Ruby on Rails Thursday, January 31, 2013

I have a synchronization problem that I am not sure how to solve in Ruby/Rails. I have a mental model of what I need to do and how it should work, but can't find the way to do it in Ruby/Rails. (I've learned all about Mutexes and transactions and lock! but despite my research, am still stuck). I come from the kernel programming world, so I may be framing this incorrectly. 

Here's the deal - 
1. I have a database object, call it a Meter. Meters have all sorts of associated objects.
2. My users can do things to meters and their associated objects via standard web requests.
3. I have a cron taks that runs on the server (Heroku) periodically and also does things to meters and their associated objects.
4. If a user is mucking about with a specific meter *or* it's associated objects at the same time as the cron task is trying to muck with the same meter or associated objects, then there's potential for a data corruption problem.

So - I need to block the cron task from mucking about with a Meter and all its associated objects while a user is doing so (and vice versa).

In my old kernel world, I would have defined a lock on each meter. Wherever I want to assure that a section of code that mucks with a meter is not running concurrently with another section of code that mucks with the same meter, I would wrap each code section that mucks with meters in a Lock/Unlock pair as follows:

m = get_my_meter
# do section 1 stuff to meter and associated objects

m = get_my_meter
# do section 2 stuff to meter and associated objects

The resulting behavior is that (assuming that two code sections tried to operate on the same meter concurrently), one would get there first and the second would be blocked until the first called the OSUnlock. At that time, the second would run and I would be assured that the code sections would always run in sequence for any given meter (as opposed to concurrently).

In Ruby/Rails, I see that there are mutexes, but they seem to be applicable to multiple threads running on the same in memory objects. In my case, the user request is going to load its copy of the meter from the database and the cron task is going to load its copy, so - mutexes don't help. Transactions and pessimistic locking seemed promising, but from my experimentation with them they will only prevent concurrent operations on the specific object that is locked. Operations on associated objects, even if they are inside transaction blocks, can still run concurrently. So - it seems that the executing code is not waiting, rather, the database access, for the locked object only, is waiting.

I need to block the cron task from executing any further than where the lock is taken until the user task releases the lock (and vice versa). How do I do that?


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