# Sporadic clustering + caching problem

**URL:** <https://www.ruby-forum.com/t/sporadic-clustering-caching-problem/109726>\
**Category:** Rails\
**Created:** [July 18, 2007, 12:12pm UTC](https://www.ruby-forum.com/t/sporadic-clustering-caching-problem/109726 "2007-07-18T12:12:58Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![nsputnik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/b3f665/32.png) [@nsputnik](https://www.ruby-forum.com/u/nsputnik)\
**Post date:** [July 18, 2007, 12:12pm UTC](https://www.ruby-forum.com/t/sporadic-clustering-caching-problem/109726/1 "2007-07-18T12:12:58Z")

</div>

So, I’ve seeing some quirking behavior with my app after launching it  
into my production environment (railsmachine). The weirdness only  
(sporadically) occurs when I’m running a mongrel cluster. The app runs  
fine (locally and in production) when I run on a single mongrel. I’m  
hoping somebody on the forum can help me out, as I’ve run out of things  
to try given my limited rails experience 🙂

Environment

My railsmachine environment consists of Apache load-balancing 2 mongrel  
instances connecting to a single (non-clustered) MySQL instance. All  
these processes run in a single VPS. I set up the VPS per the  
RailsMachine “operating manual”:  
[https://support.railsmachine.com/index.php?pg=kb.book&id=4](https://support.railsmachine.com/index.php?pg=kb.book&id=4) . Rails  
version is 1.2.3 (I can list my gems if requested, but I don’t think  
they’re applicable to the problem)

The Problem

The problem is sporadic and difficult to reproduce, but when it does  
occur, it usually looks like this:

- Go to the “edit” page and update an existing record (e.g. set one of  
the boolean flags on the record from true to false or vice-versa). The  
changes are correctly persisted to the DB via ActiveRecord (I verified  
this with mysql client).
- Load the “view” page that displays all records: the OLD version of the  
record is displayed
- Refresh (via the browser’s refresh button) the “view” page: The  
changes are now reflected in the view (i.e. the view is in sync with  
the DB state)
- Refresh the “view” page again: The view toggles back to the OLD  
version of the record.
- and so on…

Given this behavior, I’m guessing the problem is due to one of the  
mongrels caching a stale version of the page (or model?). Perhaps the  
mongrel that performed DB update flushed it’s cache while the other  
mongrel kept the stale version of the page/model? In this case, the  
fact that the view page toggles between stale and fresh on browser would  
suggest that the Apache load balancing is performed round-robin.  
However, this is just a hypothesis…

Debugging

My application is fairly simple

- 4 database tables
- no session state
- no page or action caching
- simple (hand-coded) caching of static database tables

But, I wanted to boil the problem to something really simple for  
debugging purposes. SO, I wrote the following controller:

### START games\_controller.rb

class GamesController \< ApplicationController  
session :off

# View Games Status Page

def view  
user\_id = params[:user\_id]

```
# This is an ActiveRecord "dynamic finder"
@participant = Participant.find_by_user_id(user_id)

# a Participant has_many Games. I'm using (true) here to force
# a collection refresh (just to be safe)
@games = @participant.games(true)

```

end  
end

### END games\_controller.rb

I can reproduce the bug even with this simple controller, leading me to  
believe the problem must lie in my environment and not my code. I  
verified (via the logs) the controller code is executing on each request  
(i.e. rails isn’t caching my action). After some futile  
logging/debugging efforts, I then tried turning the following knobs  
(testing between each step, of course):

- Turn off rails action caching
- Turn off the MySQL query\_cache
- Restart MySQL, Apache. Redeploy mongrels (via capistrano)

However, none of this seemed to help. Here is the production environment  
configuration file I ended up with (excluding app-specific ENV entries)

### START production.rb

config.cache\_classes = true  
config.action\_controller.consider\_all\_requests\_local = false  
config.action\_controller.perform\_caching = false

### END production.rb

So, I’m at a loss at this point. I’m a rails noob (Java’s my main  
thing), so I can only assume that I’m doing something stupid on my VPS.  
I contacted railsmachine support, and, unfortunately, they were of  
little help (“Did you check the logs?”). Any help you could offer would  
be greatly appreciated. Solutions, suggestions, anything 🙂

Thanks in advance!

-nick