“Try” is a way to “try out” a proposed change safely before review, without officially landing it. This functionality has been around for a long time in various forms, and can sometimes show its age.

Access to “push to try” is typically available to a much larger group of developers than those who can land changes in integration and release branches. Specifically, try pushes are allowed for anyone with SCM Level 1, while integration branches are at SCM level 3.

Scheduling a Task on Try

There are three methods for scheduling a task on try: legacy try option syntax, try task config, and an empty try.

Try Option Syntax

The first, older method is a command line string called try syntax which is passed into the decision task via the commit message. The resulting commit is then pushed to the repository. 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.

Try Task Config

The second, more modern method specifies exactly the tasks to run. That list of tasks is usually generated locally with some local tool and attached to the commit pushed to the try repository. This gives finer control over exactly what runs and enables growth of an ecosystem of tooling appropriate to varied circumstances.


This method uses a checked-in file called try_task_config.json which lives at the root of the source dir. The JSON object in this file contains a tasks key giving the labels of the tasks to run. For example, the try_task_config.json file might look like:

  "version": 1,
  "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. For example:

$ ./mach try fuzzy

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

Modifying Tasks in a Try Push

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:

  "version": 1,
  "tasks": [...],
  "templates": {
    env: {"FOO": "bar"}

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 contains attributes from the taskgraph.task.Task class. It looks like this:

  "attributes": task.attributes,
  "kind": task.kind,
  "label": task.label,
  "target_tasks": [<tasks from try_task_config.json>],
  "task": task.task,
  "taskId": task.task_id,
  "input": ...

The input context can be any arbitrary value or object. What it contains depends on each specific template. Templates must return objects that have have either attributes or task as a top level key. All other top level keys will be ignored. See the existing templates for examples.

Empty Try

If there is no try syntax or try_task_config.json, the try_mode parameter is None and no tasks are selected to run. The resulting push will only have a decision task, but one with an “add jobs” action that can be used to add the desired jobs to the try push.

Complex Configuration

If you need more control over the build configuration, (staging releases, for example), you can directly specify parameters to override from the try_task_config.json like this:

    "version": 2,
    "parameters": {
        "optimize_target_tasks": true,
        "release_type": "beta",
        "target_tasks_method": "staging_release_builds"

This format can express a superset of the version 1 format, as the version one configuration is equivalent to the following version 2 config.

    "version": 2,
    "parameters": {
        "try_task_config": {...},
        "try_mode": "try_task_config",