This document discusses concurrency in JRuby and presents several approaches for managing concurrent operations, including Java's java.util.concurrent utilities, Jetlang fibers and channels, and the Akka actor framework. It notes some challenges with using green threads in JRuby due to shared mutable state and provides examples demonstrating thread safety issues and solutions using these other concurrency models.

1. JRuby
Concurrency
Nick Sieger
EMRubyConf @ RailsConf 2011

2. Mental
Model
http://www.flickr.com/photos/solar_decathlon/4524503892/

3. http://twitter.com/RoflscaleTips/status/61135427813384192

4. Uncertainty
/* ruby/struct.c */
static VALUE rb_struct_equal(VALUE s, VALUE s2) {
/* ... */
if (RSTRUCT_LEN(s) != RSTRUCT_LEN(s2)) {
rb_bug("inconsistent struct"); /* should never happen */
}
/* ... */
}
O RLY?!

5. How could this code fail?
class ExpensiveToCreate
def self.instance
@instance ||= ExpensiveToCreate.new
end
end

6. How could this code fail?
class ExpensiveToCreate
def self.instance
unless defined?(@instance)
@instance = ExpensiveToCreate.new
end
end
end

7. How could this code fail?
class ExpensiveToCreate
def self.instance
T2 unless defined?(@instance)
@instance = ExpensiveToCreate.allocate
T1 @instance.initialize # big pause here
@instance
end
end
end

8. Avoid shared mutable state,
lazy initialization
MutableConstant.merge! :key => value
class MemoryCache
def self.cache
@cache ||= {} $global_lock.do_something!
end
end

9. Difficult to observe
with green threads

10. data = []
M.times do |m|
Thread.new do
N.times do |n|
data << m * n
end
end
end

11. OK NOT OK
instance vars
method def
String
class def
Array
class vars
Hash

12. class WorkerTask
def run
@thread = Thread.new do
50.times do
digest = Digest::MD5.new
@range.step(1024) do |idx|
digest.update(@data[idx...idx+1024])
end
end
end
end
end

13. Other Concurrency
Approaches

14. java.util.concurrent

15. require 'java'
java_import java.util.concurrent.Executors
@count = java.util.concurrent.atomic.AtomicInteger.new
def send_email(executor)
executor.submit do
puts "email #{@count.incrementAndGet} sent"
end
end

16. executor = Executors.newCachedThreadPool
send_email(executor)
executor =
Executors.newFixedThreadPool(2)
loop do
send_email(executor)
end

17. jetlang/jretlang
http://code.google.com/p/jetlang/
https://github.com/reevesg/jretlang

18. require 'jretlang'
fiber = JRL::Fiber.new
channel = JRL::Channel.new
fiber.start
channel.subscribe_on_fiber(fiber) do |msg|
puts msg
end
channel.publish "Hello"

19. Akka
http://akka.io
http://j.mp/jruby-akka

20. require 'akka'
class HelloWord
def hi
puts "hello actor world"
end
end
Actors.actorOf(HelloWord.new).hi
puts "initiating shutdown..."
Actors.delayedShutdown 1