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# Define an index of components which use storage
Note: This guide uses the [components v1](/docs/glossary.md#components-v1)
architecture.
## Terminology
- [Component Moniker](/docs/glossary.md#moniker)
- [Component Instance ID](/docs/glossary.md#component-instance-id)
## 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:
```json5
// 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()](/build/component/component_id_index.gni) GN template:
```gn
component_id_index("my_component_id_index") {
source = "my_component_id_index.json"
}
```
## Add a component to the index {#add-to-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](/src/sys/appmgr/config/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](/src/sys/appmgr/config/core_component_id_index.json)'s
`instances` list:
```json5
{
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:
```bash
$ 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 {#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()`.