我正在使用 Rails 5 和 Ruby 2.4。我如何弄清楚，或者您可以通过查看下面的内容来判断是否有多个线程同时运行？

pool = Concurrent::FixedThreadPool.new(1)
promises = links.map do |link|
  Concurrent::Promise.execute(executor: pool) do
    result = process_link(link)
    if result
      if result.kind_of?(Array)
        result.each do |my_obj|
          my_obj.update_attributes({ :a => a })
          records_processed = records_processed + my_obj.matches.count
        end
      else
        records_processed = records_processed + result.matches.count
        result.update_attributes({ :a => a })
      end
    end
  end
end
promises.map(&:wait).map(&:value!)

因为我已将我的池设置为“1”，所以我的假设是没有任何东西同时运行，但我不断收到此错误...

Error during processing: (ActiveRecord::ConnectionTimeoutError) could not obtain a connection from the pool within 5.000 seconds (waited 5.002 seconds); all pooled connections were in use
/Users/nataliab/.rvm/gems/ruby-2.4.0@global/gems/activerecord-5.0.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:202:in `block in wait_poll'
/Users/nataliab/.rvm/gems/ruby-2.4.0@global/gems/activerecord-5.0.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:193:in `loop'
/Users/nataliab/.rvm/gems/ruby-2.4.0@global/gems/activerecord-5.0.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:193:in `wait_poll'
/Users/nataliab/.rvm/gems/ruby-2.4.0@global/gems/activerecord-5.0.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:154:in `internal_poll'
/Users/nataliab/.rvm/gems/ruby-2.4.0@global/gems/activerecord-5.0.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:278:in `internal_poll'
/Users/nataliab/.rvm/gems/ruby-2.4.0@global/gems/activerecord-5.0.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:148:in `block in poll'
/Users/nataliab/.rvm/rubies/ruby-2.4.0/lib/ruby/2.4.0/monitor.rb:214:in `mon_synchronize'
/Users/nataliab/.rvm/gems/ruby-2.4.0@global/gems/activerecord-5.0.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:158:in `synchronize'
/Users/nataliab/.rvm/gems/ruby-2.4.0@global/gems/activerecord-5.0.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:148:in `poll'
/Users/nataliab/.rvm/gems/ruby-2.4.0@global/gems/activerecord-5.0.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:717:in    `acquire_connection'
/Users/nataliab/.rvm/gems/ruby-2.4.0@global/gems/activerecord-5.0.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:490:in `checkout'
/Users/nataliab/.rvm/gems/ruby-2.4.0@global/gems/activerecord-5.0.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:364:in `connection'
/Users/nataliab/.rvm/gems/ruby-2.4.0@global/gems/activerecord-5.0.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:883:in `retrieve_connection'
/Users/nataliab/.rvm/gems/ruby-2.4.0@global/gems/activerecord-5.0.1/lib/active_record/connection_handling.rb:128:in `retrieve_connection'

如果我修改我的代码以在我确定没有并发发生的地方运行，我不会收到上述错误...

links.each do |link|
  result = process_link(link)
  if result
    if result.kind_of?(Array)
      result.each do |race|
        my_obj.update_attributes({ :a => a })
        records_processed = records_processed + my_obj.matches.count
      end
    else
      records_processed = records_processed + result.matches.count
      result.update_attributes({ :a => a })
    end
  end
end

编辑: 这是我的开发环境的数据库配置。另请注意，所有这些都在 Rails 控制台中运行。

development:
  adapter: postgresql
  encoding: utf8
  database: sims 
  username: postgres
  password: password
  pool: 5
  timeout: 15000
  host: 127.0.0.1

最佳答案

您认为多个线程必须并发运行只是因为连接池正在耗尽的假设是不正确的。仅仅因为一个连接仍然从连接池中“ checkout ”并不意味着当前正在线程中 checkout 的连接上执行查询，它只是意味着线程的连接还没有被签回。线程可能处于空闲状态，但只要未明确终止，它仍会保持连接池中的连接。

由于 ActiveRecord 连接是线程本地的，您可以通过在多个线程上运行 ActiveRecord 查询来耗尽连接池，就像您在本例中所做的那样。 (每次调用 Concurrent::FixedThreadPool.new(1) 时，都会创建一个新线程。)即使您一次只在单个线程上运行查询，默认情况下也会创建一个连接仍然在每个线程上保持打开状态，直到它们终止。

为避免这种情况，您可以在使用连接后手动检查连接，或者确保您的线程已终止(终止)，以便池可以恢复(回收)它们的连接。

• 要手动检查连接，请参阅 ConnectionPool documentation供您选择。最简单的方法是将 ActiveRecord 代码包装在 with_connection block 中:

  Concurrent::Promise.execute(executor: pool) do
    ActiveRecord::Base.connection_pool.with_connection do
      # update_attributes, etc
    end
  end
  

• 为确保所有线程都终止，请调用 #shutdown其次是 #wait_for_termination使用完毕后在线程池上:

  values = promises.map(&:value!)
  pool.shutdown
  pool.wait_for_termination
  

关于ruby-on-rails - 即使线程池中只有一个线程，也会发生并发吗？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/42034132/