node-syntheticalert

Drive a synthetic alert metric from Node.js, so a Triple Pat check-in timer can verify your alerting pipeline end to end. Works with Prometheus and OpenTelemetry.

Why

A broken alerting pipeline looks exactly like a healthy system. No alerts might mean nothing is wrong, or it might mean your alerting is down, and your alerting system is the one thing that cannot alert you about itself.

This library provides a time-based callback to drive a synthetic alert metric. You register the callback as a gauge in your existing metrics setup, alert on the gauge like any other metric, and route the alert to a Triple Pat check-in timer. Every delivered alert then becomes a check-in, and every firing is another fire drill for the whole path from metric to notification. If the check-ins ever stop, your alerting pipeline is broken, and the Triple Pat app raises an alarm through a separate channel to tell you so. An example alert rule and Alertmanager route are below.

Usage

npm install @triplepat/syntheticalert

The library has no dependencies and starts no timers. It is a single function that answers the question "should the synthetic alert be firing right now?", and you hand it to your metrics client as a gauge callback. It is an ES module for Node 22.18 and later, where CommonJS code can require() it too.

Prometheus

Alongside your existing Prometheus client setup. The official client, @prometheus-io/client, was previously published as prom-client, and the same code works with both:

import { Gauge } from '@prometheus-io/client';
import { syntheticAlert } from '@triplepat/syntheticalert';

const alert = syntheticAlert(); // throws only for invalid options
new Gauge({
  name: 'triplepat_synthetic_alert',
  help:
    'Set to 1 when the synthetic alert should fire and 0 otherwise. Alert on ' +
    'this metric and route the alert to a Triple Pat check-in timer to ' +
    'continuously test your alerting pipeline.',
  collect() {
    this.set(alert());
  },
});

OpenTelemetry

The same callback serves OpenTelemetry. The OTel-to-Prometheus exporter turns the dotted metric name into triplepat_synthetic_alert:

const gauge = meter.createObservableGauge('triplepat.synthetic.alert', {
  description: 'Set to 1 when the synthetic alert should fire and 0 otherwise.',
});
gauge.addCallback((result) => result.observe(alert()));

The schedule

Each firing holds the gauge at 1 for exactly 10 minutes. The silent gap between firings, from the end of one to the start of the next, is memoryless: exponentially distributed with a mean of one hour.

Memoryless gaps make the firings an attempt at a Poisson process, which cannot synchronize with cron jobs or scrape cycles, and which by the PASTA theorem sees your pipeline as it typically is rather than at some special moment.

As a nod to practicality the gap is truncated. It is never less than 10 minutes, so the alert visibly resolves between firings, and never more than two hours, so the check-in timer can be sized. The truncation pulls the realized mean gap down to about 49 minutes and makes the process only roughly Poisson. If you need the PASTA property and can tolerate wider variation in start times, set a lower min and a higher max, then size the timer for the larger max. That recovers most of the Poisson behavior; for the last few percent, use a mean much longer than the firing duration, since the interval between firing starts is the firing plus the gap.

The schedule advances lazily, at scrape time, from Date.now(). If nobody scrapes for a while, the next scrape replays every transition it missed, so the process stays honest whatever your scrape interval.

There is no magic here: one line is a serviceable substitute, firing for the first ten minutes of every hour:

new Gauge({
  name: 'triplepat_synthetic_alert',
  help: 'A synthetic alert metric, firing ten minutes per hour.',
  collect() {
    this.set(new Date().getMinutes() < 10 ? 1 : 0);
  },
});

But that version fires at the top of every hour, exactly when your cron jobs are doing something interesting. The memoryless schedule cannot synchronize with anything, and that is the point of the library. If you want a deterministic schedule anyway, the snippet above is all you need.

Options

All durations are numbers, in milliseconds, of at least 1.

Option	Effect	Default
meanInterval	Mean silent gap between firings	3_600_000 (1 hour)
minInterval	Lower bound on the silent gap	600_000 (10 min)
maxInterval	Upper bound on the silent gap	7_200_000 (2 hours)
firingDuration	How long each firing holds the gauge at 1	600_000 (10 min)

const alert = syntheticAlert({ meanInterval: 30 * 60 * 1000, maxInterval: 60 * 60 * 1000 });

The firing duration must be shorter than the mean interval, and the min and max intervals must bracket the mean. Bad options throw when syntheticAlert() is called, never at scrape time: a RangeError for a number out of range and a TypeError for anything that is not a number. Setting all three intervals equal is allowed: every gap is then exactly that long and the schedule is periodic, which is pointless in production but handy for deterministic debugging. The defaults are exported as DEFAULT_MEAN_INTERVAL, DEFAULT_MIN_INTERVAL, DEFAULT_MAX_INTERVAL, and DEFAULT_FIRING_DURATION.

Alert on the metric

groups:
  - name: synthetic
    rules:
      - alert: SyntheticAlert
        expr: triplepat_synthetic_alert == 1
        labels:
          severity: synthetic
        annotations:
          summary: Synthetic alert exercising the alerting pipeline.

Route the alert to a check-in timer

Create a check-in timer at Triple Pat, then point the alert at it. Prefer email delivery: mail transfer agents queue, retry, and try every backend listed in DNS, so a check-in email is more likely to arrive than a single webhook request to a single destination. Send to the same timer at both the .com and .net addresses. The two domains are served by independent DNS providers, so if one zone cannot be resolved the other address still delivers, and extra simultaneous check-ins are harmless. Merge this into your existing Alertmanager config (the fragment assumes you already have a default receiver and working smtp_* defaults):

route:
  routes:
    - matchers:
        - alertname="SyntheticAlert"
      receiver: triplepat
      group_wait: 0s
receivers:
  - name: triplepat
    email_configs:
      - to: YOUR-TIMER-UUID@checkin.triplepat.com
        send_resolved: false
      - to: YOUR-TIMER-UUID@checkin.triplepat.net
        send_resolved: false

send_resolved: false keeps the resolve notification from counting as an extra check-in, so each firing checks in when it starts and not again when it resolves.

If you cannot send email, replace the triplepat receiver above with this webhook receiver instead. Alertmanager rejects a configuration that defines the same receiver name twice:

receivers:
  - name: triplepat
    webhook_configs:
      - url: https://triplepat.com/api/v1/checkin/YOUR-TIMER-UUID
        send_resolved: false

Sizing the timer

Set the check-in timer's interval to at least max interval + firing duration + your alerting pipeline's latency. With the defaults (silent gaps of at most two hours, plus 10 minutes of firing), a three-hour timer is comfortable.

License

Apache-2.0. See LICENSE.