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Introduction to the Fuchsia component framework

This document offers a brief conceptual overview of the component framework along with links to more detailed documents on specific topics.

Components and the component framework

A component is a unit of software that is sandboxed and interacts with other components through capabilities.

The component framework is a framework for developing component-based software for Fuchsia.

The component framework is responsible for running nearly all software on Fuchsia so it is important for developers to learn how it works and how to use it effectively.

Purpose

The component framework empowers developers to write software for Fuchsia with an emphasis on separation of concerns, modularity, and composition.

Each component typically has a small number of responsibilities and exposes services that other components use to perform more complex tasks. For example, the ethernet driver component exposes a service for sending ethernet frames. The network stack component connects to the ethernet driver component and uses its services to communicate with the hardware. These two components can be authored by different parties and be distributed separately because they agree on a common set of protocols that let them work together.

Emphasizing software composition has numerous advantages for a modern operating system:

  • Configurability: The behavior of the system can be changed easily by adding, upgrading, removing, or replacing individual components.
  • Extensibility: As components are added, the functionality of the system grows.
  • Inclusion: Anyone can author new components.
  • Reliability: The system can recover from faults gracefully by stopping or restarting individual components.
  • Reuse: Existing components can be reused and composed with other components to solve new problems.
  • Testability: Prior to integration, each component can be verified separately so it is easier to isolate bugs.
  • Uniformity: All components describe their capabilities in the same way independent of their origin, purpose, or implementation language.

Overall, the component framework makes it easier to update and improve the system incrementally as new software is created.

A component is a hermetic composable isolated unit of software

A component is a unit of software.

  • It may be executable (a program), or it may present or delegate capabilities without containing any executable code.
  • It is identified by the URL from which its declaration and assets are retrieved.
  • It can be implemented with any runtime framework provided by a component runner.
  • It has a declaration that describes what it can do and how to run it.

A component is an isolated unit of software.

  • Each instance of a component runs in its own sandbox with its own lifecycle, state, and capabilities.
  • It cannot access capabilities other than those it has been granted.
  • Its capabilities cannot be accessed by other components unless they are explicitly granted.
  • It primarily communicates with other components via IPC.
  • It cannot compromise the integrity of the entire system, even if it crashes.

A component is a composable isolated unit of software.

  • It can combine and be combined with other components to form composite components with parent-child relationships.
  • It declares the capabilities it needs to use and those it needs to delegate to its children with capability routing.

A component is a hermetic composable isolated unit of software.

  • It encapsulates its implementation, state, capabilities, and children.
  • It can be seamlessly replaced with a different implementation as long as the new implementation uses and exposes the same capabilities.
  • The component's assets and runner together include everything needed to run the component; the system does not provide language-specific assets such as the C runtime library.

Everything is a component (almost)

Components are ubiquitous. They are governed by the same mechanisms and they all work together seamlessly.

Almost all software that runs on Fuchsia is a component, including:

  • Command-line tools
  • Device drivers
  • End-user applications
  • Filesystems
  • Media codecs
  • Network stacks
  • Tests
  • Web pages

There are only a few exceptions, notably:

  • Bootloaders
  • Device firmware
  • Kernels
  • Bootstrap for the component manager itself
  • Virtual machine guest operating systems

Further Reading