Background Checks

In some cases, you might not want to manually trigger checks with every performCheck call. Instead, you may prefer to run checks on a consistent schedule. This is where background checks come into play.

Setting Up Background Checks

You can configure Medicus to automatically run checks at a specified interval. By setting the backgroundCheckInterval option, you can ensure a consistent schedule for performing health checks without needing to call performCheck manually.

import { Medicus } from 'medicus';

const medicus = new Medicus({
  // Automatically calls performCheck every 60 seconds
  backgroundCheckInterval: 60_000
});

After setting up the background check interval, you can retrieve the result of the last performed check using getLastCheck(). If the check has never been run, it will return null.

const check = medicus.getLastCheck(true);

// Prints `null` if the check has never been run
// or the details of the last check
console.log(check);

With background checks enabled, Medicus will automatically run health checks on a consistent interval, but you can still manually trigger a check whenever necessary using performCheck().

Listening for Background Checks

You can also set up an event listener to monitor when a background check is performed. The listener can be either sync or async and will be triggered every time a check is completed.

import { Medicus } from 'medicus';

const medicus = new Medicus({
  backgroundCheckInterval: 60_000,
  onBackgroundCheck(check) {
    console.log('Background check performed', check);
  }
});

The above example will log the check object every time a background check is performed, i.e., every 60 seconds.

If an error is thrown inside the event listener, it will be captured and sent to the errorLogger, if one has been configured.

setTimeout vs setInterval

Under the hood the medicus uses the setTimeout method to perform its pooled background checks. The choice is based on the fact that we do not want to add additional pressure to the system.

In fact, it is known that setInterval will call repeatedly at the scheduled time regardless of whether the previous call ended or not, and if the server is already under load, this will likely increase the problem, because those setInterval calls will start piling up. setTimeout, on the other hand, is called only once and does not cause the mentioned problem.

One note to consider is that because the two methods are not identical, the timer function is not guaranteed to run at exactly the same rate when the system is under pressure or running a long-running process.