If I have a singleton class (by using the Singleton mixin), I understand
it's thread-safe in that it ensures only a single instance of this class
will be created (by putting a mutex around the instance method). BUT
(in theory), the instance methods of my singleton instance could be
accessed concurrently by various threads, thus causing a concurrency
issue. Right?
BUT Rails is essentially single-threaded (only a single request
processed at a time), so if I have only a single call to my singleton
instance method per http request, then it should be impossible to get
concurrency problems with it. Right?
If I have a cluster 4 servers, then I could have as many as 4 instances
of my singleton created. But that's OK -- each instance will have its
own io it writes to. What I don't want is 20 requests coming in at the
same time and all trying to use the singleton to write at the exact same
time.
The only caveat I can see is that
you'd use Passenger in smart spawning mode
Thank you for the response Roderick!
Also thanks for pointing out the possible concurrency in Passenger. I
have yet to dive into Passenger, currently using Thin. Anyway the the
singleton is not created in the environment, so I should be OK even if I
do choose to leverage Passenger.
BUT Rails is essentially single-threaded (only a single request
processed at a time), so if I have only a single call to my singleton
instance method per http request, then it should be impossible to get
concurrency problems with it. Right?
It's actually possible to write various kinds of concurrent code in
Rails -- it really ALWAYS was possible to have some concurrency, but not
concurrent request handling, and whatever concurrency you added you'd
add yourself.
But in more recent versions of Rails, even some concurrent request
handling is supported. allow_concurrency!. And perhaps Passenger in
certain modes as Roderick says, I'm not familiar with Passenger.
But you're basically right about the 'ordinary' mode of Rails execution,
that only a single request will be processed at a time. This is
increasingly not the only option for Rails though.
But, yeah, you're also right that if you had Rails executing in a mode
that allowed concurrent request handling, you'd need to take care of
making sure your singleton object itself is concurrent-access safe --
the singleton pattern will take care of _instantiation_ of the Singleton
object being concurrency-safe, but can't take care of it's own internal
logic. So you can either do that -- or you can note in comments that
this thing isn't concurrency-safe, and shouldn't be used in a
concurrent-request environment.
