From: Charles Oliver Nutter <charles.nutter@...>
Date: 2008-12-24T07:34:36+09:00
Subject: Re: Are all Ruby built-in objects thread safe?

J旦rg W Mittag wrote:
> A couple of months ago, this actually became an issue. Originally, the
> JRuby developers had implemented Arrays to be not safe. One of the big
> selling points of JRuby was and still is the promise of true
> concurrency and better scalability. So, naturally, people wanted to
> take advantage of this feature and started running their concurrent
> programs on JRuby. And those programs crashed left and right, because
> they didn't lock their Arrays properly. So, the JRuby team decided to
> implement thread-safe data structures on their end, so that code that
> didn't crash on MRI could be run unmodified on JRuby.

Actually, we made a minimal attempt to ensure that concurrent operations 
against Array were usually safe across threads but also would raise a 
Ruby-land "ConcurrencyError" when concurrent changes could not be 
reconciled. It's a trade-off; adding locks to all the core collections 
would severely penalize performance for what's generally the rare case 
of concurrent access. And really there should be a separate set of 
classes with guaranteed concurrency that people can use if the 
performance considerations of locking are acceptabe for safe concurrent 
access.

So in short, you're absolutely right; nobody should ever rely on the 
core collections to be thread-safe, even if they happen to be 
thread-safe by accident in the C implementations right now. That won't 
be the case on all implementations, and may not even be the case on 
future versions of the C impls. The safe answer is to ensure you're 
watching your own back and properly synchronizing access to shared data 
structures.

- Charlie