This package provides interfaces for client identification and session support and their implementations for zope.publisher's request objects.
Contents
Sessions provide a way to temporarily associate information with a client without requiring the authentication of a principal. We associate an identifier with a particular client. Whenever we get a request from that client, we compute the identifier and use the identifier to look up associated information, which is stored on the server.
A major disadvantage of sessions is that they require management of information on the server. This can have major implications for scalability. It is possible for a framework to make use of session data very easy for the developer. This is great if scalability is not an issue, otherwise, it is a booby trap.
Sessions introduce a number of issues to be considered:
Clients have to be identified. A number of approaches are possible, including:
which is stored in a cookie. This technique is the most straightforward, but can be defeated if the client does not support HTTP cookies (usually because the feature has been disabled).
stored in the URL. This makes URLs a bit uglier and requires some care. If people copy URLs and send them to others, then you could end up with multiple clients with the same session identifier. There are a number of ways to reduce the risk of accidental reuse of session identifiers:
requires all requests to be POST requests and requires the maintenance of the hidden variables.
o Use the client IP address
This doesn't work very well, because an IP address may be shared by many clients.
Data storage
Data can be simply stored in the object database. This provides lots of flexibility. You can store pretty much anything you want as long as it is persistent. You get the full benefit of the object database, such as transactions, transparency, clustering, and so on. Using the object database is especially useful when:
If writes are frequent, then the object database introduces scalability problems. Really, any transactional database is likely to introduce problems with frequent writes. If you are tempted to update session data on every request, think very hard about it. You are creating a scalability problem.
If you know that scalability is not (and never will be) an issue, you can just use the object database.
If you have client data that needs to be updated often (as in every request), consider storing the data on the client. (Like all data received from a client, it may be tainted and, in most instances, should not be trusted. Sensitive information that the user should not see should likewise not be stored on the client, unless encrypted with a key the client has no access to.) If you can't store it on the client, then consider some other storage mechanism, like a fast database, possibly without transaction support.
You may be tempted to store session data in memory for speed. This doesn't turn out to work very well. If you need scalability, then you need to be able to use an application-server cluster and storage of session data in memory defeats that. You can use "server-affinity" to assure that requests from a client always go back to the same server, but not all load balancers support server affinity, and, for those that do, enabling server affinity tends to defeat load balancing.
Session expiration
You may wish to ensure that sessions terminate after some period of time. This may be for security reasons, or to avoid accidental sharing of a session among multiple clients. The policy might be expressed in terms of total session time, or maximum inactive time, or some combination.
There are a number of ways to approach this. You can expire client ids. You can expire session data.
Data expiration
Because HTTP is a stateless protocol, you can't tell whether a user is thinking about a task or has simply stopped working on it. Some means is needed to free server session storage that is no-longer needed.
The simplest strategy is to never remove data. This strategy has some obvious disadvantages. Other strategies can be viewed as optimizations of the basic strategy. It is important to realize that a data expiration strategy can be informed by, but need not be constrained by a session-expiration strategy.
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'
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 >>> 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. An unnamed ISessionDataContainer is automatically created for you, which may replaced with a different implementation if desired.
>>> 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.
>>> 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
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()
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>
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 users session will actually time out sometime between 50 and 60 minutes.
>>> data_container.resolution 600
New features:
New features:
Added a 'secure' option on CookieClientIdManagers to cause the secure set-cookie option to be used, which tells the browser not to send the cookie over http.
This provides enhanced security for ssl-only applications.
Only set the client-id cookie if it isn't already set and try to prevent the header from being cached. This is to minimize risk from broken caches handing the same client id out to multiple users.