On Nov 20, 2005, at 12:16 PM, Dennis Allison wrote:

The structure of the naviagation method is simple enough. Everything is
wrapped in a <dtml-let> which sets a number of parameters mostly by
reading them from the SESSION (with an interface function) or plucking
them from the relational database with a query.

In the scope of the let is dtml code which, when rendered, provides the
various navigation links.  In various sections there are additional
<dtml-let> blocks and additional queries to the relational database
and several <dtml-in> loops.

Looking at the code, I don't understand why I am seeing conflicts.
As I understand things, neither variables in the <dtml-let> space nor
the REQUEST/RESPONSE space are stored in the ZODB so modifications to
them don't look like writes to the conflict mechanism.  Am I incorrect
in my understanding?

Yes, but that's understandable.  It's not exactly obvious.

The sessioning machinery is one of the few places in Zope where it's necessary for the code to do what's known as a "write on read" in the ZODB database.

Even if you're just "reading" from a session, looking up a session, or doing anything otherwise related to sessioning, it's possible for your code to generate a ZODB write. This is why you get conflicts even if you're "just reading"; whenever you access the sessioning machinery, you are potentially (but not always) causing a ZODB write. All writes can potentially cause a conflict error.

While this might sound fantastic, it's pretty much impossible to avoid when using ZODB as a sessioning backend. The sessioning machinery has been tuned to generate as few conflicts as possible, and you can help it by doing your own timeout, resolution, and housekeeping tuning as has been suggested. MVCC gets rid of read conflicts. But it's not possible to completely avoid write conflicts under the current design.

Here's why. The sessioning machinery is composed of three major data structures:

- an index of "timeslice" to "bucket". A timeslice is an integer representing
  some range of time (the range of time is variable, depending on the
"resolution", but out of the box, it represents 20 seconds). This mapping
  is an IOBTree.

- A "bucket" is a mapping from a browser id to "session data object" (aka
  transient object).  This mapping is an OOBTree.

- three "increasers" which mark the "last" timeslice in which something was done
  (called the garbage collector, called the finalizer, etc).

The point of sessioning is to provide a writable namespace assigned to a single user that expires after some period of inactivity by that user. To this end, we need to keep track of when the last time the user "accessed" the session was. This is the point of the index.

When a user accesses his session, we may need to move his session data object (identified by his browser id) from one bucket (representing an older timeslice) to another (representing a newer timeslice). This needs to happen *even if your code doesn't write anything to his session*, because it represents a session access, and the session is defined by total inactivity (not just write inactivity). Likewise, when a user runs code that requires access to a session, but that user does not yet have a session data object, a write may need to occur. So seemingly innocuous accesses to session data can cause a write. Consider, in a Python script:

req = context.REQUEST
REQUEST.SESSION

Looks pretty harmless and unlikely to cause a write. However, that's not true. If the "bucket" in which the user's session data object is found is not associated with the "current" timeslice, we need to move his data object to the bucket that *is* associated with the current timeslice, which is a write operation in order to make note of the fact that his session is now "current".

Likewise with:

req = context.REQUEST
a = REQUEST.SESSION.get('foo')

Even though this appears to be "only a read", the sessioning machinery itself may need to perform a write operation to move the user's data object to the current bucket.

Jacking up the resolution time increases the period of time represented by a single timeslice, so fewer total writes need to be performed to keep a session "current". Turning on "external housekeeping" doesn't prevent this normal movement of data objects between buckets, it just causes another process that cleans up "stale" data from happening during normal sessioning operations.

The sessioning machinery attempts to minimize conflicts. The 2.8 version of the temporarystorage does MVCC, which essentially eliminates read conflict errors. The transience machinery includes significantly complicated logic to attempt to prevent conflict errors from occurring including code that attempts to prevent two threads from doing housekeeping at once as well as application level conflict resolution for simultaneous writes to the same session data object. However, the machinery uses BTrees to hold indexes. BTrees also have a limited number of conflict avoidance strategies, but under certain circumstances (a "bucket split" is the canonical case) it cannot be avoided so not all write conflicts can be prevented without using a different kind of data structure to hold sessioning data.

A more detailed description of how "transience" works is available within the file named "HowTransienceWorks.txt" in the Products/ Transience package within Zope in case you're interested.

I hope this explains why you see conflict errors even if your code "doesn't do any writes", because actually it probably does by virtue of accessing a session. Tuning the knobs that come with the machinery helps. Causing transactions to be as short as possible also helps (by not using ZEO to back the sessioning database or by making your code just generally faster) because then there is less of a chance of a conflicting change.

- C

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