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That's exactly how I do it for my Node programs. Any service that I want to have 100% is using the cluster module, resulting in multiple processes listening to the same port. When I want to update, I replace the files and kill the processes one by one.

You could already do this by having a way for the new version to connect an AF_UNIX socket to the old version and request that the listening file descriptor be passed from old to new.

That seems correct to me.