Adding a "subsystem" to log messages?

[Lord-Kamina]

Hi. I'm currently looking into replacing a very clunky in-house logger with spdlog. In our current logger, we have "subsystems" (which are really just implemented as arbitrary strings) to mark what section of the app a message is coming from.
With our current format we might have messages reading like this:

locale/debug: Checking if language is set in config.
locale/notice: Setting language to: 'en_US.UTF-8'

I understand I could replicate such a thing by creating a different logger for each subsystem and using the logger's name, but we really have a lot of those and I think it would end up being a tremendous mess.

I was wondering if there's any way I can just pass the subsystem in as a parameter to each logger call, and have the default log format be something like: [%Y-%m-%d @ %T] %^*SUBSYSTEM HERE*/%l%$ -- %v

[tt4g]

Context (PR #2907) and custom formatter (https://github.com/gabime/spdlog/wiki/3.-Custom-formatting#extending-spdlog-with-your-own-flags) may be used.

[Lord-Kamina]

MDC won't work because I want to use an async logger.

@gabime Had originally suggested just the custom flags (hadn't had time to reply) but I believe that wouldn't work either because, if I understand correctly, what that essentially does is provide a shorthand for not having to repeat the logging format in more than one place, but what I need is to be able to provide an extra parameter that I can then reference in my format string.

[tt4g]

Your idea sounds like #3176.

[Lord-Kamina]

Indeed, it seems like we want to achieve much the same thing.