Zope3 Session Implementation
============================

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.

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:

    >>> 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:

    >>> 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.

    >>> sdc = zapi.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. An unnamed `ISessionDataContainer` is automatically created
for you, which may replaced with a different implementation if desired.

    >>> ISession(request)['unknown'] \
    ...     is zapi.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.

    >>> 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.

    >>> session['oops'] = open(__file__)
    >>> import transaction
    >>> transaction.commit()
    Traceback (most recent call last):
        [...]
    TypeError: can't pickle file objects

    Clean up:
    >>> transaction.abort()


Page Templates
--------------

    Session data may be accessed in page template documents using
    TALES::

        <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>