Is active record 3.1.1 supposed to be threadsafe?

Hi There

Is active record 3.1.1 supposed to be threadsafe?
In other words, should the following (concurrent access to a model)
work?

Thread.new do MyModel.where(some_condition).first end
Thread.new do MyModel.where(some_condition).first end

I ask because upon the answer, it depends which component should handle
synchronization, and in consequence, which component to patch/fix.

A bit of background: We stumbled over a concurrency issue when using
capybara-webkit with rails3.1. We traced the issue down to one thread
querying and trying to accessing the cache while another thread cleared
the same cache.
Assuming active record 3.1.1 is supposed to be threadsafe, then the
culprit for the issue would be
ActiveRecord::ConnectionAdapters::QueryCache, which doesn't synchronize
access to its @query_cache. See
https://github.com/awd-switzerland/rails/commit/22aeda0f0553fdcabca156012b67f2aa2add293c
for a possible way to resolve that. However, I'm not sure in which layer
the rails team wants to handle synchronization.
Also notice that the patch is naive in that it simply assumes thread
support in ruby to be enabled.

Best regards
Stefan Rusterholz

Hi There

Is active record 3.1.1 supposed to be threadsafe?
In other words, should the following (concurrent access to a model)
work?

Thread.new do MyModel.where(some_condition).first end
Thread.new do MyModel.where(some_condition).first end

I ask because upon the answer, it depends which component should handle
synchronization, and in consequence, which component to patch/fix.

Rails added thread safety back in 2.1 or 2.2 so current versions
should be threadsafe.

A bit of background: We stumbled over a concurrency issue when using
capybara-webkit with rails3.1. We traced the issue down to one thread
querying and trying to accessing the cache while another thread cleared
the same cache.
Assuming active record 3.1.1 is supposed to be threadsafe, then the
culprit for the issue would be
ActiveRecord::ConnectionAdapters::QueryCache, which doesn't synchronize
access to its @query_cache. Seehttps://github.com/awd-switzerland/rails/commit/22aeda0f0553fdcabca15…
for a possible way to resolve that. However, I'm not sure in which layer
the rails team wants to handle synchronisation.

In theory I don't think this is needed because different threads
should be using different connection objects (there's supposed to be a
pool of connection objections, and any member of the pool is only ever
used by one thread at a time. You might also want to ask on the rails-
core list

Fred

Frederick Cheung wrote in post #1028961:

In theory I don't think this is needed because different threads
should be using different connection objects (there's supposed to be a
pool of connection objections, and any member of the pool is only ever
used by one thread at a time. You might also want to ask on the rails-
core list

Seems practice disagrees with that theory. At least we had exceptions
due to unsynchronized cache access. And we ourselves did not introduce
any threads in the code.
I'll see whether I get answers here. If not, I'll gladly accept your
advice and take it on to rails-core.

Best regards
Stefan

Frederick Cheung wrote in post #1028961:

> In theory I don't think this is needed because different threads
> should be using different connection objects (there's supposed to be a
> pool of connection objections, and any member of the pool is only ever
> used by one thread at a time. You might also want to ask on the rails-
> core list

Seems practice disagrees with that theory. At least we had exceptions
due to unsynchronized cache access. And we ourselves did not introduce
any threads in the code.
I'll see whether I get answers here. If not, I'll gladly accept your
advice and take it on to rails-core.

It might also be related to https://github.com/rails/rails/pull/1670

Fred