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sysmgr (Components v1)

sysmgr is one of the two major pieces of Components v1 (appmgr being the other). It is responsible for hosting the sys realm that contains global system services. (The term realm is used throughout this, but note that in v1 it is a synonym for 'environment'.)

Most v1 components on Fuchsia today still run directly in the 'sys' realm, especially those responsible for system-wide and non-user-facing functionality. (Eventually, these will be migrated to the newer Components v2 runtime.) There also exist many (often user-facing) components in child realms under 'sys', such as components launched and managed by the Modular framework, that sysmgr is not involved in launching or managing.

The services available in the 'sys' realm and the components that sysmgr launches to provide those services are determined based on a set of sysmgr configuration files. This doc describes how to add to sysmgr's configuration, the different configration options sysmgr supports, and the configuration format.

Adding to sysmgr's configuration

At runtime, sysmgr loads all files present under /config/data in its namespace and parses them using the format described below. This directory is provided to sysmgr because it uses the 'config-data' feature in its component manifest, sysmgr.cmx. For more details, see the docs on the config-data feature.

You can make a new service available in the 'sys' realm by adding to sysmgr's configuration. There are two supported ways to do so:

  1. Unless there is a specific reason to do otherwise, the centralized services.config file that is included in the core product config (core.gni) and all derivative products can be updated.

  2. Alternatively, a new GN config_data target can be defined and included in product configurations to include an extra file in sysmgr's /config/data directory.

    A common reason to do this is to change the component that provides a given service depending on the product config, e.g. to use fuchsia-pkg://fuchsia.com/foo#meta/foo.cmx in product1.gni and fuchsia-pkg://fuchsia.com/bar#meta/bar.cmx in product2.gni to provide service fuchsia.some.Service. A similar reason is to change the command line arguments passed to the component depending on the product config in order to modify its behavior.

TODO(fxbug.dev/48215): There are recognized deficiencies with the centralized services.config file, such as the fact that it is difficult to understand which services are actually available on a given build since it depends on whether the relevant package is available. We plan to address this during the migration to the newer v2 Component Runtime, but in the meantime using services.config is still the recommended default.

If there are conflicts present, they will be caught at build time and you will see an error similar to this:

Error: conflicts detected in sysmgr configuration
Duplicate configuration for service fuchsia.my.Service in files: ../../some/path/default.config, ../../some/path/alternative.config

TODO(fxbug.dev/48223): As a TEMPORARY workaround for build errors like this, you can set the dangerous_allow_sysmgr_config_conflicts GN variable to true. This variable will be removed shortly and should only be used locally by developers, not in product configs, as it results in non-deterministic behavior (sysmgr may pick any of the conflicting configs to use).

Option #2 Example

In your BUILD.gn:

config_data("my_service_config") {
  for_pkg = "sysmgr"
  sources = "my_service.config"
}

And then in the appropriate product.gni:

base_package_labels += [
  ...
  "//path/to:my_service_config",
  ...
]

sysmgr configuration format

sysmgr's configuration files are in JSON format. Each config file should have a single top-level JSON object, and the following keys are supported:

  • services
  • startup_services
  • apps
  • optional_services
  • update_dependencies
  • critical_components

The contents of all sysmgr config files are read from sysmgr's /config/data directory and merged at runtime to form sysmgr's overall configuration.

services

This is the most common configuration key and where most of sysmgr's configuration comes from. Each entry in the services map consists of a service name and the component URL which provides it. Optionally, command line arguments can be provided to the component by using an array instead of a string value.

{
  "services": {
    "fuchsia.foo.Service": "fuchsia-pkg://fuchsia.com/foo#meta/foo.cmx",
    "fuchsia.bar.Service": [
        "fuchsia-pkg://fuchsia.com/bar#meta/bar.cmx", "arg1", "arg2", "arg3"
    ]
  }
}

The combined services map across all config files defines the list of services that are available in the sys realm and that are available for other components running in the sys realm to request in their component manifests. It is not possible to add services to the sys realm in any other way; the list of services is fixed at realm creation time.

Components in the services map are started lazily as the services they provide are connected to. In other words, in the example above foo.cmx will not be started until another component attempts to connect to fuchsia.foo.Service. If your component needs to be started eagerly, see startup_services or apps below.

startup_services and apps

Both of these keys perform similar functions; they cause sysmgr to eagerly launch components right after it creates the sys realm. The key difference is that startup_services should be used to eagerly launch a component which is providing services to the sys realm (i.e. there is an entry in the services map that uses the component), whereas apps should be used to eagerly launch a component which does not provide any services to sys.

For example, the following configuration would cause sysmgr to eagerly launch the component that provides fuchsia.foo.Service (the same as if another component attempted to connect to the service) as well as an instance of bar.cmx and baz.cmx with the given arguments:

{
  "services": {
    "fuchsia.foo.Service": "fuchsia-pkg://fuchsia.com/foo#meta/foo.cmx"
  },
  "startup_services": [
    "fuchsia.foo.Service"
  ],
  "apps": [
    "fuchsia-pkg://fuchsia.com/bar#meta/bar.cmx",
    [ "fuchsia-pkg://fuchsia.com/baz#meta/baz.cmx", "arg1", "arg2", "arg3" ]
  ]
}

It is important to not mix up usage of startup_services and apps. Using apps instead of startup_services can result in sysmgr starting two separate instances of your component, which is likely unintended. For example, the example below would result in two instances of foo.cmx, one started eagerly and the other started lazily when another component connects to fuchsia.foo.Service:

// WARNING: This is an example of what NOT to do.
{
  "services": {
    "fuchsia.foo.Service": "fuchsia-pkg://fuchsia.com/foo#meta/foo.cmx"
  }
  "apps": [
    "fuchsia-pkg://fuchsia.com/foo#meta/foo.cmx"
  ]
}

optional_services

optional_services causes sysmgr to treat the corresponding services entry as optional. In concrete terms all this means is that sysmgr will skip printing error logs if the launching the component that provides the service fails because it was not present or if the component crashes or exits.

// WARNING: This is an example of what NOT to do.
{
  "services": {
    "fuchsia.foo.Service": "fuchsia-pkg://fuchsia.com/foo#meta/foo.cmx"
  },
  "optional_services: [
    "fuchsia.foo.Service"
  ]
}

update_dependencies

update_dependencies is a list of services that the package resolver depends on. sysmgr implements the link between component resolution (through fuchsia.sys.Loader) and the package resolver and needs this information to break dependency cycles, e.g. so that starting the resolver does not first attempt to resolve or update the resolver.

{
  "services": {
    "fuchsia.pkg.PackageResolver": "...",
    "fuchsia.needed.for.Resolver": "..."
  },
  "update_dependencies: [
    "fuchsia.pkg.PackageResolver",
    "fuchsia.needed.for.Resolver"
  ]
}

The implementation details are subject to change, but as of 2020-04-29 update_dependencies currently does two things:

  1. All services listed in update_dependencies must be present somewhere in the combined services map. If any service is missing, ephemeral package updates will be disabled.

  2. The fuchsia.sys.Loader implementation which sysmgr provides to ephemerally update packages in the sys realm - PackageUpdatingLoader - is configured to not use the package resolver (to avoid attempting to update) any package which provides a service listed in update_dependencies.

    This avoids cycles, since the fuchsia.pkg.PackageResolver service that PackageUpdatingLoader uses to ephemerally resolve packages is itself part of the sys realm.

If the auto_update_packages GN arg is set to false, update_dependencies has no effect. It is only relevant for ephemeral package updates.

critical_components

Disclaimer: this feature is not intended for general use. Please consult before using.

Critical components are a list of components which cause the system to reboot if they ever terminate. The primary use-case are components which are critical to the system's functionality and must always be running. Note that listing a component under critical_components does not mean it will necessarily launch when sysmgr starts; the component may initially launch through other means, such as via startup_services.

Here is an example configuration snippet:

{
  "startup_services": [
    "fuchsia.update.Manager"
  ],
  "services": {
    "fuchsia.update.Manager": "fuchsia-pkg://fuchsia.com/system-update-checker#meta/system-update-checker.cmx"
  },
  "critical_components": [
    "fuchsia-pkg://fuchsia.com/system-update-checker#meta/system-update-checker.cmx"
  ]
}

In the example above, system-update-checker.cmx is launched when sysmgr starts up because it provides a startup service. If system-update-checker.cmx ever exits, the system is rebooted.