what about

- single point of failure of rooms (server creating the chat room)

- media access via 3rd party servers

- gossiping encrypted message access across multiple devices

- encrypted audio-visual (group) communication

- good clients for iOS

- reliable server-side storage of messages (of all(!) chats)

Maybe for friends and family? But then, if their phone might be compromised by the state...

And telling friends and family they need to setup a Jabber connection to talk you will make you "That guy/girl who...". And if the idea spreads, then talking to n people requires n accounts on n servers. At least Pidgin (last time I used it, which was last decade) supports such a configuration. I wonder if the mobile apps can do the same.

I cannot say, that I have problems with it. I use monal on iOS and whenever someone mentions me, I get a notification.

If you’re running a single machine, you’ll be limited by the number of available ports. It’s a TCP limitation and nothing to do with XMPP. You’d need a cluster of XMPP servers to handle 40M users. Even for just text. Port limits are port limits.

Here's a blog post on scalability from the front page (well, sort of, see last paragraph): https://www.process-one.net/blog/ejabberd-massive-scalabilit... This includes not just textual claims about scalability but a lot of hooks you can use to followup on, like a reference to the Tsung benchmarking tool you can use. If you're asking about this for serious reasons, at this scale you're obviously running your own tests anyhow. You may also want to speak directly to Process One about this because it sounds like you're at a scale where you should probably be looking at paid support anyhow.

I'm not necessarily endorsing this, just giving it to you as some first stabs at answers and some ways to follow up.

If there's anyone reading this from the ejabberd project, note that the link to this on your front page under "Massively Scalable" is completely broken; that links to "blog.process-one.net" but that domain is completely dead, so it doesn't redirect to the link I gave above. (It's also part of why I posted this; my post here is not a "just read these docs" because I had to do non-trivial work to find them.) I had to pull that out of archive.org. Should probably check for any other links to that domain too.

I recently tried to move people over, and the iOS situation is bleak. It seems like notifications don't work at all in any app, which is a big problem for chat. Also, one of them (I think Siskin?) doesn't display sender name/avatar in group chats, so you can't tell who sent what.

There's also some weird behavior with how E2EE works, which is causing me problems in group chats. Initially it worked, but now people are unable to read some messages with errors like "this message was not encrypted for this device"

FWIW, everything works great on Android with the Conversations app, and also I admit I haven't really RTFM'd too much so it may be my fault as a lazy admin.

Edit: this is with ejabberd btw