Perf: coalesce audit-log writes per request

src/middleware/audit.ts

@@ -0,0 +1,44 @@
+/**
+ * Audit logging middleware
+ *
+ * Wires up request-scoped audit-log buffering so that every admin action
+ * during a single request is coalesced into a single batch write rather than
+ * written individually. This reduces I/O when the audit backend is a remote
+ * database.
+ *
+ * Usage (in app.ts or a router):
+ *   app.use(auditBufferMiddleware);
+ */
+
+import type { Request, Response, NextFunction } from 'express';
+import { auditLogService } from '../services/audit/index.js';
+
+/**
+ * Express middleware that:
+ * 1. Starts an audit-log buffer at the beginning of every request.
+ * 2. Flushes the buffer after the response is sent (on `finish`).
+ * 3. Discards the buffer if the response status >= 500 (server error paths
+ *    should not commit partial audit trails).
+ *
+ * Only admin endpoints need buffering, but the overhead of starting/stopping
+ * a buffer is negligible, so it is safe to apply globally.
+ */
+export function auditBufferMiddleware(
+  _req: Request,
+  res: Response,
+  next: NextFunction,
+): void {
+  auditLogService.startBuffer();
+
+  res.on('finish', () => {
+    if (res.statusCode >= 500) {
+      // Server error — discard the partial buffer since the action may not
+      // have completed reliably.
+      auditLogService.discardBuffer();
+    } else {
+      auditLogService.flush();
+    }
+  });
+
+  next();
+}