Hello maybe a stupid question. what is the rigth place or better in which file must i copy the your solution. thx christian
The slowdowns result when various transactions are trying to modify the same data. When a conflict happens, one of the transactions is rolled back and attempted again from the beginning. If the transaction takes a long time to execute, and this happens many times, the delays can be considerable. Only after some number of attempts, with continuing conflicts occurring, will the transaction give up and allow the Conflict Error to actually be raised back to the level of producing an error page, but each one will be logged in your event.log file.
I wrote the following to explain conflicts to my own satisfaction: -------------------------------------------------------------------
There are two types of conflict errors:
Read Conflict: -------------- Transaction 1 reads object A. Transaction 2 modifies object B. Transaction 1 attempts to read object B, but notices that object B has been modified since Transaction 1 began to read data. Since the data it read from A, and the data that it now finds in B are not necessarily consistent, a Read Conflict Error is produced. Transaction 1 rolls back and restarts.
Write Conflict: --------------- Transaction 1 reads object A. Transaction 2 modifies object A. Transaction 1 attempts to modify object A, but notices that object A has been modified since it was read. Since overwriting the changes made by Transaction 2 would cause "lost update problems" and possible data inconsistency, a Write Conflict Error is produced. If conflict is not handled (see below), Transaction 1 rolls back and restarts.
Solutions: ---------
Read Conflicts - If you do not care about consistency during your read, you can execute the statement "get_transaction().commit()" after each new object accessed. This breaks your transactions up into smaller transactions that are MUCH less likely to cause conflicts. I understand that Zope 2.8 (alpha any day now) will eliminate Read Conflicts by including the ability to read old constant states. In the example above, Transaction 1 would then just read the old state of B before Transaction 2 had modified it.
Write Conflicts - You can define a method on any ZODB object called "_p_resolveConflict". When a write conflict occurs, the first thing that happens is that Zope tries to call this method to handle the problem, passing it all the possible states of the object: OLD, FOUND, and TRIED. OLD is the state that the transaction first read. FOUND is the new modified state that some other transaction saved the object as. TRIED is the state that the transaction wanted to save the object as when it realized that there was a conflict. This method is intended to return a merged version of the two objects. If you do not care about lost updates, this method could be as simple as:
def _p_resolveConflict(self,old,found,tried): return tried
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Here is an example of some code that I think works to handle write conflicts in an order independent list. The object in question is a ZODB wrapper for a list. The [] is stored in the "data" element referred to in saved.data, old.data, etc:
def _p_resolveConflict(self,old,saved,new): """ Merges two possible list states Accounts only for additions and deletions, not changes of order Prevents duplicate add or remove of same Item during conflict """
#get changes made in saved state saved_added = [] saved_removed = [] for item in saved.data: for i in range(saved.count(item) - old.count(item)): saved_added.append(item) for item in old.data: for i in range(old.count(item) - saved.count(item)): saved_removed.append(item)
#get changes made in new state new_added = [] new_removed = [] for item in new.data: for i in range(new.count(item) - old.count(item)): saved_added.append(item) for item in old.data: for i in range(old.count(item) - new.count(item)): new_removed.append(item)
#get duplicate changes both_added=[] for item in new_added: if saved_added.count(item): both_added.append(item) new_added.remove(item) saved_added.remove(item) both_removed=[] for item in new_removed: if saved_removed.count(item): both_removed.append(item) new_removed.remove(item) saved_removed.remove(item)
#apply changes old.extend(saved_added) old.extend(new_added) old.extend(both_added) for item in saved_removed: old.remove(item) for item in new_removed: old.remove(item) for item in both_removed: old.remove(item)
return old
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