But the messaging mandate here is not about apps, it is about connectivity, so about users being able to communicate across platform boundaries; So the APIs in question are between platform servers, not between apps and OSes.
I have substantial concerns about the messaging provisions... From the article:
Immediately, gatekeepers will be required to support messaging between users on different platforms, but the DMA includes provisions to expand to group chats after two years, and video and audio calls after four years.
Depends a bunch on the politician's notion of what "immediately" means. This makes it sound like, "well, starting
right now, you're going to have to allow other services to use your messaging API". Which assumes that the various services already
have existing APIs that are suitable for outside use. I expect for many of them this is not the case. It will take time to implement something. It'll take
more time to implement something that really works well.
And what really should be done would be for the various companies to get together (ideally under the auspices of one of the existing standards bodies that do such work, like the ISO) to work out a uniform message exchange standard that can be implemented across the various platforms, that supports a workable subset of the capabilities that are present in all or most of the platforms (with probably a lot of arguments along the way about what vendor-specific features should be included/excluded, what fields should be included, what lengths each field should support, how attachments are handled, what kinds of attachments are permitted, etc. - I leave out what character set/encoding should be used because the answer
must be UTF-8).
This would have substantial long-term benefits in that if there were a problem with communications between any two of the companies, there would be a standard to consult to see which bit of software was working incorrectly. As well, it would massively reduce the amount of work for any new player entering the arena (which, presumably, is exactly what the backers of the DMA/DSA want) - instead of getting "here's Apple's API documentation, here's Google's API documentation, here's Facebook's API documentation, etc." (which each being a dozens or hundreds of pages long), the new entrant would get "here's
the messaging exchange standard document", and they just need to build their end of it
once.
Unfortunately, I expect the backers of the DMA/DSA will interpret anything more than a couple weeks for implementing their "immediately" requirement as being the various companies dragging their feet, and will push for "something" to be thrown into service right away.