The session object is a very useful feature of pylons and is core to
any web application. In pylons the session objects acts like a
dictionary that you can access and modify, as well as save it to a
storage manager which can be memory, file, dbms, memcached, etc.
Normally you want to save whatever changes you made to the session
object, so you have to explictly call session.save(). The most logical
place to do that is at the end of processing a request, but it's
annoying to do that at the end of every request and other wrappers
can't easily tell if the session was modified and needs to be saved.
Since the session can be modified by different parts of your code
during a request, and you are not always sure if those changes will be
saved at the end, what ends up happening is that the session is saved
multiple times during one request, which is wasteful.
Therefore, I have a suggestion to have a "session.autosave" setting in
the config file that will have the following options:
off: don't autosave
on: save the session object at the end of the request, if it was
modified during the request
if_accessed: saves the session object at the end of the request, if
it was accessed during the request
compat: saves the session object when it's first accessed during the
request, updating the _accessed_time attribute.
The difference between "if_accessed" and "on" options is that for
"if_accessed" the session is saved at the end of the request if it was
accessed or modified, but for "on" options it's only saved if it was
modified. In either case this means you can remove all the explicit
"session.save()" statements from your code. Applications that don't
care about the "_accessed_time" attribute can use the "on" setting so
the session will be saved only if you actually modified it (this will
produce better performance).
The last option ("compat") sounds kind of silly, but it's actually the
way the session object currently works: the first time you try to
access the session object, it is loaded using the storage manager, the
"_accessed_time" attribute is updated to now, and then the session is
immediately saved back. So any subsequent changes you make to the
session object need to be saved explicitly. The new option "if
accessed" gives you almost the exact same behavior by making sure the
session is saved if it was accessed, but since it saves at the end of
the request - the session is only saved once (and you don't need to do
it explicitly anymore).
A related note to all those automatic "_something_time" attributes -
there is a bug with "creation_time" that it gets set to "now" when you
do a session.save(). It will be better to have a "_modified_time"
attribute that records when the session was changed, but leave the
original "_creation_time" unchanged so it reflects the original
creation time.
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