Node.js — alert on rising event-loop lag
Node app stalls under CPU-blocking operations; user latency creeps up. Want an endpoint that exposes the live event-loop-lag value.
Recipe
// /healthz endpoint that exposes event-loop lag in ms
const express = require('express');
const { monitorEventLoopDelay } = require('perf_hooks');
const h = monitorEventLoopDelay({ resolution: 20 });
h.enable();
const THRESHOLD_MS = parseInt(process.env.LAG_THRESHOLD_MS || '50', 10);
const app = express();
app.get('/healthz', (_req, res) => {
const p99 = Math.round(h.percentile(99) / 1e6); // ns → ms
h.reset();
res.type('text/plain');
res.send(p99 >= THRESHOLD_MS ? `lag ${p99}` : `ok ${p99}`);
});
app.listen(8080);Same thing in Enterno.io
Point an Enterno HTTP monitor with body-keyword "ok". Multi-region check (Pro+) tells you whether it is a global stall or a single-node problem.
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