[Tickets #9173] Deadlocks in mysql SessionHandler under heavy loads

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Ticket URL: http://bugs.horde.org/ticket/9173
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  Ticket             | 9173
  Created By         | luc.germain at usherbrooke.ca
  Summary            | Deadlocks in mysql SessionHandler under heavy loads
  Queue              | Horde Groupware Webmail Edition
  Version            | 1.2.6
  Type               | Enhancement
  State              | New
  Priority           | 1. Low
  Milestone          |
  Patch              | 1
  Owners             |
+New Attachment     | sessionhandler.patch
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luc.germain at usherbrooke.ca (2010-08-11 16:05) wrote:

We use the mysql Sessionhandler of Horde on our installation, and  
under heavy loads (> 500 users), we start having a lot of deadlocks  
messages in the logs:

Aug 05 09:45:47 HORDE [error] [horde] Error writing session data:  
Deadlock found when trying to get lock; try restarting transaction  
[pid 27607 on line 185 of  
"/htdocs/horde/lib/Horde/SessionHandler/mysql.php"]

When this happens, users have to try to login many times before  
getting access.

We added the following statement to the mysql driver and it solved the  
problem nicely:

mysql_query('SET TRANSACTION ISOLATION LEVEL READ COMMITTED;', $this->_db);

ref: http://dev.mysql.com/doc/refman/5.0/en/set-transaction.html

No more deadlock seen even for > 2000 active users