Weird DB Session error, "Mysql::Error: Lost connection to MySQL server during query"

I've discovered something really weird in rails 1.2.3 (It could be fixed in 2.0? but I don't know).

Recently I started getting the error "Mysql::Error: Lost connection to MySQL server during query" in my application. It seemed like it had popped up almost without relation to anything. There was a page in the RoR wiki with possible solutions but none of them had worked (http://, it was driving me crazy!

Anyway, one day the error message changed, and turned into something like "data too long for field:". Now this message was a bit more obvious what was goin on! I was storing too much in my session! And I hadn't realised that there was a limit to that... Specifically what I was doing was storing my cart in the session, and this was overloading the db field sometimes, but not all the time, and only after I started playing with my site a lot.

I fixed the cart problem by turning it into an active record object, and just storing the id in session. BAM! there were no more DB errors! magic! no more lost connections, nada.

Until... I found another place I was storing objects in session - In my wizards. Sometimes, if I am creating a particularly complex object, I use a 'wizard' to do it. I do this by storing my object in session, and slowly build onto it with each step/page. I'm trying to figure out some way to do this without storing the whole object in session. Any suggestions?

In Ruby on Rails, four options come to mind for storing data between HTTP requests:

* use Rails' session * store the data in the database * store the data on a file on the server * store the data on the client's computer, such as in a JavaScript variable

Why do you want to avoid storing the whole object in the session? Is it slow? Or is it causing errors?

Cheers, Wyatt Greene

Yeah, I discovered that when I store ActiveRecord objects in the session, and the sessions are stored in the database, i get the following error a lot:

Mysql::Error: Lost connection to MySQL server during query

I might be crazy, I know I have been in the past, but it went away after I stopped storing so much stuff in session, and If I deliberately start creating objects in session (by accessing all of my wizards) then the errors start occurring again. The errors don't discriminate against controllers/actions, once I have active record objects stored in session I get them all over the site, like chicken pox, only less chicken.

I might see if I can store the objects in the database instead... and have some kind of "complete" flag that is set on save, after validation, or something.

Thanks, Jonathan.

Hi Jonathan,

You probably already thought of this, but have you tried changing the type of column that stores the session data (to, for example, mediumtext)? Another idea is to check the MySQL error log when this happens.

Good luck! Wyatt Greene

Yeah I thought of doing that, but I don't think I will because it's really only prolonging the problem for a little longer rather than solving it...

Thanks for the luck! I'll probably need it :wink:

Jonzo wrote:

Yeah I thought of doing that, but I don't think I will because it's really only prolonging the problem for a little longer rather than solving it...

Thanks for the luck! I'll probably need it :wink:

Any "luck" getting this solved? I'm storing a simple object in session (attr_accessors only) and this started happening as soon as I did.

Well, yes and no.

No I never figured out the exact problem, but yes I worked around it, and actually ended up with a better application because of it!

My main solution was that I stopped storing any kind of *active record* object in session. Instead I only store id's in session and retrieve the object from the database when it is required.

For things like wizards it was a little more complicated. I developed a technique that I quite liked, but isn't totally DRY. I created an extra model in my application called xyz_builder. So if you are building an account it would be called account_builder. This allowed me to completely separate complete and incomplete accounts, so I don't need to worry about checking conditions every time I deal with accounts.

This had a few other useful side effects: - I can now save all changes made to builders without worrying about consequences, which makes the wizard more usable. - I can define a lot more wizard based logic and keep it separate from the account model. E.g. I defined separate validation methods for each step such as step_one_complete? So you can do whatever you want in the controller and the view based on which steps are complete. - It became much easier to test the creation of accounts and the wizard through unit tests

and finally my controllers became very thin:

def step_1   if     account_builder.update_attributes(params[:account]) # account_builder is a method that fetches the correct builder     redirect_to :action => :step_2 if account_builder.step_1_complete?   end end

def step_2   if     account_builder.update_attributes(params[:account])     if account_builder.complete?       account = account_builder.create_account       redirect_to :action => :show, :id => account     end   end end

Note: I just made up this example, so there may be errors, but it's essentially what my controllers ended up looking like.