feat: Implement Telegram alerts for catalog updates and failures

server/watch.js

@@ -0,0 +1,43 @@
+import { getCatalog, catalogFailure } from "./source.js";
+import { syncIndex } from "./directory.js";
+import { checkCatalog, checkSource } from "./alerts.js";
+
+// The server checks the university's catalog by itself, not only when a request asks: a
+// republished document is parsed within minutes, the clients' next refresh gets it, and the
+// owner hears about problems (see alerts.js). An unchanged catalog costs two listings.
+
+const INTERVAL = Number(process.env.PSU_WATCH_INTERVAL_MS) || 5 * 60 * 1000;
+
+/** For /api/health. */
+export const watchStatus = { intervalMinutes: INTERVAL / 60000, lastAt: null, lastError: null };
+
+let running = null;
+/** One check: the catalog (refreshed), the index of a changed catalog, the messages. */
+export function watchOnce() {
+  // A slow university must not stack checks on top of each other.
+  running ||= check().finally(() => {
+    running = null;
+  });
+  return running;
+}
+
+async function check() {
+  try {
+    const catalog = await getCatalog(true);
+    // A failed crawl answers with the saved catalog: nothing new to read or report then.
+    if (!catalog.stale) await checkCatalog(await syncIndex(catalog), catalog);
+    watchStatus.lastError = catalog.stale ? catalog.error || null : null;
+  } catch (error) {
+    watchStatus.lastError = error.message;
+  }
+  await checkSource(catalogFailure());
+  watchStatus.lastAt = new Date().toISOString();
+}
+
+let timer = null;
+/** Checks every few minutes (PSU_WATCH_INTERVAL_MS) for as long as the process runs. */
+export function startWatch() {
+  if (timer) return;
+  timer = setInterval(watchOnce, INTERVAL);
+  timer.unref();
+}