You have your normal application, accessible on some port serving HTTP requests (or whatever). Then you start a thread (with `bt`, bordeaux-threads, a very common multi-threading library) and run a Swank server listening on some other port on it. Swank is the backend used by SLIME, it allows manipulation of the Lisp environment. A remote server is still the same Lisp environment as when you run it on your machine, so you can use Emacs to connect to it (M-x slime-connect) and debug live.
Swank doesn't allow remote connections though, otherwise anyone could connect and mess with your server. You must connect through the loopback interface. Easiest way is to set up an ssh tunnel, like: ssh abc@example.com -L 4200:localhost:4200
Whether you can call the Swank server a second server or not is a somewhat philosophical question I guess. There's one application serving different kinds of requests on different ports.
Does that mean the swank thread still gets all the inputs of the live process and you can debug in isolation there and then push changes to the main thread? Sorry if I’m stupid (no need to reply), I need to read into this more.
You don't need to "push changes to the main thread", you can mutate the state of the whole process during debugging, and in Lisp that includes compiling and loading new code that gets used immediately by whatever application you're trying to debug.
It's probably worth mentioning that you usually don't want to make changes in a live production environment, and even when you do, you probably want to capture those changes in a file as well, so that you have a record of what you did. It's possible to make arbitrary changes to a live, running system in Lisp with no other record of what changes you made, but it's not usually a good idea to do that.
Swank doesn't allow remote connections though, otherwise anyone could connect and mess with your server. You must connect through the loopback interface. Easiest way is to set up an ssh tunnel, like: ssh abc@example.com -L 4200:localhost:4200
Whether you can call the Swank server a second server or not is a somewhat philosophical question I guess. There's one application serving different kinds of requests on different ports.