Define an index of components which use storage

Note: This guide uses the components v1 architecture.



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


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 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 which use storage capabilities must to be included in the index. The following class of components need not be included in the index:

  • Test components
  • Components whose storage is not managed by appmgr.

Defining an index

An index file is a JSON5 formatted file, mapping a component's instance ID to its moniker. There may be multiple index files, but a system build will merge them together into a single index file and make it 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).
      instance_id: "11601233aef81741f7251907d4d2a1a33aa6fec6b2e54abffc21bec29f95fec2",
      // The `instance_id` above is associated to the following moniker:
      appmgr_moniker: {
        // This the URL of the component.
        url: "fuchsia-pkg://",

        // This is the realm path where this component runs.
        realm_path: [
          "sys",     // This the parent realm of "session"
          "session"  // This is the realm the component runs under

    // 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 `realm`
      instance_id: "644a7f0f66f8994d894c5f78b5b879911fee6c185c6aadd29d52888812d20ac4",
      appmgr_moniker: {
        url: "fuchsia-pkg://",
        realm_path: [

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.json"

Including 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 which builds on top of the core product already includes a component ID index, so the following instructions are not necessary.


All component_id_index()s in a system build are merged together using the component_id_index_config_package() template, which produces a config_data(for_pkg=appmgr). appmgr then consumes the merged index using this mechanism.

To include a component_id_index_config_package() target in a system assembly:

a) Define it with a dependency on any component_id_index() targets which you want included in the system. For example, //build/images:universe_packages is a good dependency candidate because it transitively includes all component_id_index() specified in the build.

b) Add your component_id_index_config_package() target to the system assembly. Currently, a good method is to include your component_id_index_config_package() target as a dependency to your system assembly's config_package().