Every request comes with a session, a dictionary that is persisted on
the filesystem and that saves various information such as the user
cart on the ecommerce.
When a user simply visits the website, a default session is created and
saved on disk, this bloats the filestore with many sessions. Creating
the session on-the-fly is cheaper than loading it from the filesystem.
With this work the default session is not saved on disk anymore unless
explicitly asked via `session.touch()`.
An exception to the statement "creating the session on-the-fly is
cheaper" is geoip, the ip geolocalization is not cheap. In this work,
geoip have been moved from http_routing/request.session.geoip to a
lazy property core/request.geoip. When requested the info is persisted
on the session. Like other keys from the default session, geoip will not
be persisted unless there is non-default stuff in the session.
Because the CSRF-TOKEN is based on the session-id, it is important the
session-id stays the same across multiples requests even when the
session is not persisted on disk. Even when a session is not persisted
on disk, the session-id cookie is still set so that the next session
created on-the-fly uses the same session-id.
Technical note regarding the session, it has been decided to drop the
session-snapshot protocol and to reintroduce a "modified" flag. It has
been decided not to use werkzeug's session (which natively comes with a
"modified" flag) and to keep our own session object. We decided to
extend MutableMapping instead of dict; using MutableMapping we only
have to override __setitem__ and __detitem__; using dict we would had to
override update()/pop()/... too.
Task: 2789035
Part-of: odoo/odoo#86015
Julien Castiaux
04e972660b