Using zope.session

Overview

Caution

Session data is maintained on the server. This gives a security advantage in that we can assume that a client has not tampered with the data. However, this can have major implications for scalability as modifying session data too frequently can put a significant load on servers and in extreme situations render your site unusable. Developers should keep this in mind when writing code or risk problems when their application is run in a production environment.

Applications requiring write-intensive session implementations (such as page counters) should consider using cookies or specialized session implementations.

Setup

This package provides a configure.zcml for use with zope.configuration.xmlconfig that provides the default adapters for IClientId (ClientId), ISession (Session) and the zope.traversing.interfaces.IPathAdapter named session.

It also provides zope.security declarations and marks CookieClientIdManager and PersistentSessionDataContainer as implementing zope.annotation.interfaces.IAttributeAnnotatable if that package is installed.

This document assumes that configuration has been completed:

>>> from zope.configuration import xmlconfig
>>> import zope.session
>>> _ = xmlconfig.file('configure.zcml', zope.session)

Note that it does not install any ISessionDataContainer or IClientIdManager utilities. We do that manually:

>>> from zope.component import provideUtility
>>> from zope.session.interfaces import IClientIdManager
>>> from zope.session.interfaces import ISessionDataContainer
>>> from zope.session.http import CookieClientIdManager
>>> from zope.session.session import RAMSessionDataContainer
>>> provideUtility(CookieClientIdManager(), IClientIdManager)
>>> sdc = RAMSessionDataContainer()
>>> for product_id in ('', 'products.foo', 'products.bar'):
...    provideUtility(sdc, ISessionDataContainer, product_id)

Sessions

Sessions allow us to fake state over a stateless protocol - HTTP. We do this by having a unique identifier stored across multiple HTTP requests, be it a cookie or some id mangled into the URL.

The IClientIdManager Utility provides this unique id. It is responsible for propagating this id so that future requests from the client get the same id (eg. by setting an HTTP cookie). This utility is used when we adapt the request to the unique client id:

>>> from zope.session.interfaces import IClientId
>>> from zope.publisher.http import HTTPRequest
>>> from io import BytesIO
>>> request = HTTPRequest(BytesIO(), {}, None)
>>> client_id = IClientId(request)

The ISession adapter gives us a mapping that can be used to store and retrieve session data. A unique key (the package id) is used to avoid namespace clashes:

>>> from zope.session.interfaces import ISession
>>> pkg_id = 'products.foo'
>>> session = ISession(request)[pkg_id]
>>> session['color'] = 'red'

>>> session2 = ISession(request)['products.bar']
>>> session2['color'] = 'blue'

>>> session['color']
'red'
>>> session2['color']
'blue'

Data Storage

The actual data is stored in an ISessionDataContainer utility. ISession chooses which ISessionDataContainer should be used by looking up as a named utility using the package id. This allows the site administrator to configure where the session data is actually stored by adding a registration for desired ISessionDataContainer with the correct name.

>>> import zope.component
>>> from zope.session.interfaces import ISessionDataContainer
>>> sdc = zope.component.getUtility(ISessionDataContainer, pkg_id)
>>> sdc[client_id][pkg_id] is session
True
>>> sdc[client_id][pkg_id]['color']
'red'

If no ISessionDataContainer utility can be located by name using the package id, then the unnamed ISessionDataContainer utility is used as a fallback.

>>> ISession(request)['unknown'] \
...     is zope.component.getUtility(ISessionDataContainer)[client_id]\
...         ['unknown']
True

The ISessionDataContainer contains ISessionData objects, and ISessionData objects in turn contain ISessionPkgData objects. You should never need to know this unless you are writing administrative views for the session machinery.

>>> from zope.session.interfaces import ISessionData
>>> from zope.session.interfaces import ISessionPkgData
>>> ISessionData.providedBy(sdc[client_id])
True
>>> ISessionPkgData.providedBy(sdc[client_id][pkg_id])
True

The ISessionDataContainer is responsible for expiring session data. The expiry time can be configured by settings its timeout attribute.

>>> sdc.timeout = 1200 # 1200 seconds or 20 minutes

Restrictions

Data stored in the session must be persistent or picklable. (Exactly which builtin and standard objects can be pickled depends on the Python version, the Python implementation, and the ZODB version, so we demonstrate with a custom object.)

>>> import transaction
>>> class NoPickle(object):
...     def __reduce__(self):
...          raise TypeError("I cannot be pickled")
>>> session['oops'] = NoPickle()
>>> transaction.commit()
Traceback (most recent call last):
    [...]
TypeError: I cannot be pickled

Page Templates

Session data may be accessed in page template documents using TALES thanks to the session path adapter:

<span tal:content="request/session:products.foo/color | default">
    green
</span>

or:

<div tal:define="session request/session:products.foo">
    <script type="text/server-python">
        try:
            session['count'] += 1
        except KeyError:
            session['count'] = 1
    </script>

    <span tal:content="session/count" />
</div>

Session Timeout

Sessions have a timeout (defaulting to an hour, in seconds).

>>> import zope.session.session
>>> data_container = zope.session.session.PersistentSessionDataContainer()
>>> data_container.timeout
3600

We need to keep up with when the session was last used (to know when it needs to be expired), but it would be too resource-intensive to write the last access time every, single time the session data is touched. The session machinery compromises by only recording the last access time periodically. That period is called the “resolution”. That also means that if the last-access-time + the-resolution < now, then the session is considered to have timed out.

The default resolution is 10 minutes (600 seconds), meaning that a user’s session will actually time out sometime between 50 and 60 minutes.

>>> data_container.resolution
600