Healthchecks

Express middleware that runs health checks on your application. Allows you to manage a list of healthchecks as plain text files, keep them in source control next to the application. Provides a single endpoint you can access from a mobile device, or from a monitoring service.

What and Why?

A health check is a simple ping to a resource of your web application that checks that your application is up and running, responding to network requests, and behaving correctly.

It can be as simple as pinging the home page of a web site, checking that the page includes the company's name in the title.

If you have a complex application, there are multiple things that can break independently, and so you want a good health coverage by checking multiple resources (see What Should I Check?).

If your application has got one page that's accessing the database, and another page that just storing requests in a queue, you want to check both[1].

Whether you're using a service like Pingdom or internal tool like Nagios, if you store your checks there, funny thing is they never get updated when you roll out new features.

You want checks to be part of the code base, in source control, right next to the application code that's getting checked, where it can be versioned and code reviewed.

And that's what this module does. It lets you write your checks as a plain text file that lives in the same repository as your application.

And it gives you a single endpoint that you can open in a browser, to see a list of all passed or failed checks. The same endpoint you can also use with a monitoring service like Pingdom or Nagios.

Checks File

The checks file lists one or more resources to check, and the expected content of each resource (which may be empty).

For example:

# Check a web page
/                       My Amazing App

# Check stylesheets and scripts
/stylesheets/index.css  .body
/scripts/index.js       "use strict"

# Check the image exist
/images/logo.png

All URLs are relative to the server on which they are deployed, but must consist of at least an absolute pathname. You can include the hostname and protocol if you need to, for example, if your tests are mounted at http://example.com and you want to test static.example.com and test the HTTPS admin page:

# Check a different hostname than example.com
//static.example.com/logo.png

# Check with a different protocol and hostname
https://admin.example.com           Admin Dashboard

The expected content is matched literally against the body of the HTTP response. Only 2xx responses are considered successful (however, redirects are followed).

Usage

Install:

npm install --save healthchecks

Include the checks file with your web server, then configure the middleware to read the checks file, for example:

const healthchecks = require('healthchecks');

// This file contains all the checks
const CHECKS_FILE = __dirname + '/checks';

// Mount the middleware at /_healthchecks
server.use('/_healthchecks', healthchecks(CHECKS_FILE));

Now point your monitoring at http://example.com/_healthchecks.

You can also open this page with your browser to see a list of passing and failed tests.

Note this endpoint is publicly accessible. You can use another middleware to add access control, or add this feature and send us a pull request.

You can initialize the middleware with the checks file name, or with an object containing the following options:

• filename -- The name of the checks file
• onFailed -- Called with array of failed checks
• timeout -- Timeout slow responses

You can specify the timeout in milliseconds or as a string, e.g. "3s" for 3 seconds.

Each failed check reported to onFailed is an object with the following properties:

• url -- The absolute URL
• reason -- One of 'error', 'timeout', 'statusCode' or 'body'
• error -- Connection or timeout error
• timeout -- True if failed due to timeout
• statusCode -- HTTP status code (if no error)
• body -- Response body

For convenience, the value of the reason property is the name of one of the other properties. Also, when you call toString() you get a URL with the reason, e.g. "http://example.com => statusCode".

For example:

const options = {
  filename:   CHECKS_FILE,
  timeout:    '5s',    // 5 seconds, can also pass duration in milliseconds
  onFailed:   function(checks) {
    checks.forEach(function(check) {
      log('The following check failed:', check.url, 'reason:', check.reason);
      // ... or ...
      log('The following check failed: %s', check);
    });
  }
};
server.use('/_healthchecks', healthchecks(options);

What Should I Check?

Anything that can go wrong will go wrong.

-- Murphy's law

If two parts of your application can fail independently, you want to check both.

If you have static and dynamic page, you want to check both.

If different pages use different database servers, you want to check them all.

If some page uses a caching servers, you want to check that as well.

If another page uses a 3rd party API, also check that.

If your page is composed of multiple modules that can fail independently (say shopping cart and product list), you want to check each module.

If something can fail that's not an HTML page, you want to check that as well.

Stylesheets? Check. Client-side scripts? Checks. Images? Checks.

If they are pre-processed, you want to check what was generated.

If it's served by a 3rd party CDN, don't skip this check.

If your application dynamically generates links (to pages, CSS, JS), check those as well.

You can have too many checks, but most likely your problem is you don't have enough!

License

MIT License Copyright (c) 2014 Broadly Inc