bw-bmbarga / bw-monitoring

Express JS middleware to help push metrics and checks to Prometheus

bw-monitoring

bw-monitoring is Express middleware that exposes a collection of standardised endpoints for monitoring infrastructure (e.g. Prometheus) to consume.

npm install --save bw-monitoring

bw-monitoring exports a function getMiddleware which returns an express middleware to pass to expressApp.use. This adds standard routes /healthz, /metricz, /checkz.

var monitoring = require('bw-monitoring');
expressApp.use(monitoring.getMiddleware());

/checkz

By default /checkz returns a 404 status code. It can be configured to perform arbitrary checks of application dependencies using bwMonitoring.addCheck().

monitoring.addCheck

var postgresCheck = {
  name: 'postgres',
  check: (ok, warning, critical, unknown) => {
    pgConnection.query.raw('SELECT version();', [])
      .then(ok)
      .catch(critical);
  }
}
monitoring.addCheck(postgresCheck);

With this check added the endpoint will return 200 with a body showing the result of any checks in prometheus compatible data format. E.g...

postgres 0

/livez

The /livez endpoint is the same as /checkz, and checks are added the same way (via bwMonitoring.addCheck(). However if any check fails it will return a 500 status code. The content of the body will also be only the failed check

/healthz

By default /healthz returns a 200 status code. This can be used as a sensible default in a Kubernetes readinessProbe, and all it does is check that the express server is serving correctly. It checks no upstream dependencies for health, although it can be configured to do this using addReadinessCheck.

monitoring.addReadinessCheck

To add specific checks to a readinessProbe (e.g. can connect to postgres) use the addReadinessCheck method. addReadinessCheck takes the exact same style of check as addCheck, meaning we can re-use the postgres check from above if desirable. E.g...

monitoring.addReadinessCheck(postgresCheck);

NB: Do not configure explicit readiness checks unless you have a tier infront of your application that can serve appropriate error pages (e.g nginx). If all you have is dumb TCP/HTTP load-balancing, then keep forwarding requests to the application so it can deal with errors itself when an upstream dependency is failing.

/metricz

By default /metricz returns a 404 status code.

monitoring.addMetrics

It can be configured with a function to query for Prometheus style metrics and will expose them in a compatible data format.

var prometheus = require('prom-client');
var c = new Counter('my_counter');
setTimeout(c.inc, 200);
monitoring.addMetrics(() => prometheus.register.metrics());