Proposal for yet another MQTT library for Boost

From: Takatoshi Kondo (redboltz_at_[hidden])
Date: 2024-04-29 15:41:28

Dear Boost Developers,

I'd like to propose a MQTT library to the Boost Libraries.

Repository link: https://github.com/redboltz/async_mqtt
Document link: https://redboltz.github.io/async_mqtt/doc/5.0.0/index.html

Here is characteristics of async_mqtt:
- C++17 or later is required.
- async_mqtt supports Boost.Asio style completion tokens.
  - e.g. use_future, deferred, use_awaitable, callback functions, etc
- Not only MQTT client but also MQTT server is supported.
  - User can use this library to develop MQTT broker.
  - Sample MQTT broker application is included.
- Both MQTT v3.1.1 and v5.0 are supported.
  - In addition, undetermined protocol version is supported for server
implementation. In this case, the protocol version is decided when you
receive CONNECT packet.
- Continuous packet sending
  - Users can send the next MQTT packets before the previous sending
is not completed (completion handler is not invoked yet).
- Auto acquiring/mapping topic alias is supported.
  - In addition, when resending a PUBLISH packet after reconnecting,
the topic alias is removed and the original topic is restored. It is
required by MQTT v5.0 spec.
- Packet based send()/recv() APIs.
  - MQTT has various packets and some of their fields are optional. In
order to keep simple APIs, async_mqtt uses packet based APIs.
  - See https://redboltz.github.io/async_mqtt/doc/5.0.0/tutorial/send_recv.html
- Layered archtecture
  -  async_mqtt can have predefined TCP, TLS, WS, WSS, and user
defined underlying layer. Users can access the underlying layer using
next_layer() member function of MQTT endpoint. So you can configure
any layers by yourself. e.g. tcp_no_delay, set send/recv buffer size,
etc
  - See https://redboltz.github.io/async_mqtt/doc/5.0.0/tutorial/create_endpoint.html
- High performance
  - See https://redboltz.github.io/async_mqtt/doc/5.0.0/performance.html

Background

I have developed another mqtt library mqtt_cpp
https://github.com/redboltz/mqtt_cpp since 2015. It is widely used but
has some limitations. For example:
- The functions are directly mapped to packets(publish, subscribe,
etc), so optional features are not designed cleanly.
- Due to set_publish_hander / on_publish style handler design,
Completion Token cannot be supported.

Based on this 9years experience, I designed the new MQTT library. This
is async_mqtt.

I would appreciate if you could endorse this library.

Regards,
Takatoshi Kondo