How Tos

All of this equipment is here to help you get your work done more efficiently. However, learning how task-graphs are generated is probably not the work you are interested in doing. This section should help you accomplish some of the more common changes to the task graph with minimal fuss.


If you cannot accomplish what you need with the information provided here, please consider whether you can achieve your goal in a different way. Perhaps something simpler would cost a bit more in compute time, but save the much more expensive resource of developers’ mental bandwidth. Task-graph generation is already complex enough!

If you want to proceed, you may need to delve into the implementation of task-graph generation. The documentation and code are designed to help, as are the authors - hg blame may help track down helpful people.

As you write your new transform or add a new kind, please consider the next developer. Where possible, make your change data-driven and general, so that others can make a much smaller change. Document the semantics of what you are changing clearly, especially if it involves modifying a transform schema. And if you are adding complexity temporarily while making a gradual transition, please open a new bug to remind yourself to remove the complexity when the transition is complete.

Hacking Task Graphs

The recommended process for changing task graphs is this:

  1. Find a recent decision task on the project or branch you are working on, and download its parameters.yml artifact. Alternately, you can simply take note of the artifact URL, or just the decision task’s task-id. This file contains all of the inputs to the task-graph generation process. Its contents are simple enough if you would like to modify it, and it is documented in Parameters.

  2. Run one of the mach taskgraph subcommands (see TaskGraph Mach Command) to generate a baseline against which to measure your changes, passing the parameters you found in the previous step. For example:

    ./mach taskgraph tasks --json -p parameters.yml > old-tasks.json
    ./mach taskgraph tasks --json -p url/to/parameters.yml > old-tasks.json
    ./mach taskgraph tasks --json -p task-id=<task-id> > old-tasks.json
  3. Make your modifications under taskcluster/.

  4. Run the same mach taskgraph command, sending the output to a new file, and use diff to compare the old and new files. Make sure your changes have the desired effect and no undesirable side-effects.

  5. When you are satisfied with the changes, push them to try to ensure that the modified tasks work as expected.

Hacking Actions

If you are working on an action task and wish to test it out locally, use the ./mach taskgraph test-action-callback command:

./mach taskgraph test-action-task \
    --task-id I4gu9KDmSZWu3KHx6ba6tw --task-group-id sMO4ybV9Qb2tmcI1sDHClQ \
    -p parameters.yml --input input.yml \

This invocation will run the hello world callback with the given inputs and print any created tasks to stdout, rather than actually creating them.

Common Changes

Changing Test Characteristics

First, find the test description. This will be in taskcluster/ci/*/tests.yml, for the appropriate kind (consult Task Kinds). You will find a YAML stanza for each test suite, and each stanza defines the test’s characteristics. For example, the chunks property gives the number of chunks to run. This can be specified as a simple integer if all platforms have the same chunk count, or it can be keyed by test platform. For example:

        linux64/debug: 10
        default: 8

The full set of available properties is in taskcluster/taskgraph/transform/tests/ Some other commonly-modified properties are max-run-time (useful if tests are being killed for exceeding maxRunTime) and treeherder-symbol.


Android tests are also chunked at the mozharness level, so you will need to modify the relevant mozharness config, as well.

Adding a Test Suite

To add a new test suite, you will need to know the proper mozharness invocation for that suite, and which kind it fits into (consult Task Kinds).

Add a new stanza to taskcluster/ci/<kind>/tests.yml, copying from the other stanzas in that file. The meanings should be clear, but authoritative documentation is in taskcluster/taskgraph/transform/tests/ should you need it. The stanza name is the name by which the test will be referenced in try syntax.

Add your new test to a test set in test-sets.yml in the same directory. If the test should only run on a limited set of platforms, you may need to define a new test set and reference that from the appropriate platforms in test-platforms.yml. If you do so, include some helpful comments in test-sets.yml for the next person.

Greening Up a New Test

When a test is not yet reliably green, configuration for that test should not be landed on integration branches. Of course, you can control where the configuration is landed! For many cases, it is easiest to green up a test in try: push the configuration to run the test to try along with your work to fix the remaining test failures.

When working with a group, check out a “twig” repository to share among your group, and land the test configuration in that repository. Once the test is green, merge to an integration branch and the test will begin running there as well.

Adding a New Task

If you are adding a new task that is not a test suite, there are a number of options. A few questions to consider:

  • Is this a new build platform or variant that will produce an artifact to be run through the usual test suites?
  • Does this task depend on other tasks? Do other tasks depend on it?
  • Is this one of a few related tasks, or will you need to generate a large set of tasks using some programmatic means (for example, chunking)?
  • How is the task actually excuted? Mozharness? Mach?
  • What kind of environment does the task require?

Armed with that information, you can choose among a few options for implementing this new task. Try to choose the simplest solution that will satisfy your near-term needs. Since this is all implemented in-tree, it is not difficult to refactor later when you need more generality.

Existing Kind

The simplest option is to add your task to an existing kind. This is most practical when the task “makes sense” as part of that kind – for example, if your task is building an installer for a new platform using mozharness scripts similar to the existing build tasks, it makes most sense to add your task to the build kind. If you need some additional functionality in the kind, it’s OK to modify the implementation as necessary, as long as the modification is complete and useful to the next developer to come along.

Tasks in the build kind generate Firefox installers, and the test kind will add a full set of Firefox tests for each build task.

New Kind

The next option to consider is adding a new kind. A distinct kind gives you some isolation from other task types, which can be nice if you are adding an experimental kind of task.

Kinds can range in complexity. The simplest sort of kind uses the transform loader to read a list of jobs from the jobs key, and applies the standard job and task transforms:

implementation: taskgraph.task.transform:TransformTask
   - taskgraph.transforms.job:transforms
   - taskgraph.transforms.task:transforms
   - ..your job description here..

Job descriptions are defined and documented in taskcluster/taskgraph/transforms/job/

Custom Kind Loader

If your task depends on other tasks, then the decision of which tasks to create may require some code. For example, the test kind iterates over the builds in the graph, generating a full set of test tasks for each one. This specific post-build behavior is implemented as a loader defined in taskcluster/taskgraph/loader/

A custom loader is useful when the set of tasks you want to create is not static but based on something else (such as the available builds) or when the dependency relationships for your tasks are complex.

Custom Transforms

Most loaders apply a series of “transforms” that start with an initial human-friendly description of a task and end with a task definition suitable for insertion into a Taskcluster queue.

Custom transforms can be useful to apply defaults, simplifying the YAML files in your kind. They can also apply business logic that is more easily expressed in code than in YAML.

Transforms need not be one-to-one: a transform can produce zero or more outputs for each input. For example, the test transforms perform chunking by producing an output for each chunk of a given input.

Ideally those transforms will produce job descriptions, so you can use the existing job and task transforms:

   - taskgraph.transforms.my_stuff:transforms
   - taskgraph.transforms.job:transforms
   - taskgraph.transforms.task:transforms

Try to keep transforms simple, single-purpose and well-documented!

Custom Run-Using

If the way your task is executed is unique (so, not a mach command or mozharness invocation), you can add a new implementation of the job description’s “run” section. Before you do this, consider that it might be a better investment to modify your task to support invocation via mozharness or mach, instead. If this is not possible, then adding a new file in taskcluster/taskgraph/transforms/jobs with a structure similar to its peers will make the new run-using option available for job descriptions.

Something Else?

If you make another change not described here that turns out to be simple or common, please include an update to this file in your patch.

Schedule a Task on Try

There are two methods for scheduling a task on try.

The first method is a command line string called try syntax which is passed into the decision task via the commit message. An example try syntax might look like:

try: -b o -p linux64 -u mochitest-1 -t none

This gets parsed by taskgraph.try_option_syntax:TryOptionSyntax and returns a list of matching task labels. For more information see the TryServer wiki page.

The second method uses a checked-in file called try_task_config.json which lives at the root of the source dir. The format of this file is either a list of task labels, or a JSON object where task labels make up the keys. For example, the try_task_config.json file might look like:

  "tasks": [

Very simply, this will run any task label that gets passed in as well as their dependencies. While it is possible to manually commit this file and push to try, it is mainly meant to be a generation target for various tryselect choosers.

A list of all possible task labels can be obtained by running:

$ ./mach taskgraph tasks

A list of task labels relevant to a tree (defaults to mozilla-central) can be obtained with:

$ ./mach taskgraph target

It’s possible to alter the definition of a task with templates. Templates are JSON-e files that live in the taskgraph module. Templates can be specified from the try_task_config.json like this:

  "tasks": [...],
  "templates": {
    artifact: {"enabled": 1}

Each key in the templates object denotes a new template to apply, and the value denotes extra context to use while rendering. When specified, a template will be applied to every task no matter what. If the template should only be applied to certain kinds of tasks, this needs to be specified in the template itself using JSON-e condition statements.

The context available to the JSON-e render aims to match that of actions. It looks like this:

  "task": {
    "payload": {
      "env": { ... },
    "extra": {
      "treeherder": { ... },
    "tags": { "kind": "<kind>", ... },
  "input": {
    "enabled": 1,
  "taskId": "<task id>"

See the existing templates for examples.