Taskcluster is not one singular entity that runs a script with output in a pretty interface or a github hook listener, but rather a set of decoupled interfaces that enables us to build various test infrastructures while optimizing for cost, performance and reliability. The focus of this post is Linux. I will have more information how this works for OSX/Window soon.

Some History

Mozilla has quite a few different code bases, most depend on gecko (the heart of Firefox and FirefoxOS). Getting your project hooked up to our current CI infrastructure usually requires a multi-team process that takes days or more. Historically, simply merging projects into gecko was easier than having external repositories that depend on gecko, which our current CI cannot easily support.

It is critical to be able to see in one place (TBPL) that all the projects depend on gecko are working. Today TBPL current this process is tightly coupled to our buildbot infrastructure (which together make up our current CI). If you really care about your project not breaking when a change lands in gecko, you really only have one option: hosting your testing infrastructure under buildbot (which feeds TBPL).

Where Taskcluster comes in

Treeherder resolves the tight coupling problem by separating the reporting from the test running process. This enables us to re-imagine our workflow and how it's optimized. We can run tests anywhere using any kind of utility/library assuming it gives us the proper hooks (really just logs and some revision information) to plug results into our development workflow.

A high level workflow with taskcluster looks like this:

You submit some code (this can be patch or a pull request, etc...) to a "scheduler" ( I have started on one for gaia ) which submits a set of tasks. Each task is run inside a docker container the container's image is specified as part of your task. This means anything you can imagine running on linux you can directly specify in your container (no more waiting for vm reimaging, etc...) this also means we directly control the resources that container uses (less variance in test) AND if something goes wrong you can download the entire environment that test ran on locally to debug it.

As tasks are completed the task cluster queue emits events over AMQP (think pulse) so anyone interested in the status of tests, etc.. can hook directly into this... This enables us to post results as they happen directly to treeherder.

The initial taskcluster provisions AWS spot nodes on demand (we have it capped to a fixed number right now) so during peaks we can burst to an almost unlimited number of nodes. During idle times workers shut themselves down to reduce costs. We have additional plans for different clouds (and physical hardware on open stack).

Each component can be easily replaced (and multiple types of workers and provisioners can be added on demand. Jonas Finnemann Jensen has done a awesome job documenting how taskcluster works in the docs at the API level.

What the future looks like

My initial plan is to hook everything up for gaia the FirefoxOS frontend. This will replace our current travis CI setup.

As pull requests come in we will run tests on taskcluster and report status to both treeherder and github (the beloved github status api). The ability to hook up new types of tests from the tree itself (and test new types from the tree itself) will continue on in the form of a task template (another blog post coming). Developers can see the status of their tests from treeherder.

Code landing in master follows the same practice and results will report into a gaia specific treeherder view.

Most importantly immediately after treeherder is launched we can run all gaia testing on the same exact infrastructure for both gaia and gecko commits Jonas Sicking (b2g overload) has some great ideas about locking gecko <-> gaia versions to reduce another kind of failure which occurs when developing against the ever changing landscape of gecko / gaia commits.

When is the future? We have implemented the "core" of taskcluster already and have the ability to run tests. By the end of the month (March) we will have the capability to replace the entire gaia workflow with taskcluster.

Why not X CI solution

Building a brand new CI solution is non-trivial why are we doing this?

  • To leverage LXC containers (docker): One of the big problems we hit when trying to debug test failures is the vairance of testing locally and remotely. With LXC containers you can download the entire container (the entire environment which your test runs in) and run it with the same cpu/memory/swap/filesystem as it would run remotely.

  • On demand scaling. We have (somewhat predictable) bursts throughout the day and the ability to spin up (and down) on demand is required to keep up with our changing needs throughout the day.

  • Make in tree configuration easy. Pull requests + in tree configuration enable developers to quickly iterate on tests and testing infrastructure

  • Modular extensible components with public facing APIs. Want run tasks to do things other then test/build or report to something other then treeherder? We have or will build an api for that.

    Hackability is imporant... The parts you don't want to solve (running aws nodes, keeping them up, pricing them, etc...) are solved for you so you can focus on building the next great mozilla related thing (better bisection tools, etc...).

  • More flexibility to test/deploy optimizations... We have something like a compute year of tests and 10-30+ minute chunks of testing is normal. We need to iterate on our test infrastructure quickly to try to reduce this where possible with CI changes.

Here are a few potential alternatives below... I list out the pros & cons of each from my perspective (and a short description of each).

Travis [hosted]

TravisCI is an awesome [free] open source testing service that we use for many of our smaller projects.

Travis works really well for the 90% webdev usecase. Gaia does not fit well into that use case and gecko does so even less.


  • Dead simple setup.
  • Iterate on test frameworks, etc... on every pull request without any issue.
  • Nice simple UI which reports live logging.
  • Adding tests and configuring tests is trivial.


  • Difficult to debug failures locally.
  • No public facing API for creating jobs.
  • No build artifacts on pull requests.
  • Cannot store arbitrarily long logs (this is only an issue for open source IIRC).
  • On demand scaling.

Buildbot [build on top of it]

We currently use buildbot at scale thousands~ of machines for all gecko testing on multiple platforms. If you are using firefox it was built by our buildbot setup.

(NOTE: This is a critique of how we currently use buildbot not the entire project). If I am missing something or you think a CI solution could fit the bill contact me!


  • We have it working at a large scale already.


  • Adding tests and configuring tests is fairly difficult and involves long lead times.
  • Difficult to debug failures locally.
  • Configuration files live outside of the tree.
  • Persistent connection master/slave model.
  • Its one monolithic project which is difficult to replace components of.
  • Slow rollout of new machine requirements & configurations.


We are using Jenkins for our on device testing.


  • Easy to configure jobs from the UI (decent ability to do configuration yourself).
  • Configuration (by default) does not live in the tree.
  • Tons of plugins (with varying quality).


  • By default difficult to debug failures locally.
  • Persistent connection master/slave model.
  • Configuration files live outside of the tree.

Drone.io [hosted/not hosted]

Drone.io recently open sourced... It's docker based and shows promise. Out of all the options above it looks the closest to the to what we want for linux testing.

I am going to omit the Pros/Cons here the basics look good for drone and it requires some more investigation. Some missing things here are:

  • A long term plan for supporting multiple operating systems.
  • A public api for scheduling tasks/jobs.
  • On demand scaling.