First crack at concepts

Author: itowlson

component-model/.gitignore

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+book

component-model/book.toml

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+[book]
+authors = ["itowlson"]
+language = "en"
+multilingual = false
+src = "src"
+title = "The WebAssembly Component Model"

component-model/src/SUMMARY.md

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+# Summary
+
+- [Concepts](./concepts.md)

component-model/src/concepts.md

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+# Concepts
+
+The Component Model is predicated on a small number of fundamental concepts:
+
+## Interfaces
+
+An **interface** describes a single focused, composable contract, through which components can interact with each other and with hosts. Interfaces describe the types and functions used to carry out that interaction.
+
+* A "receive HTTP requests" interface might have only a single "handle request" function, but contain types representing incoming requests, outgoing responses, HTTP methods and headers, and so on.
+* A "wall clock" interface might have two functions, one to get the current time and one to get the granularity of the timer. It would also include a type to represent an instant in time.
+
+## Worlds
+
+A **world** is a higher-level contract that describes a component's capabilities and needs. In a sense, a world _defines_ a component - it says what interfaces the component exposes for other code to call, and what interfaces the component depends on - but it defines only the surface of a component, not the internal behaviour.
+
+A world is composed of interfaces, but each interface is _directional_ - it indicates whether the interface is available for outside code to call, or outside code must fulfil the interface for the component to call. These interfaces strictly bounds the component. A component cannot interact with anything outside itself except by having its exports called, or by calling its imports. This provides very strong sandboxing: for example, if a component does not have an import for a secret store, then it _cannot_ access that secret store, even if the store is running in the same process.
+
+For a component to run, its imports must be fulfilled, by a host or by other components.  Connecting up one component's imports to another component's matching exports is called _composition_.
+
+* A (trivial) "HTTP proxy" world would export a "receive HTTP requests" interface, and import a "send HTTP requests" interface. A host, or another component, would call the exported "receive" interface, passing an HTTP request; the component would forward it on via the imported "send" interface. To be a _useful_ proxy, the component would also need to import interfaces such as I/O and clock time - without those imports the component could not perform, for example, caching.
+
+## Components
+
+Physically, a **component** is a specially formatted WebAssembly file. Internally, the component could include multiple traditional WebAssembly modules, and sub-components, composed via their imports and exports.
+
+The external interface of a component - its imports and exports - corresponds to a world. The component, however, defines behaviour and internal state.
+
+## Packages
+
+A **package** is a set of one or more WIT (Wasm Interface Type) files containing a related set of interfaces and worlds. WIT is an IDL (interface definition language) for Wasm. Packages provide a way for worlds and interfaces to refer to each other, and thus for an ecosystem of components to share common definitions.
+
+A package is not a world. It's a way of grouping related interfaces and worlds together for ease of discovery and reference, more like a namespace.
+
+* The WebAssembly System Interface (WASI) defines a number of packages, including one named `wasi:clocks`. Our HTTP proxy world could import the `wall-clock` interface from the `wasi:clocks` package, rather than having to define a custom clock interface.