I don't believe one would need a commercial license just to test a product in any way? They are not making that part of any product at that point, so no concerns here.
That puts you about 2/3rds of the way down the list of things to try. You're before all the products that have no evaluation license at all, but after all the open source options that developers can test at their leisure.
If I'm trying a products evaluation license, you can be sure I've tried literally every other option under the sun first, including investigating the possibility rolling my own if situationally appropriate. No form of development is slower than the kind where I have to wait for a company in another timezone to give me permission to use their software, so it's always last on my options list unless the company has frankly amazing reviews that pique my curiosity.
Instagram doesn't operate any user-facing Cassandra servers, though. They run user-facing web servers that talk to Cassandra internally.
I don't like the AGPL because it's unclear on this exact sort of thing, but it does seem to me like the obvious reading of "all users interacting with it remotely through a computer network" does not encompass the connection between Instagram end users and their internal Cassandra.
And, in any case, they released sources for the thing they came up with - which is all that the AGPL requires. If they're okay with doing that, they can definitely use the AGPL for production commercial software.
Counldn't sticking a proxy in front of any AGPL software defeat its purpose then? If you don't consider transitive connections it seems pointless to me.
That's probably a question for a lawyer, but I would not be surprised at an interpretation that a proxy that just mirrors the API of the thing it proxies doesn't insulate you from license compliance, in the same way that a library that just wraps a GPL library doesn't insulate you from license compliance. The question is whether the user is interacting with the AGPL product - if you're talking to software via a proxy, you'd likely say you're interacting with it, but if you're talking to some other software that happens to use that software, are you really interacting with it?
I guess the weird case is that when I'm using the Instagram app, I wouldn't say I'm personally interacting with even the Instagram front-end servers (the way I am in a browser), I'm just interacting with the app which happens to use the servers. And that does sound like not what the license authors would like.
Fake data, non-userfacing servers, sure.