Fuchsia is an operating system that does many things differently from traditional operating systems. Before you attempt to develop for Fuchsia, it is important to understand how the software model works in Fuchsia.
In Fuchsia, almost everything is a component and it is the unit of executable software. The Component framework is what runs all components on Fuchsia. For more information on components and the component framework, see Introduction to the Fuchsia component framework.
An API dependency allows different components to define a dependency on another component. These API dependencies are abstract which means that the implementation of the interface is defined by another component and resolved by the component framework. Components communicate with their dependencies through FIDL, which is the language that Fuchsia uses to describe interprocess communication (IPC) protocols of components that run on Fuchsia. For more information on FIDL, see FIDL overview.
In Fuchsia, components and their dependent files and images are often distributed through packages which is the unit of distribution in Fuchsia.
Note: For more information on packages, see Fuchsia packages
Fuchsia resolves packaged dependencies at install time, creating an ABI dependency. References to resources from an external package are resolved at runtime, creating an API dependency, but not an ABI dependency. (Runtime resolution is similar to a web services architectural model.)
Components are organized to keep critical dependencies in a package; and this extends to subpackages which are bound to their containing package at build time, allowing ABI dependencies to be resolved statically.
Note: Executable components (programs) implement API dependencies through FIDL
A logical way to package components that launch other components is to use subpackages to mirror the component parent-child relationship hierarchy such that, if a component declares a child component, the child is loaded from a declared subpackage of the parent component's package. This encapsulates the ABI dependencies and ensures the presence of the expected ABI version of the child component. Components model API dependencies through capability routing (services exposed, routed, and used, by capability name, such as a FIDL protocol). Package dependencies are less relevant to capability routing, except that a parent component can orchestrate the creation of independently-packaged peer components (subpackaged or not) and declare the capability connections between them.