[ZODB-Dev] concurrent list modification problems

[Diez B. Roggisch]

Hi,

I'm using zodb as I've written a few or-mappings too much when I was using 
java, and for a single threaded app I'm quite satisfied.

But as I'm in the process of writing a corba-based app server that deals with 
several clients which are dispatched on several worker-threads, I ran into 
trouble. 

My data is basically structured as two-level tree - parent and one level of 
childs. So I keep the childs in a PersistentList. The reason for using a list 
was that I wanted the childs to be kept in insertion order so I can use their 
index as unique key and enumerate them in a defined order.

But I had to find that when two threads acces their copy of a parent that at 
the time it was instantiated from the connection had a child list like this:

[a,b,c]

and then append childs

Thread-1: [a,b,c,d,e]
Thread-1: [a,b,c,f,g]

and afterwards commit the changes, that results in the child list beeing 
persisted thats been committed last.

first of all I wonder that this is not detected and a ConflictError is raised. 
And second, I'd like to know what the comonly used idiom for such a case is - 
are PersistenMappings with keys generated from a synchronized sequence better 
suited, or whatelse can I do?

Thanks for your help,

Diez B. Roggisch