SSE Stream Resilience
Redis-backed stream management with heartbeat monitoring and completion recovery.
When to Use This Skill
- SSE streams can fail silently (client disconnects mid-stream)
- Completion events get lost and users never see results
- Need visibility into stream health
- Want to prevent resource leaks from abandoned streams
Core Concepts
The solution provides:
- Stream registry (track all active streams in Redis)
- Heartbeat monitoring (detect orphaned streams)
- Completion store (persist terminal events for recovery)
- Stream guardian (background cleanup process)
Client ←→ SSE Endpoint ←→ Stream Registry (Redis)
↓
Completion Store (Redis)
↓
Stream Guardian (Background)Implementation
TypeScript
// lib/sse/types.ts
export enum StreamState {
ACTIVE = 'active',
COMPLETED = 'completed',
FAILED = 'failed',
ORPHANED = 'orphaned',
}
export interface StreamMetadata {
streamId: string;
streamType: string;
userId: string;
startedAt: Date;
lastHeartbeat: Date;
state: StreamState;
metadata: Record<string, unknown>;
}
export interface CompletionData {
streamId: string;
terminalEventType: string;
terminalEventData: Record<string, unknown>;
completedAt: Date;
}
// lib/sse/stream-registry.ts
const STREAM_KEY_PREFIX = 'sse:stream:';
const ACTIVE_STREAMS_KEY = 'sse:active';
const STREAM_TTL = 3600; // 1 hour max lifetime
const STALE_THRESHOLD = 30; // 30 seconds = stale
export class StreamRegistry {
constructor(private redis: Redis) {}
async register(metadata: StreamMetadata): Promise<boolean> {
const streamKey = `${STREAM_KEY_PREFIX}${metadata.streamId}`;
if (await this.redis.exists(streamKey)) {
return false;
}
const pipeline = this.redis.pipeline();
pipeline.hset(streamKey, {
streamId: metadata.streamId,
streamType: metadata.streamType,
userId: metadata.userId,
startedAt: metadata.startedAt.toISOString(),
lastHeartbeat: metadata.lastHeartbeat.toISOString(),
state: metadata.state,
metadata: JSON.stringify(metadata.metadata),
});
pipeline.expire(streamKey, STREAM_TTL);
pipeline.zadd(ACTIVE_STREAMS_KEY, metadata.lastHeartbeat.getTime(), metadata.streamId);
await pipeline.exec();
return true;
}
async heartbeat(streamId: string): Promise<boolean> {
const streamKey = `${STREAM_KEY_PREFIX}${streamId}`;
const now = new Date();
if (!await this.redis.exists(streamKey)) {
return false;
}
const pipeline = this.redis.pipeline();
pipeline.hset(streamKey, 'lastHeartbeat', now.toISOString());
pipeline.zadd(ACTIVE_STREAMS_KEY, now.getTime(), streamId);
await pipeline.exec();
return true;
}
async unregister(streamId: string): Promise<boolean> {
const streamKey = `${STREAM_KEY_PREFIX}${streamId}`;
const userId = await this.redis.hget(streamKey, 'userId');
if (!userId) return false;
const pipeline = this.redis.pipeline();
pipeline.del(streamKey);
pipeline.zrem(ACTIVE_STREAMS_KEY, streamId);
await pipeline.exec();
return true;
}
async getStaleStreams(thresholdSeconds = STALE_THRESHOLD): Promise<StreamMetadata[]> {
const cutoff = Date.now() - (thresholdSeconds * 1000);
const staleIds = await this.redis.zrangebyscore(ACTIVE_STREAMS_KEY, 0, cutoff);
const streams: StreamMetadata[] = [];
for (const streamId of staleIds) {
const stream = await this.getStream(streamId);
if (stream && stream.state === StreamState.ACTIVE) {
streams.push(stream);
}
}
return streams;
}
async updateState(streamId: string, state: StreamState): Promise<boolean> {
const streamKey = `${STREAM_KEY_PREFIX}${streamId}`;
if (!await this.redis.exists(streamKey)) return false;
await this.redis.hset(streamKey, 'state', state);
return true;
}
}
// lib/sse/completion-store.ts
const COMPLETION_KEY_PREFIX = 'sse:completion:';
const COMPLETION_TTL = 300; // 5 minutes for recovery window
export class CompletionStore {
constructor(private redis: Redis) {}
async storeCompletion(data: CompletionData): Promise<void> {
const key = `${COMPLETION_KEY_PREFIX}${data.streamId}`;
await this.redis.hset(key, {
streamId: data.streamId,
terminalEventType: data.terminalEventType,
terminalEventData: JSON.stringify(data.terminalEventData),
completedAt: data.completedAt.toISOString(),
});
await this.redis.expire(key, COMPLETION_TTL);
}
async getCompletion(streamId: string): Promise<CompletionData | null> {
const key = `${COMPLETION_KEY_PREFIX}${streamId}`;
const data = await this.redis.hgetall(key);
if (!data.streamId) return null;
return {
streamId: data.streamId,
terminalEventType: data.terminalEventType,
terminalEventData: JSON.parse(data.terminalEventData || '{}'),
completedAt: new Date(data.completedAt),
};
}
}
// lib/sse/stream-guardian.ts
export class StreamGuardian {
private intervalId: NodeJS.Timeout | null = null;
constructor(
private registry: StreamRegistry,
private completionStore: CompletionStore,
private checkIntervalMs = 30000
) {}
start(): void {
if (this.intervalId) return;
this.intervalId = setInterval(
() => this.runCheck(),
this.checkIntervalMs
);
}
stop(): void {
if (this.intervalId) {
clearInterval(this.intervalId);
this.intervalId = null;
}
}
private async runCheck(): Promise<void> {
try {
const staleStreams = await this.registry.getStaleStreams();
for (const stream of staleStreams) {
await this.handleOrphanedStream(stream);
}
} catch (err) {
console.error('Stream Guardian error:', err);
}
}
private async handleOrphanedStream(stream: StreamMetadata): Promise<void> {
console.log(`Handling orphaned stream: ${stream.streamId}`);
await this.registry.updateState(stream.streamId, StreamState.ORPHANED);
}
}SSE Endpoint
// app/api/stream/[streamId]/route.ts
export async function GET(req: Request, { params }: { params: { streamId: string } }) {
const userId = req.headers.get('x-user-id')!;
const streamId = params.streamId;
// Check for existing completion (recovery)
const existingCompletion = await completionStore.getCompletion(streamId);
if (existingCompletion) {
return new Response(
`data: ${JSON.stringify({
type: existingCompletion.terminalEventType,
data: existingCompletion.terminalEventData,
})}\n\n`,
{ headers: { 'Content-Type': 'text/event-stream' } }
);
}
// Register new stream
await registry.register({
streamId,
streamType: 'generation',
userId,
startedAt: new Date(),
lastHeartbeat: new Date(),
state: StreamState.ACTIVE,
metadata: {},
});
const encoder = new TextEncoder();
let heartbeatInterval: NodeJS.Timeout;
const stream = new ReadableStream({
start(controller) {
controller.enqueue(
encoder.encode(`data: ${JSON.stringify({ type: 'connected', streamId })}\n\n`)
);
// Heartbeat every 15 seconds
heartbeatInterval = setInterval(async () => {
try {
await registry.heartbeat(streamId);
controller.enqueue(encoder.encode(`: heartbeat\n\n`));
} catch {}
}, 15000);
},
cancel() {
clearInterval(heartbeatInterval);
registry.unregister(streamId);
},
});
return new Response(stream, {
headers: {
'Content-Type': 'text/event-stream',
'Cache-Control': 'no-cache',
'X-Stream-Id': streamId,
},
});
}Client-Side Recovery
function useResilientSSE(streamId: string) {
const [status, setStatus] = useState<'connecting' | 'connected' | 'completed' | 'error'>('connecting');
const [data, setData] = useState<unknown>(null);
const reconnectAttempts = useRef(0);
useEffect(() => {
let eventSource: EventSource | null = null;
const connect = async () => {
// First, check for existing completion
try {
const recovery = await fetch(`/api/stream/${streamId}/recover`);
const result = await recovery.json();
if (result.status === 'completed') {
setStatus('completed');
setData(result.terminalEventData);
return;
}
} catch {}
// Connect to SSE
eventSource = new EventSource(`/api/stream/${streamId}`);
eventSource.onmessage = (event) => {
const parsed = JSON.parse(event.data);
if (parsed.type === 'completed' || parsed.type === 'failed') {
setStatus('completed');
setData(parsed.data);
eventSource?.close();
} else {
setData(parsed);
}
};
eventSource.onerror = () => {
eventSource?.close();
if (reconnectAttempts.current < 3) {
reconnectAttempts.current++;
setTimeout(connect, 1000 * reconnectAttempts.current);
} else {
setStatus('error');
}
};
};
connect();
return () => eventSource?.close();
}, [streamId]);
return { status, data };
}Best Practices
- Heartbeat every 15 seconds - Keeps stream alive and detects orphans
- Store completions for recovery - 5 minute window for client reconnection
- Background guardian process - Clean up orphaned streams automatically
- Client-side reconnection - Retry with exponential backoff
- Check for completion on connect - Recover missed terminal events
Common Mistakes
- No heartbeat mechanism (can't detect orphaned streams)
- Not storing completion data (lost terminal events)
- Missing recovery endpoint (clients can't recover)
- No background cleanup (resource leaks)
- Forgetting to unregister on clean disconnect
Related Patterns
- websocket-management - WebSocket alternative
- graceful-shutdown - Drain streams on shutdown
- checkpoint-resume - Track stream progress