Re: [async-mqtt5] Review questions

From: Ivica Siladic (ivica.siladic_at_[hidden])
Date: 2024-10-21 14:32:03

Hi Seth,

> On 21.10.2024., at 00:06, Seth via Boost <boost_at_[hidden]> wrote:
> 
> On Sun, Oct 20, 2024, at 11:09 PM, Vinnie Falco via Boost wrote:
>> On Sun, Oct 20, 2024 at 1:31 PM Ivica Siladic <ivica.siladic_at_[hidden]>
>> wrote:
>> 
>>> TlsContext is indeed not a concept
>> 
>> 
>> I think that the treatment of TlsContext in the public API doesn't rise to
>> the same level of quality as the rest of the library. I'm not sure what the
>> best way to improve it is, though. 
> 
> When I read that, I was already thinking that there could be a factory for (tls) streams. Many dynamic IO frameworks (in other languages) seem to have this approach, I think I remember it from my Java past and even the Curl API seemed to have multiple callbacks to customize e.g. socket creation and configuration.
> 
> Of course, this type of customization can be modeled as a "strategy mixin" in the good old Modern C++ spirit. The principle remains the same.
> 

There are several ways to implement a factory-like approach. One option is to use a callback to create the stream. However, since the stream type cannot be type-erased, this callback would need to be a template function. This means the user would have to provide a template specialization to create the stream. During the stream creation, the user would also need access to the corresponding SSL context, which could lead to the use of a global variable or a Meyers-like singleton—neither of which is ideal.

Another option is to use a template class, such as TlsStreamFactory, and then templatize the mqtt_client class with this factory. This approach would allow the creation of a well-defined concept (i.e., prescribing the required interface). It is arguably cleaner and easier to understand, but it would require more effort from the user compared to the current TlsContext-template approach.

Honestly, I’m not sure which approach is better.

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Thanks,
Ivica Siladic