What is the proper way to save and/or autosave a vim session?

Question

You can create a session with

:mksession [sessionName.vim]

But if you later modify that session (ie. add/remove buffers) you have to manually save the session again, but doing so will generate the following error

E189: "Session.vim" exists (add ! to override)

which is an error you generally get with Save As not Save. So I have two questions:

1. Is there no Save functionality here?
2. Is there a way to autosave all changes to the session file?

Note: I am aware that I can issue mksession! sessionName.vim to overwrite an existing session file.

Answer 1

1. AFAIK mksession! is really the only way to go.
2. You can do it very easily, with just one line in your vimrc:
   au VimLeavePre * if v:this_session != '' | exec "mks! " . v:this_session | endif

Of course there are plugins that help you save/load and automate saving sessions. Try searching http://vim.org for session.