Going Faster with Balrog

Go Faster is a broad initiative at Mozilla that is focused on shipping things to users much faster than the current 6 week cycle. One important part of this project is having a mechanism to make Firefox aware of updates they need or may want to download. This is nothing entirely new of course - we've been shipping updates to users since Firefox 1.5 - but with Go Faster we will be updating bits and pieces of Firefox at a time rather than always updating the entire install. In this post I'm going to outline these new types of updates that we've identified, and talk about how things will work in the Glorious Future.

A Primer on Updates

Firefox updates work on a "pull" system, meaning Firefox regularly queries the update server (Balrog) to ask if there's an update available. For example, my Firefox is currently polling for updates by making a GET request to this URL:


All of the information in that URL is mapped against a set rules in Balrog, and eventually points to a single release. If that release is newer than the incoming one (based on the incoming version and buildid), Balrog returns the information necessary for the client to update to it:

    <update type="minor" displayVersion="41.0" appVersion="41.0" platformVersion="41.0" buildID="20150917150946" detailsURL="https://www.mozilla.org/en-GB/firefox/41.0/releasenotes/">
        <patch type="complete" URL="http://download.mozilla.org/?product=firefox-41.0build3-complete&os=linux64&lang=en-GB" hashFunction="sha512" hashValue="ea0757069363287f67659d8b7d42e0ac6c74a12ce8bd3c7d3e9ad018d03cd6f4640529c270ed2b3f3e75b11320e3a282ad9476bd93b0f501a22d1d9cb8884961" size="48982398"/>

It's important to note that Balrog only contains metadata about the update. The actual payloads of the updates are hosted on CDN networks.

New Types of Updates

We've identified three different new types of updates that we'll be implementing as part of Go Faster. They are:

  • System Addons: These are core (aka required) parts of Firefox that happen to be implemented as Addons.
  • Security Policy: This is a medium sized piece of JSON that instructs NSS about special security policies to enforce for various websites.
  • Optional Features: These are optional parts of Firefox that may be implemented as Addons or other means.

Each one of these will be implemented as an additional update request to Balrog (we may collapse these into a single request later). Eg, Firefox will look for new System Addons by making a GET request to an URL such as:


The responses will vary a bit depending on the type of update. More on that below.

System Addons

Seeing as Firefox can't function without them, System Addons may seem like a contradiction at first. The advantages are quite clear though: with them, we can ship updates to self contained pieces of Firefox at a substantionally faster rate. Shipping an update to all of Firefox takes nearly 24 hours (when we're moving as fast as we can); shipping an update to a System Addon could take as little as minutes.

Although they are implemented as Addons, we can't simply ship them through the AMO. Because Firefox cannot function without them we must ship them in the installers and full updates that happen every 6 weeks. This has the nice side effect of minimizing dependency problems -- we won't run into a case where Firefox updates but System Addons don't, which could cause incompatibilities. In between the 6 week cycles Firefox will poll Balrog for updates to System Addons and apply them as they become available. This graph may show this more clearly:

As you can see, Firefox 50.0 can be assumed to have any of Fizz 1.0, 1.1, 1.2, 1.3, or 2.0, while Firefox 51.0 is known to only have Fizz 2.0 (but may receive newer versions later).

When Firefox pings Balrog for System Addon updates, the response will look something like this:

        <addon id="fizz@mozilla.org" URL="http://download.cdn.mozilla.net/fizz-1.1.xpi" hashFunction="sha512" hashValue="abcdef123456" version="1.1"/>
        <addon id="pop@mozilla.org" URL="http://download.cdn.mozilla.net/pop-2.5.xpi" hashFunction="sha512" hashValue="abcdef123456" version="2.5"/>
        <addon id="bam@mozilla.org" URL="http://download.cdn.mozilla.net/bam-3.4.xpi" hashFunction="sha512" hashValue="abcdef123456" version="3.4"/>

Firefox will compare the list against its currently installed versions and update anything that's out of date. The exact details on where System Addons will live on disk are still being ironed out.

Security Policy

Every version of Firefox ships with the most up-to-date set of security policies that were available when it was built. However, these policies are updated more frequently than we ship, and it's crucial that we keep them up to date to keep our users safe. As with System Addons, Firefox will regularly ping Balrog to check for updated security policies. When one is found, Firefox downloads it from Kinto, which will serve it an incremental update to its security policies. The details of this process have been outlined in much more detail by the Cloud Services team.

The Balrog response for these updates is extremely simple, and simply contains a version that Firefox passes along to Kinto:

        <setting id="security" lastModified="129386427328"/>

Optional Features

These are parts of Firefox that are not core to the browser, but may be useful to subsets of users. For example: We currently ship a ton of hyphenation dictionaries as part of Firefox for Android. These are locale-specific, so only one ever gets used for each user. We can also distribute opt-in features that not everyone wants or needs, eg: Developer Tools may be a good candidate (there are no plans to do so at this time though).

Optional features may also be implemented in various ways. Hyphenation dictionaries are simple zip files, while something like Developer Tools would likely be an Addon. They will not be included in Firefox installers or update packages. Instead Firefox will regularly query Balrog to see what packages may be available to it. Some things may automatically install based on the user's environment (eg: hyphentation dictionaries for your locale), while other things may require opt-in (eg: optional features).

Balrog responses are not yet set in stone for these, but Kinto is likely to be involved, so the response may end up being similar to the one above for Security Policy updates.


While System Addons, Security Policy, and Optional Features overlap in some areas, each has its own unique combination of requirements. The chart below summarizes these:
Required? Shipped in Installer? Payload Type Payload Location
System Addons Yes Yes Addons CDN
Security Policy Yes Yes JSON Kinto
Optional Features No No Anything Kinto


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