Software Architecture
Abstract
Chromium OS consists of three major components:
- The Chromium-based browser and the window manager
- System-level software and user-land services: the kernel, drivers, connection manager, and so on
- Firmware
High-level design
We'll look at each component, starting with the firmware.
Firmware
The firmware plays a key part to make booting the OS faster and more secure. To achieve this goal we are removing unnecessary components and adding support for verifying each step in the boot process. We are also adding support for system recovery into the firmware itself. We can avoid the complexity that's in most PC firmware because we don't have to be backwards compatible with a large amount of legacy hardware. For example, we don't have to probe for floppy drives.
Our firmware will implement the following functionality:
- System recovery: The recovery firmware can re-install Chromium OS in the event that the system has become corrupt or compromised.
- Verified boot: Each time the system boots, Chromium OS verifies that the firmware, kernel, and system image have not been tampered with or become corrupt. This process starts in the firmware.
- Fast boot: We have improved boot performance by removing a lot of complexity that is normally found in PC firmware.
System-level and user-land software
From here we bring in the Linux kernel, drivers, and user-land daemons. Our kernel is mostly stock except for a handful of patches that we pull in to improve boot performance. On the user-land side of things we have streamlined the init process so that we're only running services that are critical. All of the user-land services are managed by Upstart. By using Upstart we are able to start services in parallel, re-spawn jobs that crash, and defer services to make boot faster.
Here's a quick list of things that we depend on:
- D-Bus: The browser uses D-Bus to interact with the rest of the system. Examples of this include the battery meter and network picker.
- Connection Manager: Provides a common API for interacting with the network devices, provides a DNS proxy, and manages network services for 3G, wireless, and ethernet.
- WPA Supplicant: Used to connect to wireless networks.
- Autoupdate: Our autoupdate daemon silently installs new system images.
- Power Management: (ACPI on Intel) Handles power management events like closing the lid or pushing the power button.
- Standard Linux services: NTP, syslog, and cron.
Chromium and the window manager
The window manager is responsible for handling the user's interaction with multiple client windows. It does this in a manner similar to that of other X window managers, by controlling window placement, assigning the input focus, and exposing hotkeys that exist outside the scope of a single browser window. Parts of the ICCCM (Inter-Client Communication Conventions Manual) and EWHM (Extended Window Manager Hints) specifications are used for communication between clients and the window manager where possible. The window manager also uses the XComposite extension to redirect client windows to offscreen pixmaps so that it can draw a final, composited image incorporating their contents itself. This lets windows be transformed and blended together. The window manager contains a compositor that animates these windows and renders them via OpenGL or OpenGL|ES.
