[Twisted-Python] Re: Getting Stories Straight (RPYs)
glyph at twistedmatrix.com
Wed Jun 25 15:22:37 EDT 2003
On Wednesday, June 25, 2003, at 09:40 AM, W.J. wrote:
>>> You don't want to bounce offline all users currently using the IMAP
>>> IRC servers every time you make a small change to a web page.
>> If you're doing development on the live server the users are using,
>> bouncing them off every now and then is a good way to let them know
>> they are morons for using such a sketchy server.
> Very much in the same way that moving a chair or changing a lightbulb
> is reason enough for kicking everyone out of the building.
I think there is a slight miscommunication in terms of what is meant by
'every time you make a small change'. I get the impression that what
is being said here is:
<moshez> you should not be doing your development on a live server.
<W.J.> so you're saying I shouldn't be able to load updates on a live
One of Twisted's original design goals was _fast server restart_,
because restarting the server is pretty much inevitable if you want to
make substantial changes; I wanted to make it super-easy for developers
to bounce their servers every minute while making minor changes. I
even had a 'bounce my Twisted server' key in emacs for a while when I
was working on a more deeply interactive system than I am now. This
was based on some pretty bitter experience with a server that I was
coding on at the time which took almost an hour to start up. EJB
servers also have this problem; it's not just that you have to restart
them, it's that you have to _wait_ when they restart.
Another design goal is _restart-free code reload_, which you can see
still in twisted.python.rebuild, because I know there are times when a
deployed server needs to be fixed _right now_ and although it can lose
a few seconds of time, it could not brook an interruption of service.
The choice of how you do your development with these tools is really a
matter of taste, and I don't have any feelings about it one way or
another; however, deployment is contingent upon how reliable restart
vs. reload is.
You should NEVER make live server code changes without first setting up
a test environment, making the changes, testing the changes, and then
bringing up an OLD server environment and testing the upgrade path from
one version to the other however you intend to do it live.
Python's reloading is handy, but dangerous, and Twisted's 'rebuild'
utility adds to both the power and the danger. If you use Python's
reload() thinking it will simply update your server to the most recent
code revision, you will be in for a nasty shock when you find that
objects using both the new AND the old code are still around,
isinstance() will fail sometimes for instances of a particular class,
and tracebacks have incorrect code in them.
rebuild() 'does more' for you, however, it can still be surprising.
You may find that objects which were previously initialized to a
consistent state are now missing attributes which the new code expects
the initializer to set. You may find that objects that were old-style
and are now new-style simply didn't reload. It has been too long since
I've looked at the code to remember every dark corner, but there are
definitely things that can be made to fail. You have to be careful
with what kinds of changes you load, and understand how the change will
interact with the existing code and objects you're updating.
So, despite the conveniences that Twisted can provide for you, you
can't escape the inevitable software reality that "Development",
"Test", and "Production" should be three completely separate
installations on which you can perform upgrades independently, in a way
suitable to their purpose. Anything less will result in an unstable
production environment where, whether you like it or not, your users
will all fall out of a 30th story window every time you change a
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