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[Kristina Hoeppner <1607131@xxxxxxxxxxxxxxxxxx>]

** Changed in: mahara
   Importance: Medium => Wishlist

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https://bugs.launchpad.net/bugs/1607131

Title:
  Bring SQL transaction support back to MySQL

Status in Mahara:
  Triaged

Bug description:
  To resolve Bug 1514608 (Mahara locking when multiple simultaneous
  users are active), we disabled SQL transaction support in MySQL.
  Specifically, the "db_begin()", "db_rollback()", and "db_commit()"
  methods all check "is_myslq()" and return without doing anything if
  MySQL is in use.

  As might be expected this has resulted in a few other MySQL-specific
  bugs popping up, due to race conditions.

  So, it would be good to add transaction support back to our MySQL
  driver. However, I think to do so in a way that won't again cause
  concurrency issues, we need to be much more disciplined in how we use
  transactions. The MySQL problems were caused because we had very long-
  lived transactions. For instance, when copying a collection, we would
  open a transaction at the start of the process, and commit it at the
  end. Depending on the size of the collection and the pages in it, this
  could leave that transaction open for up to several minutes. From what
  I can tell, the MySQL InnoDB storage engine doesn't handle long-
  running transactions very well, so this causes a problem.

  Ideally, then, we should use db_begin and db_commit close together
  only, like in the same function, enclosing some related SQL statements
  but *not* any calls to other functions that might balloon out in
  execution time (such as triggering an event).

  Yet more ideally, we'd have some way of detecting and/or enforcing
  this automatically in Mahara's code. But I can't see any good way to
  do that in PHP. At most, we could track the amount of time that passes
  between class to db_begin() and db_commit(), and issue a warning
  message when a transaction has a long lifetime.

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