RFC-0182: Deprecate config-data

RFC-0182: Deprecate config-data
  • Component Framework

Intent to deprecate the legacy mechanism to provide/consume configuration files between components across repositories

Gerrit change
Date submitted (year-month-day)2022-06-30
Date reviewed (year-month-day)2022-08-15


This RFC proposes:

  • Declaring the [config-data] mechanism as deprecated.
  • Establishing an allowlist for existing use cases.
  • Over time, replacing existing usages of config-data with other, better solutions.
  • Eventually removing support for config-data from Fuchsia.


The immediate goal of this RFC is to establish rough consensus for creating an allowlist for existing config-data usage, with a preference not to grow said allowlist. Additional information not pertaining to this immediate goal is provided for extra context and at the request of various stakeholders.

[config-data] is a mechanism for configuration files that modulate the behavior of a packaged component by making certain files available in that component's namespace at runtime.

config-data creates "spooky action at a distance" using an unenforced contract or convention over the package name of the target component. This is problematic because package names are not useful as stable identifiers that can become part of the Fuchsia SDK surface, be versioned with the SDK, and evolve over time such as by declaring support windows and allowing for soft transitions. Experience has shown that contracts based on package names make for brittle runtime assumptions and have high maintenance costs.

Additionally, since config-data mixes content from a developer's package with content from a base package that contains the configuration files, this creates a confusing developer experience where developers would push a new revision of their package but see old configuration files at runtime. Often, developers ask why is it that they pushed an updated version of their package but they're seeing stale files under /config/data, not realizing that these files come from a different package. Explaining this behavior to developers requires exposing them to platform implementation details such as "base packages", which demonstrates a failure of abstraction and is undesirable. Not to mention the loss of productivity due to this recurring workflow inconsistency.

config-data has been very valuable to solving problems where actors at different stages of product assembly needed to influence components that were outside of their direct control. Using config-data, configuration values that originate from one place (a repository, a build system, etc) could be used to modulate the behavior of components that originate from elsewhere.

In some cases components and configurations have the same origin, but users still prefer to use config-data as an integration point. This allows users for instance to generate one package and sideload different configuration values to components included in this package. This is useful when the cost of generating multiple packages is high, for instance if each package flavor needs to be uploaded individually to a subsequent integration point.

Nowadays, multiple alternatives to config-data exist. These include other approaches to routing a configuration directory based on directory capabilities, structured configuration, providing configuration via protocol capabilities, and packaging configuration files with the components that consume them.

While it is not an immediate priority to deprecate existing usages of config-data and the feature itself, we should steer new usages towards modern solutions and nudge owners of existing usages to migrate to modern alternatives.

The best practice in such cases is to establish an allowlist for the config_data() GN rule and to initialize it with existing usage. The allowlist is expected to trend towards zero over time, however new usages can be allowlisted if it's not clear that the modern alternatives are applicable. When new usages are admitted the change to allow this should state the rationale up-front, in order to reduce code review churn.


  • Component Framework team: created config-data and its present alternatives.
  • Build team: maintainers of config-data build-time logic.
  • Software Assembly team: maintainers of config-data assembly-time logic.
  • Current users of config-data, which span many teams.


  • hjfreyer@google.com


  • aaronwood@google.com
  • adamperry@google.com
  • awolter@google.com
  • ddorwin@google.com
  • fmil@google.com
  • geb@google.com
  • jsankey@google.com
  • kpozin@google.com
  • wez@google.com
  • yaar@google.com
  • ypomortsev@google.com


A draft for this RFC was reviewed by Component Framework and Software Assembly technical leads before it was published.


How config-data works

In legacy appmgr (aka CFv1), config-data was implemented by matching a launched component's package name against a subdirectory in the config-data base package. If a match was found, the matching subdirectory was made available in the launched component's namespace under the path /config/data.

Defining config-data is done by specifying files to be made available, paths at which they should be made available in a component's incoming namespace (by convention, under /config/data), and one or more package names for components that should have these files available at these paths.

For instance, a component might request to use configuration files like so:

    use: [
            directory: "config-data",
            rights: [ "r*" ],
            path: "/config/data",

This expresses that the component requires embedders to provide a read-only directory capability named "config-data", that will be presented by the framework at /config/data, to allow existing code relying on /config/data to work unmodified.

The parent component or realm might have these declarations:

    children: [
            name: "font-provider",
            url: "fuchsia-pkg://fuchsia.com/fonts#meta/font-provider.cm",
    offer: [
            directory: "config-data",
            from: "parent",
            to: [ "#font-provider" ],
            subdir: "fonts",

This accepts a config-data containing multiple sub-directories, from the parent, and routes the example sub-directory, containing configuration data for the font-provider, to that component.

The contents of config-data might come from build definitions such as this:


config_data("example_config_data") {
  for_pkg = "example"
  sources = [
  outputs = [ "{{source_file_part}}" ]

The target above, and others like it, are collected by the build system to form the contents of a package named config-data. Then, purpose-built routing rules route all the contents of the config-data package, organized into subdirectories as indicated by the for_pkg parameter shown above, to the core realm and elsewhere, where they are distributed as further configured to various components that consume them.

How config-data is used today

config-data has many use cases. Below are some illustrative examples:

  • ICU data and tzdata: data for the ICU library, and specifically timezone data ("tzdata"), is provided at runtime as config-data. Defining a single source of truth for these data files in the Fuchsia platform sources and providing the files to components from various sources (such as Chromium, Flutter, internal Google repositories etc) is used to ensure that exactly one revision of these files is used. This achieves consistency (e.g. all components agree on a single tzdata regardless of their origin) and efficiency (all components share the same ICU blobs regardless of when or where they were built).
  • Values for the buildinfo and hwinfo components are provided as config-data. These components are built from platform source code but may need to be configured by products. Currently config-data serves as this configuration mechanism.
  • The Settings UI component, defined in platform sources, can be configured to behave differently on different devices that expose different hardware toggles. For instance SetUI's behavior is different on Astro vs on Sherlock devices in a manner currently governed by config-data.
  • The platform font provider component can be configured to serve product-specific fonts. The font files and a manifest describing their properties are provided as config-data that is added in out-of-tree product assembly.

Design & implementation

A build-time regression stop will be established to prevent new uses of config-data without explicit approval. An allowlist of existing usages will be checked in, and changes to this allowlist will be governed by an OWNERS file. Owners will be assigned from the Component Framework team to represent fuchsia.git, and from petals that use config-data to represent their usage. Representatives will be responsible to manage their respective allowlist entries, for instance assist in refactors or targeted burndowns.

It's not important how the regression stop is implemented. A common and likely implementation strategy is to change the config-data GN template to add a dependency on a target with a set visibility list. Notably, this only covers in-tree usage, but restricting new usage of config-data in out-of-tree product assembly is also important. An appropriate mechanism may be developed here as well, in coordination with the PDK team.

An in-depth of the alternatives to config-data is outside the scope of this RFC. Review the documentation linked above when referring to these alternatives.

Establishing best practices for providing configuration to components, or seeding a migration guide away from config-data, are outside the scope of this RFC. There is ongoing work to produce such documentation, and it will be published separately from this RFC.


A specific and interesting aspect of the config-data package is that all files are packaged under the path meta/. This means that the files are archived in the meta.far file. When stored in the underlying blobfs filesystem, file sizes on disk are rounded up to a block size, typically 8KiB. By archiving configuration files together, the added overhead of rounding up is removed. This is important because configuration files are often numerous and small, so the total overhead can actually be greater than the sum of compressed sizes of these files.

When using alternative solutions, in instances where storage space matters, the same technique or an equivalent should be used to ensure parity on storage efficiency.


Alternatives to config-data have better ergonomics, most importantly because they don't rely on brittle contracts based on package names and on "action at a distance".

Backwards Compatibility

Migrations away from config-data will sometimes need to be done as soft transitions. During the transition period, the component that is consuming the configuration data must be able to accept both forms of input, config-data and the chosen alternative.

Security considerations

This RFC is not introducing any new configuration mechanisms, all the mechanisms we would use as an alternative to config-data are already present in the system and have been through their own security review. Component authors should consult with security when designing or changing the configuration for security-relevant features.

Privacy considerations

This RFC is not introducing any new configuration mechanisms, all the mechanisms we would use as an alternative to config-data are already present in the system and have been through their own privacy review. Component authors should consult with privacy when designing or changing the configuration for privacy sensitive features.


The alternatives presented to config-data all have established best practices for testing. Refer to documentation for specific features for testing information. For instance, see the guide for testing structured configuration using Realm Builder. Replacing structured configuration values under test is as easy as:

realm_builder.SetConfigValue(child_name, key, value_for_test);

When we need to provide configuration data as files to a component under test, such as in an integration test, the files can be packaged with the test and then routed from the test component (at the test realm root) to the component under test as a data directory. For instance:

    capabilities: [
            directory: "test_config",
            rights: [ "r*" ],
            path: "/test_config",
    children: [
            name: "component_under_test",
    offer: [
            directory: "test_config",
            from: "self",
            as: "config",


To support transitions away from config-data, a migration guide will be provided. The migration guide will help developers choose the best alternative for their use case, and refer them to additional documentation.

Notes on staffing and churn

Accepting this RFC does not impose any sort of requirement on existing users of config-data to migrate to modern alternatives. If a mandate for a migration is given, then there should be a core migration team staffed to perform the majority of the work. The author proposes that no such work should begin until after the CFv2 migration is complete, but we should hurry up and land a regression stop.

Until such a time that there is dedicated staffing for a migration, updates to the allowlist such as to support refactors should be reviewed and approved within one business day.

Drawbacks, alternatives, and unknowns

Structured configuration is a new and evolving mechanism. For instance some features that are needed for expressing configurations, such as additional data types, are not yet implemented.

Capability routing is not currently subject to platform versioning. It is the plan of record to build support for modulating capability routes on ABI revisions, but this mechanism has not been designed yet.