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ZODB based heavy duty Zope session implementation.

Project description

This package contains a replacement for Zope’s Products.Transience, which is typically used to implement Zope’s sessions. While Products.Transience has quite a high risk for ConflictError if many requests access the Zope session, the current package should be responsible only for very few ConflictErrors, even if every request accesses the Zope session.

This is achieved by a changed organization of the sessions set. Products.Transience organizises this set into a list of generations. If a session is accessed, it is moved into the newest generation (if not already there). This way, a session access (even a read only session access) can cause a write and result in a ConflictError. dm.zope.session maintains the sessions in an OOBTree (which has partial conflict resolution). To support session timeout, the last access time is maintained on the individual session object in a data structure with 100 % conflict resolution. This way, read access to the session should not result in a ConflictError. To reduce ZODB bloat, the access time is not updated on every access but employs a limited resolution, by default about 1/8 of the session timeout.

Products.Transience performs an inline cleanup for outdated sessions. It can do that because its cleanup is very fast (drop the oldest generation and create a new newest one). dm.zope.session’s cleanup involves visiting all sessions and check whether they are outdated. This can be expensive. Therefore, it performs the cleanup offline - in a separate thread. The cleanup uses very short transactions to reduce the risk of conflicts.

If you use Zope in a ZEO setup, i.e. serveral Zope processes use the same ZODB, then only one of the Zope processes will run the cleanup thread. It may take until the (old) period of this thread has finished before some configuration changes become effective (especially cleanup_period_sec). In general, this should not make a big problem. Should it be problematic in special cases, you may need to restart this process. You will find its process id in a lockfile under clienthome with a name derived from the session container path.

ATTENTION Like most (server side) session implementations, dm.zope.session can be vulnerable to denial of service attacks. It is often not difficult to make the server create a new session and each session needs resources; an attacker can try to massively create sessions in order to overload this kind of resource. dm.zope.session puts its sessions into a ZODB which typically uses a file as storage. This kind of resource usually has a large capacity and therefore is difficult to overload. However, should an overload happen, the consequences can be severe. Please read the section DOS attacks to learn about some of your options to counter those attacks.


dm.zope.session supports the following features. Corresponding configuration variables are specified in parenthesis.

Maximal session lifetime (session_max_lifetime_min)

This feature allows to limit the maximal lifetime of the session. It can be interesting for security reasons: should an attacker have been able to hijack a session, he should only be able to use it for a limited period.

In Zope’s session architecture, the session id is managed by the so called browser_id_manager. It is using a cookie to pass the session id between browser and server. To be effective, the maximal session lifetime must be combined with a limited lifetime for the session id; otherwise, an attacker can use the known session id to hijack a followup session. In modern Zope versions, the session id is by default stored in a (browser) session cookie. In this setup, an attacker can hijack followup sessions until the user terminates his browser session.

The maximal session lifetime is controlled via the configuration variable session_max_lifetime_min. It specifies the maximal lifetime in minutes. A non positive value deactivates the feature.

Session inactivity timeout (session_timeout_min)

A session is deleted when is it not used for about session_timeout_min minutes. If session_timeout_min has a non positive value, the feature is deactivated.

The timeout feature is not implemented precisely: a session may be deleted slightly before the specified inactivity and may live slightly longer than the specified timeout. There are two reasons for this:

  1. to avoid ZODB bloat, the session access time is stored with limited (usually about 1/8 of the session timeout) resolution.

  2. session inactivation is performed by a periodic cleanup process. Nothing happens until the next run of this process.

The inactivity timeout is an inportant feature for sessions implemented by Products.Transience, because it typically stores sessions in (precious) RAM. dm.zope.session sessions are typically stored in a FileStorage for which early freeing resources is of far less importance. However, if you store sensible information in the session, you may still want to use this feature for security reasons: it forces a potential attacker to keep a hijacked session alive for continued exploitation.

Session access time resolution (session_access_time_resolution_min)

This controls whether the session access time is updated: an update is suppressed if the last update was more recent then the access time resolution. session_access_time_resolution_min specifies an access time resolution in minutes. The effective access time resolution is the minimum of this resolution and about 1/8 of the session timeout where non positive values are ignored. If both resolutions are non positive, the access time is not maintained.

Session target number (session_target_number)

A periodic cleanup tries to keep the number of sessions below session_target_number by deleting the oldest sessions. A non positive value for session_target_number deactivates the feature.

Session maximal number (session_max_number)

session_max_number limits the number of session objects. The exception dm.zope.session.MaximalSessionNumberExceeded is raised when the limit would be exceeded. A non positive value deactivates the feature.

This option can be used to limit the thread posed by DOS attacks.

Periodic session cleanup (cleanup_period_sec)

A periodic cleanup process deletes sessions whose maximal lifetime or inactivity timeout is reached. It also deletes the oldest sessions to keep the session number below the session target number. cleanup_period_sec specifies the period for this process in seconds. If it has a non positive value, 10 is used instead.


For customization purposes, some events are notified. Their interfaces are all defined in dm.zope.session.interfaces. All events are “object events” with either the session or the session container as event object.


Notified when a new session object is created. Can be used to transfer information from the request to the session, e.g. information potentially helpful to detect DOS attacks.


Notified when the maximal session number is reached. Can be used to implement a custom policy to delete sessions in such a case in order to let the operation succeed. Note, however, that the deletion of the same session by concurrent requests leads to an unresolvable conflict (only one of those requests will succeed); it is therefore advicable to choose the session to be deleted with considerable randomness.


Notified during each cleanup round. Can be used to implement a custom cleanup policy, e.g. check for DOS attacks or enforce a “One session per user” policy. Note that this event is notified in a separate thread outside the typical Zope request context; only very few “services” are available.


Unfortunately, it is more difficult to use Zope sessions implemented by dm.zope.session than those implemented by Products.Transience. In the latter case, all you need to do is installing Products.Sessions and (maybe) (slightly) extending your Zope configuration file. To use dm.zope.session, you must install both it and Products.Sessions, ensure that the ZCML configuration of dm.zope.session is executed during startup and then [re]configure Zope’s session machinery via the ZMI (= “Zope Management Interface”). To facilitate this configuration, we sketch first Zope’s session architecture.

Zope’s session architecture

The various subtasks related to session management are delegated to different cooperating Zope objects:


This objects makes the session available via the request object. It uses the browser_id_manager to generate a session id and a session container (called “transient object container”) for the storage of the session objects. By default, the session container is located at /temp_folder/session_data; this can be changed via the ZMI.


This object is responsible to assign session ids (called browser ids, because they in fact identify the browser).

session container

This is an object used to store (and manage) the sessions. Its path is determined by the session_data_manager; by default, it is /temp_folder/session_data.


By default, this is a mount point. The mount point is configured in the Zope configuration file (zope.conf). Typically, it mounts a Products.TemporaryFolder.TemporaryContainer from a tempstorage with name temporary. A tempstorage maintains its data in RAM; therefore, the data is lost on restart. To (partially) counter this data loss, Zope creates (typically) a session container named session_data with values from the Zope configuration file on startup.

ZMI configuration for dm.zope.session

There are various options to configure for the use of dm.zope.session.

The easiest way is to create a dm.zope.session:Container (via the ZMI) somewhere (but not below temp_folder where it would be lost on restart) and then let the session_data_manager’s Transient Object Container Path point to its location. This puts the sessions into the main ZODB.

You might want to maintain the sessions in their own ZODB, e.g. to use different parameters (such as e.g. cache size) for sessions and the main content or use different pack parameters and/or frequencies. To do this, you would configure a mount point in the Zope configuration file (likely similar to the definition for the main ZODB – do not use temporarystorage!), add a corresponding mount point in the main ZODB (via the ZMI) and then put there the session container.

Should you use Plone, you likely would need to disable its CSRF protection or put the session container into a mounted ZODB with name temporary: Plone’s CSRF protection looks for ZODB writes and usually allows them only, if the request is “authenticated”. To support session access (which might cause writes even for a read access), it allows writes to objects in temporary also for unauthenticated requests. Likely, your Zope configuration file already contains a definition for this ZODB, adapted for Zope’s standard sessions and based on temporarystorage; you would need to replace it by one for dm.zope.session (i.e. similar to the definition for the main ZODB and especially not using temporarystorage).


The typical session setup (by Products.Sessions) makes the current session available as request["SESSION"] where request represents the request object. The session behaves similar to a Python dict. In addition, you can use the methods set and delete to set a session “variable” or delete one, respectively.

dm.zope.session’s sessions are (like those of Products.Transience) ZODB based. This has an important implication: the request does not work with the session directly but with a local copy of the session object maintained in the ZODB. When the request changes the session, then in the first place it only changes the local copy. Some changes are recognized by the ZODB – this includes changes to the session itself and changes to other “persistent object”s – and cause the modifications to update the affected persistent objects in the ZODB when the request finishes successfully. In this case, followup requests will see the modifications. Other changes (to non persistent objects, e.g. lists, dicts, most class instances, …) are NOT recognized automatically and are not written to the ZODB. In such a case, some followup requests may see the modifications but others will not. If you use complex (“mutable”) values for your session variables, try to use “persistent objects” (the package persistent has persitent variants of lists and mappings) or reassign a the value to the session variable after you have changed it, e.g.:

complex_value = session["var"]
# modify complex_value via its methods
session["var"] = complex_value

The code above will ensure that the change will be written to the ZODB if the request succeeds (does not fail with an exception).

DOS attacks

dm.zope.session can be vulnerable to denial of service (DOS) attacks. In such an attack, the attacker would try to force the server to massively create new sessions. Each of those sessions needs resources (typically file space for dm.zope.session). The attacker’s aim would be to overload this kind of resource and get the site (and maybe other services using the same resource kind) in trouble.

dm.zope.session has not enough knowledge to prevent this kind of DOS attacks. However, it has features to limit the potential damage and to help detecting and handling such attacks.

The simplest approach is to limit the number of sessions by giving the configuration parameter session_max_number a positive value. This limits resource usage. However, if you have put data vital for the use of your site into the session, your site may still no longer function properly because no new sessions can be created during the attack. Therefore, try not to use the session for the most fundamental services of your site - use cookies instead.

An alternative approach would be to try to detect and handle attacks. dm.zope.session notifies some events to help you with this. An ISessionCreatedEvent is notified whenever a new session object is created. You can register a handler to add information to the session which helps to detect and/or handle potential attacks; potentially, the handler also performs the detection and handling itself. An IMaximalSessionNumberExceededEvent is notified when the maximal session number would be exceeded. In a correspoding handler, you can try to delete sessions based on custom policies. Potentially, you try to detect a potential attack and delete related sessions. dm.zope.session notifies an ISessionCleanupEvent during each cleanup round. DOS detection and handling could also be implemented in its handler; note, however, that it runs “offline” (not inside the typical Zope request context) and that it can use only very few Zope services.

Any attack comprises a large number of requests, all under the control of the attacker. To detect an attack, you must be able to recognize sessions which may have been created by the attacker. A fairly reliable way would be to use the session only for authenticated users (and e.g. use cookies for all services provided for anonymous users). In this case, you could store the corresponding user id in the session. If an attack is suspected, you could visit all sessions, check for users with an unexpected number of sessions and delete those, maybe even disable the user. If you must use the session for anonymous users, you can try to use the request’s IP address in place of the user id. Note, however, that this is far less reliable as many organisations grant internet access only through proxies; then all organization members use the same IP address. Therefore, it is much more difficult to distinguish normal use from an attack. Sessions have properties created (a timestamp float) and created_date (DateTime) indicating when the session has been created; they can help with this distinction. Note, that advanced attackers may use a large number of (hijacked) clients; in such a case, the attacking requests would come from different IPs; the “created” properties might still be used to find candidates for attacker sessions - again with reduced reliability.


The sessions managed by dm.zope.session are typically stored in a FileStorage. Such a storage not only keeps the current state but also historical data (important e.g. for conflict resolution, “undo” and analysis of past events). To get rid of old, no longer used information, you must regularly “pack” the storage. This is particularly important for a storage hosting sessions as sessions tend to be much more frequently changed than “normal” objects and more frequent changes mean more irrelevant historical data.



Python 3.10 compatibility

  • Session objects now have methods invalidate and isValid (used by Plone)

  • Session objects referenced by events are now acquisition wrapped

  • more expressive function names for easy recognition of dm.zope.session transactions

  • avoid cleanups (and associated transactions) if no new sessions have been created


Initial version

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