Handle exceptions in concurrent-ruby thread pool - ruby

Question

How to handle exceptions in concurrent-ruby thread pools (http://ruby-concurrency.github.io/concurrent-ruby/file.thread_pools.html)?
Example:
pool = Concurrent::FixedThreadPool.new(5)
pool.post do
raise 'something goes wrong'
end
# how to rescue this exception here
Update:
Here is simplified version of my code:
def process
pool = Concurrent::FixedThreadPool.new(5)
products.each do |product|
new_product = generate_new_product
pool.post do
store_in_db(new_product) # here exception is raised, e.g. connection to db failed
end
end
pool.shutdown
pool.wait_for_terminaton
end
So what I want to achive, is to stop processing (break loop) in case of any exception.
This exception is also rescued at higher level of application and there are executed some cleaning jobs (like setting state of model to failure and sending some notifications).

Answer 1

The following answer is from jdantonio from here https://github.com/ruby-concurrency/concurrent-ruby/issues/616
"
Most applications should not use thread pools directly. Thread pools are a low-level abstraction meant for internal use. All of the high-level abstractions in this library (Promise, Actor, etc.) all post jobs to the global thread pool and all provide exception handling. Simply pick the abstraction that best fits your use case and use it.
If you feel the need to configure your own thread pool rather than use the global thread pool, you can still use the high-level abstractions. They all support an :executor option which allows you to inject your custom thread pool. You can then use the exception handling provided by the high-level abstraction.
If you absolutely insist on posting jobs directly to a thread pool rather than using our high-level abstractions (which I strongly discourage) then just create a job wrapper. You can find examples of job wrappers in all our high-level abstractions, Rails ActiveJob, Sucker Punch, and other libraries which use our thread pools."
So how about an implementation with Promises ?
http://ruby-concurrency.github.io/concurrent-ruby/Concurrent/Promise.html
In your case it would look something like this:
promises = []
products.each do |product|
new_product = generate_new_prodcut
promises << Concurrent::Promise.execute do
store_in_db(new_product)
end
end
# .value will wait for the Thread to finish.
# The ! means, that all exceptions will be propagated to the main thread
# .zip will make one Promise which contains all other promises.
Concurrent::Promise.zip(*promises).value!

Answer 2

There may be a better way, but this does work. You will want to change the error handling within wait_for_pool_to_finish.
def process
pool = Concurrent::FixedThreadPool.new(10)
errors = Concurrent::Array.new
10_000.times do
pool.post do
begin
# do the work
rescue StandardError => e
errors << e
end
end
end
wait_for_pool_to_finish(pool, errors)
end
private
def wait_for_pool_to_finish(pool, errors)
pool.shutdown
until pool.shutdown?
if errors.any?
pool.kill
fail errors.first
end
sleep 1
end
pool.wait_for_termination
end

Answer 3

I've created an issue #634. Concurrent thread pool can support abortable worker without any problems.
require "concurrent"
Concurrent::RubyThreadPoolExecutor.class_eval do
# Inspired by "ns_kill_execution".
def ns_abort_execution aborted_worker
#pool.each do |worker|
next if worker == aborted_worker
worker.kill
end
#pool = [aborted_worker]
#ready.clear
stopped_event.set
nil
end
def abort_worker worker
synchronize do
ns_abort_execution worker
end
nil
end
def join
shutdown
# We should wait for stopped event.
# We couldn't use timeout.
stopped_event.wait nil
#pool.each do |aborted_worker|
# Rubinius could receive an error from aborted thread's "join" only.
# MRI Ruby doesn't care about "join".
# It will receive error anyway.
# We can "raise" error in aborted thread and than "join" it from this thread.
# We can "join" aborted thread from this thread and than "raise" error in aborted thread.
# The order of "raise" and "join" is not important. We will receive target error anyway.
aborted_worker.join
end
#pool.clear
nil
end
class AbortableWorker < self.const_get :Worker
def initialize pool
super
#thread.abort_on_exception = true
end
def run_task pool, task, args
begin
task.call *args
rescue StandardError => error
pool.abort_worker self
raise error
end
pool.worker_task_completed
nil
end
def join
#thread.join
nil
end
end
self.send :remove_const, :Worker
self.const_set :Worker, AbortableWorker
end
class MyError < StandardError; end
pool = Concurrent::FixedThreadPool.new 5
begin
pool.post do
sleep 1
puts "we shouldn't receive this message"
end
pool.post do
puts "raising my error"
raise MyError
end
pool.join
rescue MyError => error
puts "received my error, trace: \n#{error.backtrace.join("\n")}"
end
sleep 2
Output:
raising my error
received my error, trace:
...
This patch works fine for any version of MRI Ruby and Rubinius. JRuby is not working and I don't care. Please patch JRuby executor if you want to support it. It should be easy.