Components

Conceptually, a component is a self-describing WebAssembly binary that interacts only through interfaces instead of shared memory. Let's break down what each of these terms means:

• Self-describing: Like a WebAssembly core module, a component includes import and export declarations that declare both the names and types of imported and exported functions. Compared to core modules, components use a richer type system to describe these types, so it's easier to understand what functionality a module provides and what functionality it relies on.
• Interacts: When a component interacts with other components, that means either that it calls a function defined in a different component, or that another component calls a function defined in it. Interfaces specify what kinds of function calls are valid.
• Shared memory: In the "Why the Component Model?" section, we showed how WebAssembly core modules can only exchange compound data through shared memory. Components use memory in the same way that core modules do, except that in components, memories are never exported or imported; they are not shared.

Logically, a component is a structure that may contain core modules and/or other components. The component encodes the interfaces of these contained modules and sub-components using WebAssembly Interface Types (WIT).

For a more formal definition of a component, take a look at the Component Model specification.

The on-disk representation of a component is a specially-formatted WebAssembly binary. Internally, this file could include representations of one or many traditional ("core") WebAssembly modules and sub-components, composed together via their imports and exports. Two modules or components can be composed if the imports of one are satisfied by the exports of another. Composition can be repeated arbitarily, composing a single component out of many interlocking modules and components. Interfaces enable checking that a particular composition makes sense.

Each component is described by a world, which potentially collects together multiple interfaces to describe all the imports and exports of the component. The world only describes the types of imported and exported functions; the component internally defines the code to implement the world.

Composition

Two modules or components can be composed if the imports of one are satisfied by the exports of another. Composition can be repeated arbitarily, composing a single component out of many interlocking modules and components. Interfaces enable checking that a particular composition makes sense.