The purpose of this document is to provide short definitions for a collection of technical terms used in the Fuchsia source tree.

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A component whose lifecycle is not tied to any story, is a singleton in user scope, and provides services to other components. An agent can be invoked by other components or by the system in response to triggers like push notifications. An agent can provide services to components, send and receive messages, and make proposals to give suggestions to the user.


The Application Manager (AppMgr) is responsible for launching applications and managing the namespaces in which those applications run. It is the first process started in the fuchsia job by the DevMgr.


An implementation of a user shell.

Base shell

The platform-guaranteed set of software functionality which provides a basic user-facing interface for boot, first-use, authentication, escape from and selection of user shells, and device recovery.


Analogous to a software package, a component encapsulates a single installable resource. It may contain data, code, assets or a combination thereof. Components contain a JSON manifest that describes a number of facets. Components include, but are certainly not limited to modules and agents.


A Channel is the fundamental IPC primitive provided by Zircon. It is a bidirectional, datagram-like transport that can transfer small messages including Handles.


A Device Host (DevHost) is a process containing one or more device drivers. They are created by the Device Manager, as needed, to provide isolation between drivers for stability and security.


The Device Manager (DevMgr) is responsible for enumerating, loading, and managing the lifecycle of device drivers, as well as low level system tasks (providing filesystem servers for the boot filesystem, launching AppMgr, and so on).


The Driver Development Kit is the documentation, APIs, and ABIs necessary to build Zircon Device Drivers. Device drivers are implemented as ELF shared libraries loaded by Zircon's Device Manager.


Graphics library for compositing user interface content. Its design is inspired by modern real-time and physically based rendering techniques though we anticipate most of the content it renders to have non-realistic or stylized qualities suitable for user interfaces.


The Fuchsia Archive Format is a container for files to be used by Zircon and Fuchsia. It will replace Zircon's older BootFS container and be used in Fuchsia Packages.


The Fuchsia Interface Definition Language (FIDL) is a language for defining protocols for use over Channels. FIDL is programming language agnostic and has bindings for many popular languages, including C, C++, Dart, Go, and Rust. This approach lets system components written in a variety of languages interact seamlessly.


Flutter is a functional-reactive user interface framework optimized for Fuchsia and is used by many system components. Flutter also runs on a variety of other platform, including Android and iOS. Fuchsia itself does not require you to use any particular language or user interface framework.


GN is a meta-build system which generates build files so that Fuchsia can be built with Ninja. GN is fast and comes with solid tools to manage and explore dependencies. GN files, named, are located all over the repository.


The “file descriptor” of the Zircon kernel. A Handle is how a userspace process refers to a kernel object. They can be passed to other processes over Channels.




Launchpad is a library provided by Zircon that provides the functionality to create and start new processes (including loading ELF binaries, passing initial RPC messages needed by runtime init, etc). It is a low-level library and over time it is expected that few pieces of code will make direct use of it.


Ledger is a distributed storage system for Fuchsia. Applications use Ledger either directly or through state synchronization primitives exposed by the Modular framework that are based on Ledger under-the-hood.


Little Kernel (LK) is the embedded kernel that formed the core of the Zircon Kernel. LK is more microcontroller-centric and lacks support for MMUs, userspace, system calls -- features that Zircon added.


Zircon is the microkernel and lowest level userspace components (driver runtime environment, core drivers, libc, etc) at the core of Fuchsia. In a traditional monolithic kernel, many of the userspace components of Zircon would be part of the kernel itself.


Services to expose ambient and task-related context, suggestions and infrastructure for leveraging machine intelligence.


A component with a "moduleā€ facet, which describes the module's API in a language in which the system can reflect and compute compatibility with user data and/or other modules.


The view subsystem. Includes views, input, compositor, and GPU service.


Fuchsia's standard C library (libc) is based on Musl Libc.


ZX is an abbreviation of “Zircon” used in Zircon C APIs/ABIs (zx_channel_create(), zx_handle_t, ZX_EVENT_SIGNALED, etc) and libraries (libzx in particular).


fdio is the Zircon IO Library. It provides the implementation of posix-style open(), close(), read(), write(), select(), poll(), etc, against the RemoteIO RPC protocol. These APIs are return-not-supported stubs in libc, and linking against libfdio overrides these stubs with functional implementations.


A namespace is the composite hierarchy of files, directories, sockets, services, and other named objects which are offered to application components by their environment.



Ninja is the build system executing Fuchsia builds. It is a small build system with a strong emphasis on speed. Unlike other systems, Ninja files are not supposed to be manually written but should be generated by more featureful systems, such as GN in Fuchsia.


RemoteIO is the Zircon RPC protocol used between fdio (open/close/read/write/ioctl) and filesystems, device drivers, etc. As part of FIDL v2, it will become a set of FIDL Interfaces (File, Directory, Device, ...) allowing easier interoperability, and more flexible asynchronous IO for clients or servers.


A service is an implementation of a FIDL interface. Components can offer their creator a set of services, which the creator can either use directly or offer to other components.

Services can also be obtained by interface name from a Namespace, which lets the component that created the namespace pick the implementation of the interface. Long-running services, such as Mozart, are typically obtained through a Namespace, which lets many clients connect to a common implementation.


A user-facing logical container encapsulating human activity, satisfied by one or more related modules. Stories allow users to organize activities in ways they find natural, without developers having to imagine all those ways ahead of time.

Story Shell

The system responsible for the visual presentation of a story. Includes the presenter component, plus structure and state information read from each story.

User Shell

The user-specific and replaceable set of software functionality that works in conjunction with devices to create an environment in which people can interact with modules.


The VDSO is a Virtual Shared Library -- it is provided by the Zircon kernel and does not appear in the filesystem or a package. It provides the Zircon System Call API/ABI to userspace processes in the form of an ELF library that's “always there.” In the Fuchsia SDK and Zircon DDK it exists as for the purpose of having something to pass to the linker representing the VDSO.


A Virtual Memory Address Range is a Zircon Kernel Object that controls where and how VMOs may be mapped into the address space of a process.


A Virtual Memory Object is a Zircon Kernel Object that represents a collection of pages (or the potential for pages) which may be read, written, mapped into the address space of a process, or shared with another process by passing a Handle over a Channel.