The @ref engine is an AMQP “protocol engine”. It provides functions to manipulate AMQP endpoints and messages and generates [events](@ref event) for the application to handle. The @ref engine has no dependencies on IO or threading libraries.
The @ref proactor is a proactive, 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.
Low-level integration: The @ref connection_driver provides a simple bytes in/bytes out interface to the @ref engine for a single connection. You can use this to integrate proton with special IO libraries or external event loops. It is also possible to write your own implementation of the @ref proactor if you want to transparently replace proton's IO layer.
Old APIs: The @ref messenger and @ref reactor APIs are older APIs that were limited to single-threaded applications. @ref reactor applications can be converted to use the @ref proactor since most of the code is written to the common @ref engine API.