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field-trials.md

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Field trials

WebRTC provides some means to alter its default behavior during run-time, colloquially known as field trials. This is foremost used for A/B testing new features and are related to Chromium field trials to facilitate interoperability.

A field trial consist of a key-value pair of strings. By convention, the field trial key is prefixed with WebRTC- and each word is capitalized without spaces. Sometimes the key is further subdivided into a category, for example, WebRTC-MyCategory-MyExperiment. The field trial value is an opaque string and it is up to the author to define what it represents. There are helper functions to use a field trial as a boolean, with the string Enabled representing true and Disabled representing false. You can also use field trial parameters if you wish to encode more elaborate data types.

The set of field trials can be instantiated from a single string with the format <key-1>/<value-1>/<key-2>/<value-2>/. Note the final / at the end! In Chromium you can launch with the --force-fieldtrials flag to instantiate field trials this way, for example:

--force-fieldtrials="WebRTC-Foo/Enabled/WebRTC-Bar/Disabled/"

Policy

The policy for field trials is:

  • A field trial should only be used to test out new code or parameters for a limited time period. It should not be used for configuring persistent behavior.
  • The field trial must have an end date. The end date may be pushed back if necessary, but should not be pushed back indefinitely.
  • A field trial must be associated with a bug that
    • reserves the field trial key, and
    • is assigned to an owner.

Creating a field trial

Before creating a new field trial, make sure to read the policy.

Either create a new or reuse an existing bug and make sure it is assigned to the correct owner. Take note of the bug ID. Next, decide how long you need the field trial to last. It should be rare to have field trials lasting more than 12 months. You can use the NextAction field in the bug to help you remember the end date.

Using this information, add a new entry to ACTIVE_FIELD_TRIALS in experiments/field_trials.py. You may not add new items to POLICY_EXEMPT_FIELD_TRIALS since it is reserved for field trials that were created before the policy was in place.

Removing a field trial

Any field trial that has expired or otherwise is not needed anymore may be removed by following these steps:

  • Remove all references from the code base. You can find these by, e.g. grepping for the field trial key.
  • Clean up potential glue code that might have been added.
  • Remove the field trial from ACTIVE_FIELD_TRIALS in experiments/field_trials.py.
  • If all work is finished, also close the associated bug.