Failover

Feb 8, 2008 at 3:22 PM
Let's say I three servers setup with the memcached. And 33.3% of my users have their session stored in one of the servers. If that server goes down. Will the 33.3% of the clients be able to get their session from one of the other two servers? Bascially how does memcached handle "failures"? I know the product ncache says it handles this. They said the 33.3% of the users would be able to get their data from one of the two remaining servers... Does memcached allow for this type of failover?
Coordinator
Feb 8, 2008 at 5:02 PM
Memcached does not provide synchronization of data between different instances of the memcached servers. So if one server goes down, data for that server is lost.
Please refer to following article for detail. How to store sessions in memcached
That is why the session state provider has SQL Server 2005 backup. If the data is lost in one of the memcached server, it can be retrived from SQL Server 2005.


Feb 8, 2008 at 5:56 PM
Yes, I think I understand better after reading: http://www.socialtext.net/memcached/index.cgi?this_is_a_story_of_caching. Sorry for the confusion. Just so I understand. From my example, (the 33.3% that was lost) If I was using sql server 2005 and that one memcached server went down, would that mean, if I "asked" one of the other 2 good servers, it would go to "SQL Server 2005" and get the session/cached value?
Coordinator
Feb 8, 2008 at 7:15 PM
Edited Feb 8, 2008 at 7:16 PM
It will get the value from SQL Server 2005 and that value will then be stored in one of the other memcached servers. Next time when that Session value is needed it will be picked up from the memcached servers. So application won't exprience any session data loss.
Feb 8, 2008 at 8:29 PM
Yes.... "That got like three feet of air!"