Field Trial Testing Configuration

This directory contains the fieldtrial_testing_config.json configuration file, which is used to ensure test coverage of active field trials.

For each study, the first available experiment after platform filtering is used as the default experiment for Chromium builds. This experiment is also used for perf bots and various browser tests in the waterfall (e.g. browser_tests, components_browsertests, content_browsertests, extensions_browsertests, interactive_ui_tests and sync_integration_tests). It is not used by unit test targets.

Note: This configuration applies specifically to Chromium developer builds. Chrome branded / official builds do not use these definitions by default. They can, however, be enabled with the --enable-field-trial-config switch. For Chrome branded Android builds, due to binary size constraints, the configuration cannot be applied by this switch.

Note: Non-developer builds of Chromium (for example, non-Chrome browsers, or Chromium builds provided by Linux distros) should disable the testing config by either (1) specifying the GN flag disable_fieldtrial_testing_config=true, (2) specifying the --disable-field-trial-config switch, (3) specifying field trials using the --force-fieldtrials switch, or (4) specifying a custom variations server URL using the --variations-server-url switch. In order to apply the testing configuration as well as specify additional field trials (using --force-fieldtrials), the --enable-field-trial-config switch can be used.

Note: An experiment in the testing configuration file that enables/disables a feature that is explicitly overridden (e.g. using the --enable-features or --disable-features switches) will be skipped.

Config File Format

{
    "StudyName": [
        {
            "platforms": [Array of Strings of Valid Platforms for These Experiments],
            "experiments": [
                {
                    "//0": "Comment Line 0. Lines 0-9 are supported.",
                    "name": "ExperimentName",
                    "params": {Dictionary of Params},
                    "enable_features": [Array of Strings of Features],
                    "disable_features": [Array of Strings of Features]
                },
                ...
            ]
        },
        ...
    ],
    ...
}

The config file is a dictionary at the top level mapping a study name to an array of study configurations. The study name in the configuration file must match the FieldTrial name used in the Chromium client code.

Note: Many newer studies do not use study names in the client code at all, and rely on the Feature List API instead. Nonetheless, if a study has a server-side configuration, the study name specified here must still match the name specified in the server-side configuration; this is used to implement consistency checks on the server.

Study Configurations

Each study configuration is a dictionary containing platforms and experiments.

platforms is an array of strings, indicating the targetted platforms. The strings may be android, android_weblayer, android_webview, chromeos, chromeos_lacros, ios, linux, mac, or windows.

experiments is an array containing the experiments.

The converter uses the platforms array to determine which experiment to use for the study. The first experiment matching the active platform will be used.

Note: While experiments is defined as an array, currently only the first entry is used***. We would love to be able to test all possible study configurations, but don‘t currently have the buildbot resources to do so. Hence, the current best-practice is to identify which experiment group is the most likely candidate for ultimate launch, and to test that configuration. If there is a server-side configuration for this study, it’s typically appropriate to copy/paste one of the experiment definitions into this file.

* Technically, there is one exception: If there‘s a forcing_flag group specified in the config, that group will be used if there’s a corresponding forcing_flag specified on the command line. You, dear reader, should probably not use this fancy mechanism unless you‘re quite sure you know what you’re doing =)

Experiments (Groups)

Each experiment is a dictionary that must contain the name key, identifying the experiment group name.

Note: Studies should typically use the Feature List API. For such studies, the experiment name specified in the testing config is still required (for legacy reasons), but it is ignored. However, the lists of enable_features, disable_features, and params must match the server config. This is enforced via server-side Tricorder checks.

For old-school studies that do check the actual experiment group name in the client code, the name must exactly match the client code and the server config.

The remaining keys -- enable_features, disable_features, min_os_version, and params -- are optional.

enable_features and disable_features indicate which features should be enabled and disabled, respectively, through the Feature List API.

min_os_version indicates a minimum OS version level (e.g. “10.0.0”) to apply the experiment. This string is decoded as a base::Version. The same version is applied to all platforms. If you need different versions for different platforms, you will need to use different studies.

params is a dictionary mapping parameter name to parameter value.

Reminder: The variations framework does not actually fetch any field trial definitions from the server for Chromium builds, so any feature enabling or disabling must be configured here.

Comments

Each experiment may have up to 10 lines of comments. The comment key must be of the form //N where N is between 0 and 9.

{
    "AStudyWithExperimentComment": [
        {
            "platforms": ["chromeos", "linux", "mac", "windows"],
            "experiments": [
                {
                    "//0": "This is the first comment line.",
                    "//1": "This is the second comment line.",
                    "name": "DesktopExperiment"
                }
            ]
        }
    ]
}

Specifying Different Experiments for Different Platforms

Simply specify two different study configurations in the study:

{
    "DifferentExperimentsPerPlatform": [
        {
            "platforms": ["chromeos", "linux", "mac", "windows"],
            "experiments": [{ "name": "DesktopExperiment" }]
        },
        {
            "platforms": ["android", "ios"],
            "experiments": [{ "name": "MobileExperiment" }]
        }
    ]
}

Formatting

Run the following command to auto-format the fieldtrial_testing_config.json configuration file:

python3 testing/variations/PRESUBMIT.py testing/variations/fieldtrial_testing_config.json

The presubmit tool will also ensure that your changes follow the correct ordering and format.