It’s broken into several layers, with entry points based on your needs:
The tokio-proto layer is the easiest way to build servers and
clients. All you have to do is handle message serialization; proto takes
care of the rest. The library encompasses a wide range of protocol flavors,
including streaming and
multiplexed protocols.
The tokio-core layer is good for writing specialized, low-level
asynchronous code. It allows you to work directly with I/O objects and event
loops, but provides high-level, ergonomic APIs for doing so. Use tokio-core
if your problem doesn’t fit into tokio-proto, or you need absolute control
over the internals of your server or client.
The futures layer provides abstractions like futures, streams and
sinks, which are used throughout Tokio and the wider ecosystem. These
zero-cost abstractions are our approach for productive, asynchronous
programming in Rust.
Usually tokio-proto is the right place to start, and that’s how we’ll start
with the guide as well.