As Rails developer I'm used to Nginx + Unicorn in our servers.
Are there similar solutions for Crystal web apps / Kemal ?
I'm not a Nginx expert but I suppose I could use proxy_pass directly to the Crystal HTTP server or Kemal or fast-http-server... what I would miss is process management, GC options, re-spawing, etc.
You can use Nginx in front of your Kemal application and that's fine.
For process management and monitoring i suggest you use something like Monit, init.d e.g since it's not the responsibility of the app server.