Fuchsia Boot Sequence

This document describes the boot sequence for Fuchsia from the time the Zircon layer hands control over to the Garnet layer. This document is a work in progress that will need to be extended as we bring up more of the system.

Layer 1: appmgr

appmgr's job is to host the environment tree and help create processes in these environments. Processes created by appmgr have an zx::channel back to their environment, which lets them create other processes in their environment and to create nested environments.

At startup, appmgr creates an empty root environment and creates the initial apps listed in /system/data/appmgr/initial.config in that environment. Typically, these applications create environments nested directly in the root environment. The default configuration contains one initial app: bootstrap.

Layer 2: sysmgr

sysmgr's job is to create the boot environment and create a number of initial components in the boot environment.

sysmgr provides services that are not offered in the boot environment itself. When sysmgr receives a request for a service for the first time, sysmgr creates the appropriate application to implement that service and routes the request to that application. Subsequent requests are routed to the already running application. If the application dies, it is restarted automatically the next time a service attempts to connect to that application.

By default, sysmgr reads all configuration files from /config/data/sysmgr/, which are formatted as either an app or service. The table of which components implement which services is contained in the services.config file.

Layer 3: basemgr

basemgr's job is to setup the interactive flow for user login and user management.

It first gets access to the root view of the system, starts up Device Shell and draws the Device Shell UI in the root view starting the interactive flow. It also manages a user database that is exposed to Device Shell via the User Provider FIDL API.

This API allows the Device Shell to add a new user, delete an existing user, enumerate all existing users and login as an existing user or in incognito mode.

Adding a new user is done using an Account Manager service that can talk to an identity provider to get an id token to access the user's Ledger.

Logging-in as an existing user starts an instance of sessionmgr with that user's id token and with a namespace that is mapped within and managed by basemgr's namespace.

Logging-in as a guest user (in incognito mode) starts an instance of sessionmgr but without an id token and a temporary namespace.