openerp-india team mailing list archive

["Olivier Dony \(OpenERP\)" <992525@xxxxxxxxxxxxxxxxxx>]

Here is some extra information concerning this bug report:

As discussed on the framework mailing-list[1] and on bug 746620, a TransactionRollbackError may happen under normal circumstances when a certain series of concurrent updates happen in an order that is not safe for the proper serialization of transactions. In other words, it is a fundamental safety mechanism that prevents corrupting data in case of concurrent updates to the same information.
As an example, think of a simplistic bank management system where 2 transactions would run at the same time, read "current balance: $100" and both withdraw $100 by saving "current balance: $0" - an obvious mistake as the final result should be -100$.
This cannot happen in OpenERP as accounting is using a double-entry system, but similar conflicts may happen for other kinds of data.

It is an "optimistic locking" strategy, which means that it will not make the operations block all the time when another operation is using the same resources (which would bring poor performance), but when a conflict is actually detected it will only allow the first transaction to complete, and rollback the orther, as if they had never occurred.
The user can simply retry the failed operation and it will work perfectly fine.

There is one way we could improve this system in OpenERP: we could
automatically retry failed transactions after waiting a few
milliseconds, because the chances are very high that on the second
attempt the transaction will succeed (because the previously conflicting
transaction has completed already). This means the user would not even
see the failed transaction most of the times.

There is already an attempt to implement this (see the merge proposal
linked to this bug), but the implementation is not finished/properly
working yet.

[1] https://lists.launchpad.net/openerp-expert-framework/msg00818.html

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https://bugs.launchpad.net/bugs/992525

Title:
  TransactionRollbackError due to concurrent update could be better
  handled

Status in OpenERP Server:
  Confirmed

Bug description:
  While using openerp, psycopg2 raises TransactionRollbackError quite
  often even on small database.

  This does not seem to be easily reproduceable as it seems to be a
  conflict between two thread accessing the same table. Nevertheless, I
  provided a quick video reproducing this while installing "base_crypt"
  on my computer.

  This occurs mostly at module installation. And can completely mess up
  the module installation by giving empty wizard windows of instance.

  I guess it could also occurs in other situations (in multi-user
  context), where the bug would be quite difficult to reproduce and with
  unforeseeable consequences ;)

  I've spotted an other bug that is due to this it seems:
  https://bugs.launchpad.net/bugs/956715

  In my case (single user), it seem to hit more often on fast computers.
  To make a probable better guess, it seems to hurt more often whenever
  using a local connection between the browser and the server. It could
  be about the web module trying to update the res_users session info
  and may collide with normal operation.

  On my computer, from a new database, installing the 'base_crypt' will trigger the exception.
  When using a distant connection, the bug won't show up.

  Please check the video I've posted with the bug report if you want to
  have more detail on the procedure I used. Sorry for the bad sound
  recording. Note that the video will show you the bug occuring on my
  computer and NOT occuring on a distant computer.

  I'm providing a merge proposal along with this patch which solves the
  issue for me, but need a patient review.

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