Component Catalog HOWTO

This document explains how to use the Component Catalog to answer some questions about components in the build graph.

Producing reports

This guide assumes that you already configured your, such as using an fx set command, for instance fx set core.x64.

To regenerate all component catalog reports, use fx gen.

All reports have the filename suffix .component_catalog.json. To find all reports, you can use for instance:

find $(fx get-build-dir) -name "*.component_catalog.json"

The directory structure generally follows the target names. For instance a component target //foo/bar:qux will emit $(fx get-build-dir)/obj/foo/bar/qux_component_catalog.json.

The Component Catalog is presented in human-readable JSON for some definition of human and readable.

To clean stale metadata reports (such as from a previous build), you may use fx clean.

What information is present in the Component Catalog?

  • General identifying information about the component.
  • Information about programming languages used in compilation for code that is associated with the component.
  • Information about SDK elements used by the component.

Sample output

A report might look as follows:

    "label": "//sdk/lib/syslog/cpp:backend(//build/toolchain/fuchsia:x64)",
    "sdk_id": "sdk://pkg/syslog_cpp_backend"
    "has_cxx": true,
    "label": "//src/developer/debug/debug_agent:bin(//build/toolchain/fuchsia:x64)"
    "has_cxx": true,
    "label": "//src/developer/debug/debug_agent:launcher(//build/toolchain/fuchsia:x64)"
    "component_manifest_path": "meta/debug_agent.cml",
    "label": "//src/developer/debug/debug_agent:debug_agent-component_manifest_compile(//build/toolchain/fuchsia:x64)"

Each clause tells you something about some aspect of the component. Clauses have a label associated with them for troubleshooting reasons. This indicates where the metadata was defined. Clauses with has_* keys indicate some additive boolean aspect of the component. For instance in the example above we see two clauses that indicate that this component has C++ code. Clauses with sdk_id indicate an SDK atom that is used by this component.


Use fx gn meta for more information on how metadata is generated. For instance:

fx gn meta $(fx get-build-dir) sdk/lib/input_report_reader:input_report_test --data=component_catalog

This should print the same information as what's presented in the generated file, but additionally list all the targets that had metadata collected from.

Known issues

  1. Indicating which languages are used in a component could produce confusing results.
  • For most components you will see exactly one example used.
  • For some components you will see multiple. For instance a component written in Rust with dependencies on C++ code will indicate both Rust and C++ are used.
  • For some components you will see no languages at all. It‘s possible to construct a build graph this way, if for instance the component’s binary is a dependnecy of the package target but not a dependency of the component target itself. It's technically wrong but it works.
  1. The order of clauses within a JSON report file is unstable.

Converting to csv

Use to harvest all component catalog files in the tree and produce a single table in CSV format.