Element/Matrix, plain old IRC
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Really depends on what you're looking for. I'm a big fan of xmpp and irc but that might not fit the bill
Matrix and Element work. It has apps for desktop IOS and Android. Can have multiple channels for different topics. Not quite slack level of workspace management, but it's an option.
Is it text only? xmpp should be lightweight and smooth sailing. prosody configured to never expire messages would give you a forever archive on the server. There are good mobile clients, plus web/pwa and native desktop clients.
At 30 people, buying a refurbished rackmount server is my recommendation.
They run anywhere from $250 to a couple thousand, but I'm running a media server and ebook/comic library server on an HPE DL360 Gen 9 with 64gb DDR4 RAM and dual 8-core Xeon processors, which I paid $450 for.
That will give you so many more options for your co-op.
For 30 people? A $10us VM at Digital Ocean would be more than sufficient.
My t2.small instance on AWS runs me about $40/month when you factor in elastic IP, backup snapshots, traffic, bursting, etc. I haven't used DO before, so I don't know what kind of overheads they get beyond the raw instance compute costs, but even at $10/month you'd have paid for the server in less than 4 years, (and just 1 year at $40/mo), and you're getting a MUCH beefier server.
To run an XMPP server you don't really need a beefy instance. 2 gigs of RAM, one core, 2TB of transfer per month (which I've yet to even touch given how much I use that server), 50 gigs of storage locally, a static IPv4 address, IPv6 addresses if I want them, snapshots, and enough traffic that I don't think I've even noticed it on my invoices.
That will give you so many more options for your co-op.
Sure, if they are only ever going to do text over XMPP, that's plenty. If they ever want to do more than that, VPS will mean more money, whereas a self-hosted server won't. Shared docs, video or music streaming, e-book library, etc will murder a small or medium instance on AWS if you've got 30 people using it at once. Music and video especially, obviously.
Co-ops generally want to minimize costs and outside reliance, and "scale up on hosted cloud stacks as-needed" is sort of the opposite of that philosophy.
We use matrix with Element and set up our own video server so that we can do video meetings etc. All encrypted and self hosted.
I think xmpp
is closest to a Slack replacment, second only to Mattermost
. I think Mattermost does have a stronger commercial lean, so I wonder if it's just a matter of time till it goes closed-source to generate "more value".