)
Added conversion from `CheckpointEvent` to `WebSocketEvent`:
```rust
impl WebSocketEvent {
pub fn from_checkpoint_event(event: &CheckpointEvent) -> Self {
match event {
CheckpointEvent::Created { checkpoint_id, session_id, .. } => {
Self::checkpoint_created(checkpoint_id.to_string(), session_id.to_string())
}
// ... other variants
}
}
}
```
#### 3. Per-Connection Event Forwarding (websocket.rs)
Each WebSocket connection spawns an event forwarding task that:
1. Manages subscriptions per session
2. Polls all subscribed receivers
3. Forwards events to the client's WebSocket channel
```rust
async fn handle_connection(...) {
// ... connection setup ...
// Channel for coordinating subscriptions
let (sub_tx, mut sub_rx) = mpsc::unbounded_channel::<SubscribeCommand>();
// Event forwarding task
let event_forward_task = tokio::spawn(async move {
let mut event_receivers: HashMap<SessionId, mpsc::Receiver<CheckpointEvent>> = HashMap::new();
loop {
tokio::select! {
// Handle new subscription requests
Some((session_id, notify_tx)) = sub_rx.recv() => {
match service_for_events.subscribe(&session_id) {
Ok(rx) => { event_receivers.insert(session_id, rx); }
// ...
}
}
// Poll all receivers for events
Some((session_id, event)) = poll_receivers(&mut event_receivers) => {
let ws_event = WebSocketEvent::from_checkpoint_event(&event);
// Send to client
}
}
}
});
}
```
#### 4. Subscription Confirmation
The `handle_subscribe` command now:
1. Adds session to client state
2. Sends subscription request to event task
3. Waits for confirmation before responding
```rust
async fn handle_subscribe(...) -> WebSocketResponse {
// ... parse session_id ...
// Channel to receive confirmation
let (notify_tx, mut notify_rx) = mpsc::unbounded_channel();
// Send subscription request
sub_tx.send((session_id, notify_tx))?;
// Wait for confirmation with timeout
match timeout(Duration::from_secs(5), notify_rx.recv()).await {
Ok(Some(event)) if event.event_type == "subscribed" => {
WebSocketResponse::success(...)
}
_ => WebSocketResponse::error(...)
}
}
```
---
## Event Types
| `checkpoint_created` | `service.checkpoint()` | `checkpoint_id`, `session_id`, `timestamp` |
| `checkpoint_restored` | `service.restore()` | `checkpoint_id`, `session_id`, `timestamp` |
| `checkpoint_deleted` | `service.execute(Delete)` | `checkpoint_id`, `session_id`, `timestamp` |
| `checkpoints_compacted` | `service.execute(Compact)` | `session_id`, `remaining`, `timestamp` |
---
## Protocol Example
### Client Subscribes
```json
// Request
{
"id": "sub-1",
"method": "subscribe",
"params": { "session_id": "uuid-here" }
}
// Response
{
"id": "sub-1",
"success": true,
"result": {
"subscribed": true,
"session_id": "uuid-here"
}
}
```
### Server Broadcasts Event
```json
{
"event_type": "checkpoint_created",
"data": {
"checkpoint_id": "uuid-here",
"session_id": "uuid-here",
"timestamp": "2026-02-19T12:34:56Z"
}
}
```
---
## Test Coverage
The test verifies:
1. Multiple clients can subscribe to the same session
2. Creating a checkpoint broadcasts to all subscribers
3. Both clients receive identical event data
4. Event type is correctly "checkpoint_created"
---
## Performance Considerations
- **Event Buffering**: Each subscription uses a bounded channel (100 events) to prevent memory exhaustion
- **Non-blocking Delivery**: `try_send` is used for best-effort delivery (drops events if client is slow)
- **Polling Interval**: 10ms sleep between polls to prevent CPU spinning
- **Per-Connection Tasks**: Each client gets their own event forwarding task
---
## Future Enhancements
1. **Selective Subscriptions**: Filter by event type (only Created events)
2. **Replay**: Request historical events on subscription
3. **ACKs**: Client acknowledges receipt for guaranteed delivery
4. **Batching**: Batch rapid-fire events to reduce network overhead
---
## Files Modified
| `src/websocket.rs` | Event forwarding task, WebSocketEvent conversion, subscription handling |
| `src/service.rs` | Changed to tokio channels for async compatibility |
| `tests/websocket_tests.rs` | Enabled `test_websocket_event_broadcast` |
| `tests/integration_tests.rs` | Updated `test_checkpoint_streaming` to use async/tokio |
---
## Summary
Wave 8 completes the WebSocket API by implementing real-time event broadcasting. Clients can now subscribe to sessions and receive immediate notifications when checkpoints change, enabling reactive UIs and real-time monitoring.