Like how is that a 10x improvement on modifying a Lambda function in something like Netlify and clicking "Deploy"? And just "Rollback" if it goes wrong? For most products, which have relatively small user counts, they don't gain much from gradual rollouts.
Right now I'm looking at it for what it is: food for thought.
For me this gradual deploy matters because:
1. it generalizes local development and production and possibly also staging/testing environments in between. I don't have to setup some docker-compose environment that sets up the whole shebang locally. I don't have to setup complex CI systems that spin up an env for integration tests. And I don't have to bend over backwards to ensure that both local and automated test envs are reasonably similar so I can troubleshoot when stuff fails only on CI.
2. even if I don't have that many users, and even if my uptime is not that important; I still don't want to unduly break the application. Somebody will be watching and complaining, if only your boss. What usually happens is that people tend to batch changes and have periodic pushes to production. I've seen products with relatively small user bases have very time consuming release processes. Starting weekly, then biweekly, then monthly. The more the pain, the less frequently you ended up doing them . The less frequently you ended up doing them the more painful they become. Then you end up with somebody dedicated on that because this "release engineering" is becoming clearly a bottleneck. And you do more of it, because now it's somebody's job.
Ah, so "Dark" as in "dark launch"?
You can do that with any language and any serverless stack around, but it still requires work. I'm struggling often to make my teams accept that this little extra setup work will let you iterate faster and often it's not that obvious that it would, since the overhead of the process is non-zero (for some value of zero).
I can't wait to try darklang; I'm really curious. I just need to think of a pet-project that would be suitable for the beta.