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.
Distributing components through packages
In Fuchsia, components and their dependent files and images are often distributed through packages which is the unit of distribution in Fuchsia. Components can only use shared libraries that are included in the same package as the component. This allows multiple components in the same package to use the same shared library. This is known as an ABI dependency.
There is no concept of inter-package dependencies because transitive dependency closures have unbounded resolution time. Components are organized to keep critical dependencies in a package, and service dependencies are resolved by runtime resolution, and do not specify exact peer implementations or versions. This is known as an API dependency.
Fuchsia resolves dependencies at runtime instead of resolving at install time like most other operating systems. This applies to API dependencies, but not to ABI dependencies. This approach is similar to a web services architectural model.