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Define an index of components which use storage

Terminology

Scope

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

Overview

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

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

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, omit this
      // field and run the build; the build will fail and suggest a new
      // instance ID which you can copy-paste here.
      instance_id: "11601233aef81741f7251907d4d2a1a33aa6fec6b2e54abffc21bec29f95fec2",
      // The `instance_id` above is associated to the following moniker:
      appmgr_moniker: {
        // This the URL of the component.
        url: "fuchsia-pkg://example.com/my_package#meta/my_component.cmx",

        // 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://example.com/my_other_package#meta/my_other_component.cmx",
        realm_path: [
          "sys"
        ]
      }
    }
  ]
}

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"
}

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.json index. Components outside fuchsia.git are listed in a separate index file, probably located under a //vendor repository where the relevant product is defined.

Add an entry to the index

Append an entry to the instances list with the component's moniker. You may omit the instance_id field to have the build fail and suggest a new one you can use.

Example

Add fuchsia-pkg://example.com/my_other_package#meta/my_other_component.cmx to the index by appending this entry to core_component_id_index.json's instances list:

  {
    appmgr_moniker: {
      // The component's URL
      url: "fuchsia-pkg://example.com/my_other_package#meta/my_other_component.cmx",
      // The realm the component is run under.
      realm_path: [
        "app",
        "sys"
      ]
    }
  }

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

$ fx build
.
.
Error: Could not merge index file ../../src/sys/appmgr/config/core_component_id_index.json

Caused by:
    Some entries are missing `instance_id` fields. Here are some generated IDs for you:
[
  {
    instance_id: "47c3bf08f3e560c4dee659c28fa8d863dbdc0b1dbb74065e6cb1f38441ac759c",
    appmgr_moniker: {
      url: "fuchsia-pkg://example.com/my_other_package#meta/my_other_component.cmx",
      realm_path: [
        "app",
        "sys"
      ]
    }
  }
]

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

component_id_index_config_package()

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().