Reading data from a beacon

While Boomerang excels at collecting Real User Monitoring performance metrics, you'll probably want to do something with the data that it collects.

What can you do with the data?

Beacon results back to your server

For most cases you'll want to send the collected performance data back to your server or to the cloud so you can analyze it in aggregate.

The first thing you need to do is configure a url that beacon data will be sent to.

The beacon URL can be configured via the beacon_url parameter of BOOMR.init:

<script src="boomerang.js"></script>
  beacon_url: ""

beacon_url should be a HTTP(s) endpoint that accepts beacon data encoded in the query string (via <IMG> GET requests) and application/x-www-form-urlencoded form data (via POST requests from XMLHttpRequest or navigator.sendBeacon()).

Beacon Parameters

Boomerang and each plugin adds its own parameters to the beacon. Please refer to those documentation pages for details.

Backend Servers

There are several open-source projects that can receive and analyze Boomerang data:

Here are some articles for putting it all together:

Protecting the Beacon from Abuse

There are two types of beacon abuse you may need to protect against:

  • Denial of Service (DoS) attacks
  • Fake beacons that do not originate from a page you own

Denial of Service (DoS) attacks

There's nothing you can do in JavaScript to prevent Denial of Service (DoS) attacks, but you can configure your server to rate limit beacons originating from a single IP. You typically shouldn't be receiving beacons faster than a user can navigate through your website. However, you may need to allow for multiple users originating from the same proxy IP address.

You'll also want to do some operating system/web server level configuration to detect abusive access patterns. The majority of these are beyond the scope of this document, but we have a few tips on building your back-end to protect you from an attack.

One recommendation is to have a lightweight web server running as your beacon endpoint, and have this server quickly log the request and immediately respond without processing the data. You can send a HTTP 204 No Content response and close the connection so there is minimal overhead.

Once the data is logged, you can periodically batch process and analyze the incoming data.

More references for preventing DoS attacks:

Fake beacons that do not originate from a page you own

The most common reason for this kind of abuse is that someone liked your page design and copied it to their own server, including the boomerang JavaScript -- only they didn't update the beacon_url, so it still beacons to your server. You probably don't want these beacons.

The easiest way to fix this is to just check the HTTP referrer of all requests and block any that don't come from your own domain. This works for the clueless abuser case, but not for the intentional abuser.

The intentional abuser is someone who will try to exploit URLs on your site to see if they can get something out of it. What they try isn't really important -- there's only one legitimate way of using your beacon, and you should block all other uses. The best way to do this is through a nonce or a crumb.

This is a string that is valid for one use only. It probably includes the current timestamp and a validity period as part of its hash. You generate it on every page request and add it to Boomerang using BOOMR.addVar.

On your beacon endpoint, you then validate the nonce before accepting the beacon.

You can either include this nonce in your (non-cached) HTML page, or, fetch it from your server via a XMLHttpRequest.

The nonce also doesn't protect you from someone pulling the beacon out of your page (with a valid nonce), then modifying the beacon parameters and sending it off to your server. Protecting against this requires you to sign the parameters in the beacon, but this isn't something that you can do in JavaScript in a way that an attacker couldn't replicate.

Read beacon data from JavaScript

You can get a notification of each beacon before it is sent by subscribing to the before_beacon event:

BOOMR.subscribe('before_beacon', function(beaconData) {
  if (beaconData.u.indexOf("/some/page")) {
    // take some action based on the page

  // you can still add data to the beacon at this point
  beaconData.someData = 1;

You can also get a notification of each beacon after it was sent by subscribing to the beacon event:

BOOMR.subscribe('beacon', function(beaconData) {
  // remove beacon data that was only relevant for a single beacon