Scale display player by switching from SSE to polling

app/routes/api.py

@@ -29,10 +29,14 @@ def _enforce_and_touch_display_session(display: Display, sid: str | None):
         return True, None
 
     cutoff = datetime.utcnow() - timedelta(seconds=SESSION_TTL_SECONDS)
+    # Cleanup old sessions. Avoid committing if nothing was deleted (saves write locks on SQLite).
+    deleted = (
         DisplaySession.query.filter(
             DisplaySession.display_id == display.id,
             DisplaySession.last_seen_at < cutoff,
         ).delete(synchronize_session=False)
+    )
+    if deleted:
         db.session.commit()
 
     existing = DisplaySession.query.filter_by(display_id=display.id, sid=sid).first()

app/templates/display/player.html

@@ -38,9 +38,6 @@
       let idx = 0;
       let timer = null;
 
-      let es = null;
-      let esRetryMs = 1000;
-
       async function fetchPlaylist() {
         const qs = sid ? `?sid=${encodeURIComponent(sid)}` : '';
         const res = await fetch(`/api/display/${token}/playlist${qs}`, { cache: 'no-store' });
@@ -115,37 +112,17 @@
           // keep retrying; if a slot frees up the display will start automatically.
         }
 
-        // Open live event stream: when server signals a change, reload playlist immediately.
-        connectEvents();
-
-        // Fallback refresh (in case SSE is blocked by a proxy/network): every 5 minutes.
+        // Poll for updates instead of SSE.
+        // This scales better for 100s of displays because it avoids long-lived HTTP
+        // connections (which otherwise tie up gunicorn sync workers).
+        // Default: check every 20s (can be overridden via ?poll=seconds).
+        const pollParam = parseInt(new URLSearchParams(window.location.search).get('poll') || '', 10);
+        const pollSeconds = Number.isFinite(pollParam) && pollParam > 0 ? pollParam : 20;
         setInterval(async () => {
-          try {
-            playlist = await fetchPlaylist();
-            if (!stage.firstChild) {
-              idx = 0;
-              next();
-            }
-          } catch(e) {
-            clearStage();
-            setNotice(e && e.message ? e.message : 'Unable to load playlist.');
-          }
-        }, 300000);
-      }
-
-      function connectEvents() {
-        if (isPreview) return; // preview shouldn't consume a slot / keep a long-lived connection
-
-        try { if (es) es.close(); } catch(e) { /* ignore */ }
-
-        const qs = sid ? `?sid=${encodeURIComponent(sid)}` : '';
-        es = new EventSource(`/api/display/${token}/events${qs}`);
-
-        es.addEventListener('changed', async () => {
           try {
             const newPlaylist = await fetchPlaylist();
 
-            // If content changed, restart from the beginning.
+            // Restart if something changed.
             const oldStr = JSON.stringify(playlist);
             const newStr = JSON.stringify(newPlaylist);
             playlist = newPlaylist;
@@ -154,21 +131,16 @@
               next();
             }
 
-            esRetryMs = 1000; // reset backoff on success
-          } catch(e) {
-            // leave current playback; we'll retry via reconnect handler
+            // If player is blank (e.g. after a temporary error), kick it.
+            if (!stage.firstChild) {
+              idx = 0;
+              next();
             }
-        });
-
-        es.onerror = () => {
-          try { es.close(); } catch(e) { /* ignore */ }
-          es = null;
-
-          // Exponential backoff up to 30s
-          const wait = esRetryMs;
-          esRetryMs = Math.min(30000, Math.floor(esRetryMs * 1.7));
-          setTimeout(connectEvents, wait);
-        };
+          } catch(e) {
+            clearStage();
+            setNotice(e && e.message ? e.message : 'Unable to load playlist.');
+          }
+        }, pollSeconds * 1000);
       }
 
       start();