Comment by kragen

Yes, I agree. I meant that you can edit code on the production web server so that the customer you're on the phone with sees their application start working. PG had the advantage that he could tweak the Lisp process for that specific user, reducing the risk of breaking the site for other concurrent users.

Database transactions bridge some of the gap between "change the code for the next request" and "stop at a breakpoint and change the code": as long as your request handler code keeps failing, it will abort the transaction, so the database is unchanged, so you can restart the transaction as many times as you want to get to the same point in execution, at least if your programming language is deterministic. By providing a snapshot you can deterministically replay from, it allows you to add log entries before the point where the problem occurred, which can be very useful.

Stopping at a breakpoint can be more productive, especially with edit-and-continue, but often it isn't. A breakpoint is a voltmeter, which you can use to see one value at every node in your circuit; logs are a digital storage oscilloscope with a spectrum analyzer, where you can analyze the history of millions or billions of values at a single node in your circuit.