mahara-contributors team mailing list archive

[Aaron Wells <1492919@xxxxxxxxxxxxxxxxxx>]

Hi Russell,

Thanks for the research, and the patch! I've pushed it into our gerrit
code review system here: https://reviews.mahara.org/#/c/5335

What you've described makes a lot of sense. We haven't experienced this
issue with the large Mahara sites we host here at Catalyst IT, and those
are all on Postgres. Doing a little checking, I see that we are using
the default transaction level of READ COMMITTED on our hosted site
databases.

So, I think the best bet is probably to change the commit level to READ
COMMITTED at the application level. I've amended your commit so that it
sets that up along with the other basic database parameters when
establishing the initial connection. Since this is what we're already
doing on our many production Mahara sites, it's unlikely to cause
problems (although race conditions are always a possibility).

In answer to your item #3, we do not have any code in place to retry
when a transaction fails. We have encountered so few cases of this so
far, that we've just relied on "F5" in the web browser, as the retry
mechanism.

Anyway, please give the latest version of the patch a try and see if
that helps: https://reviews.mahara.org/#/c/5335/

Cheers,
Aaron

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https://bugs.launchpad.net/bugs/1492919

Title:
  Deadlock issues when 20-30 users copying collections & pages at the
  same time

Status in Mahara:
  In Progress
Status in Mahara 1.10 series:
  In Progress
Status in Mahara 15.04 series:
  In Progress
Status in Mahara 15.10 series:
  In Progress

Bug description:
  Multiple users on the forum have reported deadlock issues when there
  are many users (20-30) copying pages & collections at the same time.
  See
  https://mahara.org/interaction/forum/topic.php?id=7309&offset=0&limit=10

  Note that 20 to 30 users is around the number you'd hit if you're
  running a whole class of users through a training session at the same
  time.

  The three reported cases of this are all in 15.04, so it may be a
  problem introduced in that version.

  The problem has been reported in both Postgres & MySQL.

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