Hardware and performance

Hi Wathsala, have you checked that memcached isn't filling up the available memory and starting to evict data for sessions that are still in use? That would give the behaviour you're seeing. If that's the case you would have to reconfigure memcached to give it more memory (how much will depend on the number of users and the size of their session data). Unless you're running a pretty small Moodle instance the memcached default of 64Mb probably won't be enough.

If you're not already using it, it would be worth installing phpMemcachedAdmin  (https://code.google.com/p/phpmemcacheadmin/), as that gives a clear pie chart of the memory use and loads of other useful statistics.

Cheers, Michael.

I'm sure there's a tracker ticker for this somewhere (which I can't find). Basically, anything that invalidates the cache also trashes the session data, hence logging out your users. 

I'll keep looking

Hi Wathsala --

I don't know about ElastiCache, but Memcached has a default object size limit of 1MB.  This means if a session in serialized form is larger than 1MB, it will be truncated and corrupted.  A corrupted session that fails to deserialize could result in users being logged out.

I've seen users logged out because because of the 1MB limit, although not recently.  You may want to look through your logs to see if you can find any indication of problems deserializing sessions and also keep an eye on the size of sessions to see if they are getting close to the 1MB mark.

--Adam