Component ID index



This document describes how to define an index that maps instance ids to monikers. This mapping matters for components that use isolated storage capabilities.


The goal of an index of component instance IDs is to assign stable identifiers to component instances. This is done by mapping an instance ID to a moniker. When a component instance is assigned an ID, its persistent resources are identified on disk using this instance ID. This allows the component's URL or realm to be changed while its resources still remain attributed to it, so long as this index is also updated.

When the component runtime discovers an instance ID -> moniker mapping, it automatically moves the component instance's existing storage directory to be keyed under its instance ID.

Only components that use storage capabilities must to be included in the index. Test components should not be included in the index.

Define a new index

An index file is a JSON5 formatted file, mapping a component's instance ID to its moniker. There may be multiple index files in a build tree, but they will be merged together into a single index file, and this merged file will be made available to the component runtime. This merged index file is immutable, and can only be updated through another system update.

The schema for an index file is described in the following example:

// Index files are written in JSON5, so you may use comments!
  // A list of entries, where each entry maps an instance ID to a moniker.
  instances: [
    // An entry, mapping an instance ID to a moniker.
      // Instance IDs are randomly generated, 256-bits of base-16 encoded
      // strings (in lower case). To generate a new instance ID, set this to
      // an empty string (or other invalid value) and run the build; the build
      // will fail and suggest a new instance ID which you can copy-paste here.
      instance_id: "2bd6cc2bd10243354b873a4ddb8a188b1d29171e26eebac06567bcdc36614af6",
      // The `instance_id` above is associated to the following moniker:
      moniker: "/core/account/credential_manager",

    // More than one entry can be included. However, all entries must be distinct:
    // * Two entries cannot reference the same `instance_id`
    // * Two entries cannot reference the same `moniker`
      instance_id: "7db7e88479772e241229682b47f1794e12ac5d692f8d67421acd9d7ff318a975",
      moniker: "/core/account/password_authenticator",

To supply an index file to the build, use the component_id_index() GN template:

component_id_index("my_component_id_index") {
  source = "my_component_id_index.json5"

Add a component to the index

Locate the appropriate index file

In order to add a component to the index, you must insert an entry into the appropriate index file. Currently, fuchsia.git's components are listed in the core_component_id_index.json5 index file.

Add an entry to the index

The first step is to determine the component instance's moniker. You can find the moniker of a component on a particular product's eng build by running ffx component show.

Then, append an entry to the instances list with the component's moniker. Set the instance_id field to an invalid value (e.g. empty string) to have the build fail and suggest a new one you can use.


In this example, the component fuchsia-pkg:// is added to the index.

To determine the component instance's moniker, you can run ffx component show fuchsia-pkg://

$ ffx component show fuchsia-pkg://
               Moniker:  /core/my_other_component
                   URL:  fuchsia-pkg://

The above output shows us that the moniker of this instance is /core/my_other_component

Add /core/my_other_component to the index by appending this entry to core_component_id_index.json5's instances list with an invalid, empty instance_id:

    instance_id: "",
    moniker: "/core/my_other_component"

Now run the build. The build will fail, suggesting a new instance ID:

$ fx build
Product Assembly Failed
    4.  parsing obj/bundles/assembly/resources/core_component_id_index.json5
    5.  Invalid instance ID: invalid length; must be 64 characters

Here is a valid, randomly generated ID: 4251833a8d2e47b473732a23de84437e0b346a151ec1e7bdfd43b91e02f894a7

Update the entry you've added by copying the suggested instance_id field. The build should now pass.

Include a Component ID Index in a system assembly

The target audience for this section are product owners who are setting up a system assembly

This section describes how to include the component ID index in a system assembly.

A system assembly should include a component ID index if it contains components which use isolated storage. Any product that builds on top of the core product already includes a component ID index in its assembly, so the following instructions may not be necessary.

All component_id_index()s in a system build are merged together using the component_id_index_config() template. This template is currently used in assembled_system.gni, and assembly will fail if you define your own alongside the one from assembled_system.gni.


  1. Define any component_id_index()s you want included in the system.
  2. Add these targets as dependencies of base_packages in your assembled_system() target.