如何确定在Rails中随时打开哪些数据库连接？

Question

I'm running a single-threaded Rails 5.0 application through the "rails console", with the below configuration in config/database.yml for my development environment:

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

Invariably, running my code below will eventually die with a "Could not obtain a connection" error:

  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!)

How can I figure out what connections are not being closed, or what queries are still running that would cause this to occur? I am baffled because nowhere should concurrent queries be running at once.

Error during processing: (ActiveRecord::ConnectionTimeoutError) could not obtain a connection from the pool within 5.000 seconds (waited 5.003 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'

Answer 1

您可以使用以下方法从PostgreSQL获取此信息：

SELECT * from pg_stat_activity;

Answer 2

I'm running a single-threaded Rails 5.0 application

As soon as your application code creates another thread (which it is doing via Concurrent::FixedThreadPool.new(1) ), it is no longer a 'single-threaded application'.

How can I figure out what connections are not being closed or what queries are still running that would cause this to occur.

To inspect the connections that are not being closed, you can access the current connection pool's list of connections via ActiveRecord::Base.connection_pool.connections , which will return an Array of ActiveRecord::ConnectionAdapters::AbstractAdapter objects you can examine. For example, to inspect the thread owner of each connection currently in the connection pool, you can run:

ActiveRecord::Base.connection_pool.connections.map(&:owner).map(&:inspect)

I am baffled because nowhere should concurrent queries be running at once.

Your assumption that concurrent queries are running because the connection pool is being exhausted is not correct. On how to fix this issue, refer to my answer to your other (very similar) question, Does concurrency happen even when only one thread is in a thread pool?