sse-streaming

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.

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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

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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().encode(sseData));
          }

          // Signal completion
          controller.enqueue(
            new TextEncoder().encode(`data: ${JSON.stringify({ done: true })}\n\n`)
          );
        } catch (error) {
          controller.enqueue(
            new TextEncoder().encode(`data: ${JSON.stringify({ error: 'Stream failed', done: true })}\n\n`)
          );
        } finally {
          controller.close();
        }
      },
    });

    return {
      status: 200,
      headers: {
        'Content-Type': 'text/event-stream',
        'Cache-Control': 'no-cache',
        'Connection': 'keep-alive',
        'X-Accel-Buffering': 'no', // Disable proxy buffering
      },
      body: stream,
    };
  },
});