Heartbeat and nicer status line

server.js

@@ -18,6 +18,7 @@ const DATA_DIR = process.env.DATA_DIR || './data';
 const PAGE_SIZE_MAX = 48;
 const UPLOAD_MAX_BYTES = 5 * 1024 * 1024;
 const REQUEST_MAX_BYTES = 6 * 1024 * 1024;
+const SSE_HEARTBEAT_MS = 25_000;
 const events = new Set();
 const DISCOVERY_ROUTES = new Set([
   '/robots.txt',
@@ -135,11 +136,19 @@ const server = http.createServer(async (req, res) => {
       res.writeHead(200, {
         'Content-Type': 'text/event-stream; charset=utf-8',
         'Cache-Control': 'no-cache, no-transform',
-        Connection: 'keep-alive'
+        Connection: 'keep-alive',
+        'X-Accel-Buffering': 'no'
       });
+      res.write('retry: 5000\n');
       res.write('event: ready\ndata: {}\n\n');
       events.add(res);
-      req.on('close', () => events.delete(res));
+      const heartbeat = setInterval(() => {
+        res.write(': keep-alive\n\n');
+      }, SSE_HEARTBEAT_MS);
+      req.on('close', () => {
+        clearInterval(heartbeat);
+        events.delete(res);
+      });
       return;
     }