The [Engine API](@ref engine) is a “pure AMQP” toolkit, it decodes AMQP bytes into proton [events](@ref event) and generates AMQP bytes from application calls. There is no IO or threading code in this part of the library.
The [Proactor API](@ref proactor) is a pro-active, asynchronous framework to build single or multi-threaded Proton C applications. It manages the IO transport layer so you can write portable, event-driven AMQP code using the @ref engine API.
The [connection driver](@ref connection_driver) provides a simple bytes in/bytes out, event-driven interface so you can read AMQP data from any source, process the resulting [events](@ref event) and write AMQP output to any destination. It lets you use proton in in alternate event loops, or for specialized embedded applications.
It is also possible to write your own implementation of the @ref proactor if you are dealing with an unusual IO or threading framework. Any proton application written to the proactor API will be able to use your implementation.
The [Messenger](@ref messenger) [Reactor](@ref reactor) APIs are older APIs that were limited to single-threaded applications.
Existing @ref reactor applications can be converted easily to use the @ref proactor, since they share the same @engine API and @ref event set.