I've run into this before, not with sessions, but with some other data
stored in a table.
The issue, from memory, was something to do with this line in my.cnf:
max_allowed_packet = 1M
Which basically means (IIRC) that the longest query you may send to
the server is 1MB. This means that if your marhalled session data is
bigger than that (plus the overhead of the statement) then you will
get a "Packet too large" error and AR's connection will be severed.
I can't remeber if this is the exact reason the problem occurred, but
upping the value did fix it.
However, if you're storing more than a megabyte in session then you
probably have bigger problems than worrying about truncated session
data - namely the huge overhead of continuously reading and updating
this data with EVERY SINGLE REQUEST. Once you get up around a large
number of requests, that's a lot of data being piped around, which
probably ain't a good thing.
I'd guess you're probably storing AR objects in session, which is
fine, but make sure that their associated objects don't get marshalled
as well, otherwise you might end up with a massive association tree
being stored as well. Either just store the ID of the object in
question, or modify your AR classes so the associated classes aren't
marshalled - I did this by adding the following mixin to my models to
make sure only the "attributes" hash was marshalled for the ones I
stored in session:
@attributes = data
class MyModel < ActiveRecord::Base
# Make sure only local data gets marshalled
There may be further caveats to marshalling/unmarshalling only
@attributes (and accessing it directly is bad practice, but hey, it
gets the job done in this case) so be aware that it may not be fully
future proof. For now though, it seems to work pretty well.
Hope that helps!