Rails multi-tenancy, scopes, class variables and thread safety

Question

Short Version

Are class variables thread-safe for the duration of a controller action?

Long Version

I'm writing a Rails 3 app that implements multi-tenancy using a single database schema with tenant_id columns in each table to identify to whom the data belongs.

Naturally I need a way to scope all queries that are run on the database to ensure that access to data is properly isolated per-tenant.

Several people have suggested achieving this by always accessing the database through the tenant instance, e.g.:

current_tenant.associate_collection.where(...)

etc.

However, since multi-tenancy is an architectural concern (not a business domain concern), I'd prefer to keep this as transparent to the model as possible by setting it up the scope globally. It's also much DRYer.

I've found I can do this using the multitenant gem and around_filters:

class ApplicationController
  around_filter do
    Multitenant.with_tenant current_tenant
      yield
    end
  end
end

This manipulates the default_scope of the appropriate models so that all data access is automatically scoped to the current tenant.

This works great in development, but I have concerns about thread safety. Multitenant.with_tenant preserves the current_tenant reference in a class variable.

The question is can I rely on the integrity of that variable while the controller action is on the stack? or could it be corrupted by another request?

Also, what’s a good source on general concurrency issues in Rails?

Answer 1

It's not a matter of duration, and there isn't anything special about a controller that makes it an atomic (uninterruptible) sequence of code. Thread safety is about understanding locks, synchronization, and the thread scheduler.

I don't think your question is even Rails specific in this case. In a general sense, thread safety is achieved through locking data while a write is happening, and unlocking it when the write is finished. It's very similar database locking mechanisms, if you're familiar with those.

For a good reference that focuses on Ruby, I would go here: http://www.rubycentral.com/pickaxe/tut_threads.html

For more info about the concept the thread safety (threads run at the OS level, after all, so it's more about understanding how thread safety works in any program), I would go here: http://en.wikipedia.org/wiki/Thread_safety