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A history of considered changes for Feature Manifest Language

Future specifications

This document houses a number of proposals not ready for comment. They are not linked to from the spec.

Feature defaults and runtime targeting

⚠️ Unimplemented. This is intended to use the same JEXL targeting used elsewhere in Nimbus SDK.

Occasionally, we would like to vary a configuration based upon the device. In this example, the homepage feature has sections for both Pocket and Topsites.

description: If true, show stories from Pocket. This is only available in certain territories.
type: Bool
default: false
description: If true, show tiles from the users most recent and frequently visited pages.
type: Bool
default: false
- channel: nightly
value: { topsites-enabled: true }
- channel: nightly
targeting: locale in ['en-US', 'de-DE', 'en-GB']
value: { pocket-enabled: true }

In this example, pocket-enabled defaults to true only on the nightly channel and when the device locale is in US, DE or GB.

Types coerced from String

We have already seen enums being declared in the types section of the manifest.

Other types that may be derived from String:

  • Color
  • URL

Variables and fields with no defaults

Required fields

Some object fields do not have a sensible default value, but without them, the object itself doesn't make sense.

For example a MessageItem doesn't make sense unless it has a message and a deeplink. Where any required fields are missing from the JSON, the MessageItem is incomplete, and cannot be used.

Incomplete objects cannot be included in maps, lists or optional types.

In this case, a message surface is added to the new-tab, but isn't always needed to be displayed.

description: An optional message
type: MessageItem?
default: null
type: String
required: true
type: String
required: true

In these cases, the app code must deal with the cases with objects that are said to be failable, because there are no defaults.

let newTabConfig = FxNimbus.features.newTab.value()

if let message = newTabConfig.message {
// We have to check that the message exists, because the message may not exist.
displayMessage(message.label, message.deeplink)

// continue configuring the new tab.

Failable features

Feature variables may also be marked as required. In the cases where these variables are missing, then the whole feature no longer makes sense.

Features with required variables must be marked as failable.

failable: true
description: A message displayed to the user at start up.
description: The long form message to be displayed to the user
type: String
required: true
description: The background color of the screen
type: String
default: red

Failable features are implemented as Optional. If the feature is incomplete or failed, then it is returned as nil or null.

guard let message = FxNimbus.features.emergencyStartupMessage else {
// phew, there is no emergency message.

Features with required variables which should be marked failable but aren't will produce an error.

Imports and Includes


  • The importing .fml.yaml file should not have access to types from the imported one.
  • The imported file may not have an import list.

Eventually it might be necessary to have imported files recursively importing other files. Implementing this proposal will give an idea of how much extra complexity is required.

Connecting to different languages or contexts

The import block does two different roles:

  1. Giving experimenter knowledge of the imported features.
  2. Sending experimental configuration from the application singleton into the components.

For features written in a different language, the feature configuration needs to travel across an FFI.

path: @mozilla/gecko-dev/fenix.fml.yaml
channel: production
connector: gecko-view
- value: {}

Imports for the same connector are gathered up so we can get a Map<String, JSONObject> containing configuration from the experiment (and the app-specific defaults) by calling:


The app will need to send this to the right place, for example:

val nimbus = Nimbus.shared
nimbus.register(object : Nimbus.Observer {
fun onExperimentsApplied() {
val features: Map<String, JSONObject> = FxNimbus.connectors.geckoView.value()