RFC-0173: Structured Configuration in Component Framework APIs

RFC-0173: Structured Configuration in Component Framework APIs
  • Component Framework

Define the publicly-facing implementation of basic Structured Configuration features.

Gerrit change
Date submitted (year-month-day)2022-04-11
Date reviewed (year-month-day)2022-06-30


Changes to Component Framework APIs and implementation for structured configuration.


Structured configuration was approved in RFC-0127 which described an overall architecture and roadmap for the feature, but did not specify a number of implementation details that are necessary to ratify under the Component Framework RFC criteria. RFC-0146 describes the CML syntax used to declare a configuration schema and RFC-0158 describes the client libraries users will generate for accessing their configuration.

The initial prototype of structured configuration is available in fuchsia.git. The Component Framework team is waiting to make the feature generally available out-of-tree until the APIs and behaviors in this RFC are ratified.


Facilitator: leannogasawara@google.com


  • geb@google.com (Component Framework)
  • jsankey@google.com (RFC-0127 author)

Consulted: hjfreyer@google.com, xbhatnag@google.com, aaronwood@google.com, mcgrathr@google.com, shayba@google.com

Socialization: This RFC is the product of design reviews with the Component Framework team, subsequent code reviews during the prototyping process, and feedback from early adopters of the prototype.


This RFC describes changes to Component Framework to implement "Phase 1" and parts of "Phase 2" from RFC-0127's implementation plan, covering only the "static values" functionality and a limited version of "values from parents" in test environments using RealmBuilder.


Component Framework has a number of new responsibilities:

  • Compiled component manifests include config value sources
  • Component resolvers fetch a component's config values
  • Component Manager encodes a component's config as a persistent FIDL message
  • Component runners deliver the encoded configuration to components
  • RealmBuilder allows setting configuration values of launched children in tests

Configuration Values

Every component with a configuration schema must have configuration values defined. These are stored and transferred between parts of the Component Framework as fuchsia.component.config.ValuesData:

library fuchsia.component.config;

type ValuesData = table {
    1: values vector<ValueSpec>:MAX;
    2: checksum fuchsia.component.decl.ConfigChecksum;

// NOTE: table to allow defining mutability in future revisions
type ValueSpec = table {
    1: value Value;

type Value = flexible union {
    1: single SingleValue;
    2: vector VectorValue;

type SingleValue = flexible union {
    1: bool bool;
    2: uint8 uint8;
    3: uint16 uint16;
    4: uint32 uint32;
    5: uint64 uint64;
    6: int8 int8;
    7: int16 int16;
    8: int32 int32;
    9: int64 int64;
   10: string string:MAX;

type VectorValue = flexible union {
    1: bool_vector vector<bool>:MAX;
    2: uint8_vector vector<uint8>:MAX;
    3: uint16_vector vector<uint16>:MAX;
    4: uint32_vector vector<uint32>:MAX;
    5: uint64_vector vector<uint64>:MAX;
    6: int8_vector vector<int8>:MAX;
    7: int16_vector vector<int16>:MAX;
    8: int32_vector vector<int32>:MAX;
    9: int64_vector vector<int64>:MAX;
   10: string_vector vector<string:MAX>:MAX;

Values are stored in the same order as the corresponding fields in the compiled manifest.

Each config schema includes a checksum which is a hash of all field names and types. The checksum in ValuesData must match the checksum in the compiled component manifest.

We have considered and rejected an alternative to store defined values using the same encoding as is sent to the component itself.

Component value sources

For packaged components, config values are stored in a "configuration value file". A configuration value file is a persistent FIDL encoding of fuchsia.component.config.ValuesData.

Because the configuration values are not stored in the component manifest, the Component Framework needs to know where to find the values when resolving the component. A new field is added to the compiled representation of config schemas:

library fuchsia.component.decl;

type ConfigSchema = table {
    // ...existing fields...

    3: value_source ConfigValueSource;

type ConfigValueSource = flexible union {
    /// The path within the component's package at which to find config value files.
    1: package_path string:MAX;

A flexible union is used to allow for the addition of new config value sources over time.

By convention, value files are packaged at meta/${manifest_basename}.cvf. For example, if a component's manifest is packaged at meta/foo.cm, its value file will be packaged at meta/foo.cvf.

Build rules for producing structured configuration need to ensure that component manifests include the path at which the component's config will be packaged. For example, the in-tree GN build achieves this by requiring the config values target to reference the component target so they can agree about the packaged location:


# NOTE: results in a package path of `meta/my_component.cm`
fuchsia_component("my_component") {
  manifest = "meta/my_component.cml"
  deps = [ "..." ]

# NOTE: internally calls `get_target_outputs(":my_component")` to determine
# the correct packaging path
fuchsia_structured_config_values("my_config_values") {
  component = ":my_component"
  values_source = "config/my_component.json5"

fuchsia_package("my_package") {
  deps = [

An alternative has been considered and rejected to store the values in the manifest itself.

An alternative has been considered and rejected to infer the location of the value file within the package based on the packaged path of the manifest.

An alternative has been considered and rejected to add an index blob to packages which points to both the component manifest and the value file.

Component resolution

Component resolvers are responsible for retrieving a component's packaged values and returning them to Component Manager as a new field in the fuchsia.sys2.Component table:

library fuchsia.sys2;

// NOTE: This type is returned by fuchsia.component.resolution.Resolver/Resolve.
type Component = resource table {
    // ... existing fields ...

    /// Binary representation of the component's configuration values
    /// (`fuchsia.component.config.ValuesData`).
    4: config_values fuchsia.mem.Data;

ValuesData is returned as a fuchsia.mem.Data to match how component manifests are returned from resolvers. This allows for the combined size of a compiled manifest and configuration values to exceed the size of a single zircon channel message.

This requires component resolvers to parse the component manifest to interpret the value source in the manifest, while previously resolvers were able to return the raw bytes directly to Component Manager without understanding the compiled manifest representation.

Encoding configuration values

Once Component Manager has a configuration schema and values, it must pass these values and the component's config schema's checksum to the component's runner for provisioning to the component over a runtime-specific interface.

VMO contents

Configuration values are encoded for the component as a persistent FIDL struct with the fields in the same order as the compiled manifest. A rejected alternative would be to encode the final configuration as a FIDL table rather than a struct. We have also considered and rejected non-FIDL encodings for the resolved configuration.

This requires Component Manager to understand the FIDL wire format to perform runtime encoding of messages that can be successfully parsed by generated FIDL bindings.

Component Manager writes the encoded configuration into a VMO with the following contents:

  • Bytes 0..1 contain the length N of the checksum as a little-endian integer
  • Bytes 2..2+N contain the checksum
  • Bytes 3+N..ZX_PROP_VMO_CONTENT_SIZE contain a persistent FIDL message with the component's configuration values encoded as a struct

The checksum is stored as a variable-length portion of the header to decouple the VMO encoding from the size of any particular hash function's output. Changing the hash algorithm used to derive checksums should not require a change to the VMO encoding.

Passing VMO to runners

Once the config VMO has been created, it is passed to the runner as a field of fuchsia.component.runner.ComponentStartInfo:

library fuchsia.component.runner;

// NOTE: Passed to fuchsia.component.runner.ComponentRunner/Start.
type ComponentStartInfo = resource table {
    // ... existing fields ...

    /// Binary representation of the component's configuration.
    /// # Layout
    /// The first 2 bytes of the data should be interpreted as an unsigned 16-bit
    /// little-endian integer which denotes the number of bytes following it that
    /// contain the configuration checksum. After the checksum, all the remaining
    /// bytes are a persistent FIDL message of a top-level struct. The struct's
    /// fields match the configuration fields of the component's compiled manifest
    /// in the same order.
    7: encoded_config fuchsia.mem.Data;

We've also considered and rejected alternatives to defer encoding to the runner and to use FIDL to describe the checksum layout in the VMO.

Running components with encoded configuration

Each runner is responsible for establishing a contract with components it runs to provide them access to the encoded configuration. This contract is exercised by accessor libraries.

ELF runner

The ELF runner provides configuration VMOs as a startup handle:

// in //zircon/system/public/zircon/processargs.h:

Driver runner

The driver runner provides configuration VMOs as a field in the fuchsia.driver.framework.DriverStartArgs table:

library fuchsia.driver.framework;

// NOTE: Passed directly to a driver upon starting.
type DriverStartArgs = resource table {
    // ... existing fields ...
    7: config zx.handle:VMO;

Test runners

We are not yet implementing structured config support in test runners, as we have not yet discovered sufficiently-motivated use cases to constrain and guide a design.

Overriding configuration values with RealmBuilder

RealmBuilder supports controlling the configuration values for a component it launches. Users can provide values for individual fields, and by default they must specify values for all fields in the component's schema. This will encourage a component's own integration tests to fully enumerate the matrix of configuration options to test.

RealmBuilder will also allow test authors to load the packaged values for a component, either using them wholesale or overriding individual fields. This will allow tests to, for example, enable a particular testing feature that is not useful outside of a subset of tests, while still using the rest of the "production" config for that component.

These methods will be added to RealmBuilder:

    LoadPackagedConfigValues(struct {
        name fuchsia.component.name;
    }) -> (struct {}) error RealmBuilderError2;

    SetConfigValue(struct {
        name fuchsia.component.name;
        key fuchsia.component.decl.ConfigKey;
        value fuchsia.component.config.ValueSpec;
    }) -> (struct {}) error RealmBuilderError2;

RealmBuilder client libraries will be extended to expose this functionality.


The contents of this RFC as initially proposed have been experimentally implemented and made available in fuchsia.git, initially under an allowlist with the understanding from users that the Component Framework may make breaking changes while iterating. Changes to match this RFC's final design will be landed before making structured configuration available to out-of-tree customers.


The designs in this RFC could impact system runtime performance in terms of time needed to resolve and start components, which is not a metric that to the author's knowledge has thus far been a bottleneck on the quality of products built with Fuchsia. We do have continuous benchmarking of the start time for some components within the topology, and we will continue monitoring those for regressions but none have yet been identified in the process of prototyping this feature.

Structured config may increase system memory usage if copies of config are held in memory, which this design minimizes by storing configuration in a VMO which accessor libraries may close after parsing the contents into the domain-specific types used by each component's implementation.

Backwards Compatibility

This RFC assumes that platform versioning will have sufficient runtime support within the Component Framework before we discover any need to evolve how configuration is encoded into the VMOs passed to components.

RealmBuilder's overrides implementation allows tests in the same package as the launched component to take a runtime dependency on a component's configuration schema, and authors of components used in others' tests will need to exercise caution when changing the types of a configuration field or when removing a configuration field.

Security considerations

Structured configuration may be used to control security-critical features in components so it is essential that the implementation delivers the right values.

A component's configuration comes from the same source as its manifest, but in the future we may extend the existing component resolvers or build new resolvers which allow a looser relationship between the two. We will need to exercise caution to ensure that a component's configuration is always unambiguous and auditable.

Privacy considerations

Per RFC-0127, structured configuration is not designed to store user- generated data.


The most sensitive part of this design is the dynamic FIDL encoder used by Component Manager. Its prototype has been integrated as a "language backend" in FIDL's GIDL conformance suite to ensure that for the subset of types it supports it produces identically laid-out messages as the statically-typed FIDL bindings.


Documentation for the prototype is currently available for in-tree developers.

Drawbacks, alternatives, and unknowns

Alternative: storing values in a typed format

Instead of the untyped fuchsia.component.config.ValuesData, we could store and transfer a component's configuration values between portions of the Component Framework using a FIDL-encoded payload with a type that matches the component's schema, similar to the encoding used for the VMO passed directly to the component.

This would likely result in a slightly more compact on-disk representation, as well as being aesthetically aligned with the typed payloads that are eventually delivered to the component.

The design proposed in this RFC allows various tools to understand configuration without having to reimplement a dynamically-typed FIDL parser. In the future, we might reconsider this decision if the FIDL toolchain offers more general tools for "reflection" over messages.

Alternative: storing values in the manifest

Storing values inline in the manifest would simplify several elements of the implementation (removing responsibilities from component resolvers) at the cost of changed blob hashes for component manifests between products.

Alternative: finding value files by file extension

Instead of adding a value source to component manifests, we could infer from our packaging convention where to find the value file. This would create an undesirable runtime dependency on what are currently only conventions.

Alternative: finding value files by a package-component index

Adding a third blob to the package would allow us to pipeline reads of the manifest and value file, which would result in a cleaner build graph, simplify some elements of the manifest compilation process, and allow component resolvers to understand a simpler file format than having to parse the entire component manifest.

However, this would increase the system image size and would be a significant user-visible change to how components are resolved.

Alternative: encode configuration as a FIDL table

Instead of a FIDL struct, configuration fields could be encoded as a table, as was suggested as an option in RFC-0127. Tables have a number of useful properties for evolution, but they also require a parser to assume that any of the fields could be omitted. With structured configuration, Component Manager is always able to supply all fields and there is no need for the caller to handle the case where no value has been provided.

FIDL structs occupy fewer bytes to carry the same data and are slightly faster to parse.

Alternative: use a non-FIDL encoding for configuration values

An encoding other than FIDL would be viable, but the FIDL wire format works well for this use case and choosing a different encoding would be a net increase in the number of concepts needed to understand Fuchsia. Using the FIDL encoding aligns with Component Framework's desire to integrate more closely with FIDL going forward and enables using FIDL's generated bindings as the implementation for accessors, which allows structured config to reap the benefits of performance and binary size optimizations which have already been achieved there.

Alternative: runners encode configuration

Instead of encoding the configuration VMO in Component Manager, we could encode it in each runner. This would allow runners that operate at different levels of abstraction to deliver configuration in different formats. For example, a runner for Javascript code could deliver the configuration as objects in that language runtime rather than requiring each Javascript component to parse a VMO.

However, as noted in Testing, FIDL encoding is the most sensitive part of this design in terms of correctness. There are multiple runners today which would use a FIDL-based encoding for config, which means that there would be multiple binaries (and implementing programming languages) responsible for this sensitive task. Centralizing the responsibility for config encoding in Component Manager allows us to constrain the implementation and achieve greater confidence in the overall feature with extensive testing.

The proposed design would make it difficult for a runner to configure its behavior based on a component's structured configuration values, as each runner would need to parse the encoded FIDL based on the component's declared schema. This would be comparable complexity to making each runner responsible for encoding which is also undesirable. When it comes time to configure runners from a component's config, we will design a separate interface for passing values from the component's configuration to the runner. Defining an explicit interface for runner configuration also has the advantage of insulating a component's config namespace from Hyrum's Law-style effects.

Alternative: describe VMO/checksum layout with FIDL

Instead of encoding the config checksum as header bytes in the VMO, we could describe the VMO's ABI in terms of FIDL:

type ConfigVmo = struct {
    checksum bytes:MAX;
    contents bytes:MAX;

This would occupy slightly more space in each config VMO, and would also require traversing the bytes of contents twice.