docs: add an SSE example

Author: jonathanong

docs/guide.md

@@ -16,6 +16,7 @@
   - [Async operations](#async-operations)
   - [Debugging Koa](#debugging-koa)
   - [HTTP2](#http2)
+  - [Server-Side Events](#server-side-events)
 
 ## Writing Middleware
 
@@ -267,3 +268,48 @@ const serverOptions = {
 
 const server = http2.createSecureServer(serverOptions, onRequestHandler);
 ```
+
+## Server-Side Events
+
+An example of using server-side events with Koa:
+
+```js
+import PassThrough from 'stream'
+import OpenAI from 'openai'
+import Koa from 'koa'
+
+const openai = new OpenAI({
+  apiKey: process.env.OPENAI_API_KEY,
+})
+
+const app = new Koa()
+
+app.use(async (ctx, next) => { // TODO: use your favorite routing library
+  const { message } = await ctx.request.json() // this example uses koa-body-parsers
+
+  const stream = await client.chat.completions.create({
+    model: 'gpt-4o-mini',
+    messages: [{ role: 'user', content: messages }],
+    stream: true,
+  })
+
+  ctx.response.type = 'text/event-stream'
+  const body = ctx.body = new PassThrough()
+
+  // streaming needs to be handled in a separate event loop
+  ;(async () => {
+    for await (const chunk of stream) {
+      const content = chunk.choices[0]?.delta?.content
+      if (!content) continue
+
+      body.write(`data: ${JSON.stringify({
+        delta: {
+          content: chunk.choices[0].delta.content || ''
+        }
+      })}}\n\n`)
+
+      body.end()
+    }
+  })().catch((err) => app.emit('error', err))
+})
+```