node: add initial standard checks

node-health/src/checks.ts

@@ -0,0 +1,26 @@
+import { IntervalHistogram, monitorEventLoopDelay } from 'node:perf_hooks';
+import { ReadinessDetail, ReadinessFunction, ReadinessStatus } from './readiness';
+
+let hist: IntervalHistogram | null = null;
+
+/**
+ * Builds a readiness check function that monitors event loop lag.
+ * @param degradedMs - The threshold in milliseconds above which the status is 'degraded' (default 200 ms).
+ * @returns A ReadinessFunction that checks event loop lag.
+ */
+export const buildEventLoopLagCheck = (degradedMs: number = 200): ReadinessFunction => {
+	if (!hist) {
+		hist = monitorEventLoopDelay({ resolution: 10 });
+		hist.enable();
+	}
+
+	return (): ReadinessDetail => {
+		const lag = hist!.mean / 1e6; // Convert from nanoseconds to milliseconds
+		const status: ReadinessStatus = lag < degradedMs ? 'ok' : 'degraded';
+		return {
+			name: 'event-loop-lag',
+			status,
+			message: `Event loop lag is ${lag.toFixed(2)} ms`,
+		};
+	};
+};