sse-streaming

name	sse-streaming
description	POST-based Server-Sent Events streaming for Azure Functions — HTTP streaming, chunked response parsing, reconnection
tier	extended
applyTo	**/*sse*,**/*streaming*,**/*server-sent*,**/*real-time*
metadata	{"version":"1.0.0","category":"web-development","tags":["sse","streaming","azure-functions","http-streaming","readablestream","real-time"],"activation":{"triggers":["SSE","streaming","server-sent events","HTTP streaming","ReadableStream","chunked response","real-time"],"context":"Working on real-time streaming from Azure Functions to client"},"prerequisites":[]}
currency	2026-04-22T00:00:00.000Z

SSE Streaming

POST-based Server-Sent Events pattern for Azure Functions. Solves the gap where native EventSource (GET-only) can't send request bodies, and Azure Static Web Apps don't proxy WebSocket to the API layer.

---

Why POST-Based SSE

Option	SWA API Support?	POST body?	Limitation
WebSocket	No	N/A	SWA doesn't proxy WebSocket to Functions
EventSource	Yes	GET only	Can't send context in request body
POST + ReadableStream	Yes	Yes	Recommended pattern

---

Azure Functions Setup

Enable HTTP Streaming

// host.json
{
  "version": "2.0",
  "extensions": {
    "http": {
      "enableHttpStream": true
    }
  }
}

Streaming Function Pattern

import { app, HttpRequest, HttpResponseInit, InvocationContext } from '@azure/functions';

app.http('stream-response', {
  methods: ['POST'],
  authLevel: 'anonymous', // SWA handles auth via EasyAuth
  route: 'stream',
  handler: async (req: HttpRequest, context: InvocationContext): Promise<HttpResponseInit> => {
    const payload = await req.json();

    const stream = new ReadableStream({
      async start(controller) {
        try {
          // Stream from Azure OpenAI or any async source
          const aiStream = await getAIStream(payload);

          for await (const chunk of aiStream) {
            const sseData = `data: ${JSON.stringify({ text: chunk, done: false })}\n\n`;
            controller.enqueue(new TextEncoder().(sseData));
          }

          
          controller.(
             ().()
          );
        }  (error) {
          controller.(
             ().()
          );
        }  {
          controller.();
        }
      },
    });

     {
      : ,
      : {
        : ,
        : ,
        : ,
        : , 
      },
      : stream,
    };
  },
});

---

Client Consumption

POST + ReadableStream Pattern

interface SSEMessage {
  text?: string;
  done: boolean;
  error?: string;
}

async function streamResponse(
  payload: unknown,
  onChunk: (text: string) => void,
  onComplete: () => void,
  onError: (error: string) => void,
): Promise<void> {
  const response = await fetch('/api/stream', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify(payload),
  });

  if (!response.ok) {
    onError(`HTTP ${response.status}: ${response.statusText}`);
    return;
  }

  const reader = response.body!.getReader();
  const decoder = new TextDecoder();
  let buffer = '';

  while (true) {
     { done, value } =  reader.();
     (done) ;

    buffer += decoder.(value, { :  });

    
     messages = buffer.();
    buffer = messages.() || ; 

     ( msg  messages) {
       (!msg.()) ;
       json = msg.(); 

       {
         :  = .(json);
         (parsed.) { (parsed.); ; }
         (parsed.) { (parsed.); }
         (parsed.) { (); ; }
      }  {
        
      }
    }
  }

  ();
}

React Hook

function useStreamedResponse() {
  const [text, setText] = useState('');
  const [isStreaming, setIsStreaming] = useState(false);
  const [error, setError] = useState<string | null>(null);

  const stream = useCallback(async (payload: unknown) => {
    setText('');
    setIsStreaming(true);
    setError(null);

    await streamResponse(
      payload,
      (chunk) => setText(prev => prev + chunk),
      () => setIsStreaming(false),
      (err) => { setError(err); setIsStreaming(false); },
    );
  }, []);

  return { text, isStreaming, error, stream };
}

---

Cancellation with AbortController

async function streamWithAbort(
  payload: unknown,
  signal: AbortSignal,
  onChunk: (text: string) => void,
): Promise<void> {
  const response = await fetch('/api/stream', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify(payload),
    signal,
  });

  const reader = response.body!.getReader();
  signal.addEventListener('abort', () => reader.cancel());

  // ... same parsing logic
}

// Usage
const controller = new AbortController();
streamWithAbort(payload, controller.signal, onChunk);
controller.abort(); // To cancel

---

SWA Timeout Handling

Azure Static Web Apps has a 45-second API timeout. For long operations, use multi-phase streaming:

// Instead of one long stream, report progress in phases
app.http('long-operation', {
  methods: ['POST'],
  handler: async (req, context) => {
    const stream = new ReadableStream({
      async start(controller) {
        const send = (data: object) =>
          controller.enqueue(new TextEncoder().encode(`data: ${JSON.stringify(data)}\n\n`));

        send({ phase: 'step-1', progress: 0.1 });
        const result1 = await doStep1();

        send({ phase: 'step-2', progress: 0.5 });
        const result2 = await doStep2(result1);

        send({ phase: 'complete', progress: 1.0, result: result2 });
        controller.close();
      },
    });

    return { body: stream, headers: { :  } };
  },
});

---

Error Recovery

Retry Strategy

async function streamWithRetry(
  payload: unknown,
  onChunk: (text: string) => void,
  maxRetries: number = 2,
): Promise<void> {
  let lastError: string | undefined;

  for (let attempt = 0; attempt <= maxRetries; attempt++) {
    try {
      await streamResponse(payload, onChunk, () => {}, (err) => { throw new Error(err); });
      return;
    } catch (error) {
      lastError = error instanceof Error ? error.message : 'Unknown error';
      if (attempt < maxRetries) {
        await new Promise(resolve => setTimeout(resolve, 1000 * (attempt + 1)));
      }
    }
  }

  throw new Error(`Stream failed after ${maxRetries + 1} attempts: ${lastError}`);
}

---

Activation Patterns

Trigger	Response
"SSE", "streaming", "server-sent events"	Full skill activation
"Azure Functions stream", "HTTP streaming"	Azure Functions Setup section
"ReadableStream", "chunked response"	Client Consumption section
"abort", "cancel stream"	Cancellation section
"SWA timeout", "45 second"	SWA Timeout Handling section