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If your automation breaks on day 1 of your vacation, you’re now forced to spend some of your vacation fixing it. Hope you were checking email.

It’s also worth mentioning that sockets aren’t a real layer, because they’re just an API at the OS level (they’re an abstraction to let application developers use TCP/UDP/etc.). ALSO, in some interpretations, everything above Layer 4 is debatably just the application layer. There’s not necessarily a clean separation between the presentation layer and “application layer” in all applications. That said, the 7 layer model is still useful as a teaching too, but knowing these extra details will give you a more complete understanding (and be less confusing).

I’m late to the party here, but did you consider just paying for Mattermost? If it meets your needs, and your organization has 250 people, the cost for licensing is going to be relatively small compared to your IT budget (right?). They have “contact us” pricing, which means you can negotiate it.