You couldn't do everything that Terraform does today without a local state, but perhaps that would be a good thing? Call it "Terraform strict mode".
As every application developer knows, duplication of state is a primary source of bugs. To combat this, React/Flux type of architectures became extremely popular where state flows in one direction only. They dictate that, no you can't just cheat a little and use jQuery to modify some element, it _will_ get bulldozed on the next render. And a lot of Terraform headaches do come from this analogous reconciliation of what really is (our cloud env), what we want (our TF code) and this intermediate state of what TF thinks the cloud state is.
So, by saying that you cannot have resources outside those defined by TF there is actually a massive simplification with far reaching consequences possible.
How I imagine the experience would be:
- You could say that your Dev env is a shitshow and always will be, of manually created resources and only partially TFed. But your Production and Staging envs are opted in to "strict mode". This means that if there is a conflict you do only have two options: import the offender or destroy it, and the critical mindset change is that this is a good thing and will save us a lot of tears later on.
- Caching is an orthogonal concern. Terraform mixes these two together to its detriment, but the nature of a cache is such that you can safely blow it away and perhaps the next reconciliation will be slow, but it will be accurate. I also don't believe it would actually be that slow, tools like Cloudcraft map the entire metadata of your account in seconds.
- I find the excuse that some resources don't support tags intellectually lazy. Of the top of my head thinking about it for a minute, could you tag a parent resource with the child metadata you need? E.g. individual DNS records don't have tags, OK tag the Zone with childA=value. Same thing with tag length limits, you can work around it, concatenate values or whatever. However, in a truly strict mode you wouldn't even need metadata in tags because the TF code describes the entire target environment.
I hope Terraform would entertain such a strict stateless mode. Unfortunately it will probably take another tool, because the problem is not so much technical as it is an entire mindset change.