Comment by dmux

Being paused in the debugger is per-thread. If the server's using a thread-per-request model, and you're stopped in the request, then other requests can proceed just fine. If some of those requests also trigger the debugger, they'll pause and have to wait, they won't interrupt your current debugging view. Extra care should be taken in any sort of production debugging, of course. (At a Java BigCo, production debugging was technically allowed but required multiple signoffs, the engineer wasn't the one in control but had to direct someone else, lots of barriers to prevent looking at arbitrary customer data, and of course still limited to what you can do with a standard JVM restarted in debug mode. (Mainly setting breakpoints and walking stack traces.))

But the nicest part is that once you connect to the production application, apart from network lag it's no different than if you were developing and debugging locally on similarly specced hardware to the server, you have all the same tools. Many of the broader activities around "debugging" don't need to happen in a paused thread that was entered with an explicit breakpoint or error, they can happen in a separate thread entirely. You connect, then you can start inspecting (even modifying) any global state, you can define new variables, you can inspect objects, you can define new functions to test hypotheses, redefine existing functions... if you want all requests to pause until you're done, you can make it so. Or if you want to temporarily redirect all requests to some maintenance page, you can make that so instead. A simple thing I like doing sometimes when developing locally (and I could do it on a production binary too) is to define some (namespaced) global variable and redefine a singly-dispatched method to set it to the self object (possibly conditionally), and once I have it I might redefine the method again to have that bit commented out just so I know it won't change underneath me. Alternatively I can (and sometimes do) instead set this where the object is created. Then I have a nice variable independent of any stack frames that I can inspect, pass to other method calls, change properties of, whatever, at my leisure without really impacting the rest of the program's running operation. Another neat trick is being able to dynamically add/remove inherited mixin superclasses to some class, and when you do that it automatically impacts all existing objects of that class as well. Mixin classes are characterized by having aspect-oriented methods associated with them; you can define custom :before, :after, or :around methods independent of the primary method that gets called for some object.