As I could not find a clear example about how to rescue exceptions from Concurrent::Promises (part of the Concurrent Ruby gem ) I read through the documentation and here are two examples: one that documents success case and one that shows what is happening when there is an error.

Here is a simple way to execute concurrent ruby and rescue exceptions.

Success Example

require 'bundler/inline'

gemfile do
  ruby "3.0.1"
  source 'https://rubygems.org'
  gem "concurrent-ruby"
end

require "concurrent"
require "pp"

# Just a simple class that needs to execute something
class Load
  def initialize(id)
    @id = id
  end

  def work
    sleep(1)
    "Executing #{@id}"
  end
end

class ParallelExecution
  def call

    # Using here Concurrent::Array to be thread safe
    exceptions = Concurrent::Array.new
    promises = []
    10.times do |t|
      promises << Concurrent::Promises.future { Load.new(t).work }.rescue { exceptions.push(_1) }
    end

    # Calling .value on each job to wait for its execution
    values = Concurrent::Promises.zip(*promises).value

    # Raising exceptions if any of the jobs were returning an exception
    raise exceptions.first if exceptions.length > 0

    pp values
  end
end


ParallelExecution.new.call

This will print:

["Executing 0",
 "Executing 1",
 "Executing 2",
 "Executing 3",
 "Executing 4",
 "Executing 5",
 "Executing 6",
 "Executing 7",
 "Executing 8",
 "Executing 9"]

Failure example

require 'bundler/inline'

gemfile do
  ruby "3.0.1"
  source 'https://rubygems.org'
  gem "concurrent-ruby"
end

require "concurrent"
require "pp"

# Just a simple class that needs to execute something
class Load
  def initialize(id)
    @id = id
  end

  def work
    raise MyException.new
  end
end

class MyException < Exception ; end

class ParallelExecution
  def call

    # Using here Concurrent::Array to be thread safe
    exceptions = Concurrent::Array.new
    promises = []
    10.times do |t|
      promises << Concurrent::Promises.future { Load.new(t).work }.rescue { exceptions.push(_1) }
    end

    # Calling .value on each job to wait for its execution
    values = Concurrent::Promises.zip(*promises).value

    # Raising exceptions if any of the jobs were returning an exception
    raise exceptions.first if exceptions.length > 0

    pp values
  end
end


ParallelExecution.new.call

This code will raise an exception MyException.

To notice in the code example

One can get all exceptions and do various stuff based on their type for example:

require 'bundler/inline'

gemfile do
  ruby "3.0.1"
  source 'https://rubygems.org'
  gem "concurrent-ruby"
end

require "concurrent"
require "pp"

# Just a simple class that needs to execute something
class Load
  def initialize(id)
    @id = id
  end

  def work
    exception = [MyException, SecondException].sample
    raise exception.send(:new)
  end
end

class MyException < Exception ; end
class SecondException < Exception ; end

class ParallelExecution
  def call

    # Using here Concurrent::Array to be thread safe
    exceptions = Concurrent::Array.new
    promises = []
    10.times do |t|
      promises << Concurrent::Promises.future { Load.new(t).work }.rescue { exceptions.push(_1) }
    end

    # Calling .value on each job to wait for its execution
    values = Concurrent::Promises.zip(*promises).value

    # Raising exceptions if any of the jobs were returning an exception
    if exceptions.length > 0
      count_my_exception = exceptions.count { _1.instance_of?(MyException) }
      count_second_exception = exceptions.count { _1.instance_of?(SecondException) }

      puts "MyException generated #{count_my_exception}"
      puts "SecondException generated #{count_second_exception}"
    end
  end
end


ParallelExecution.new.call