How to manage opening and closing database connections while working with activerecords and multiple threads

Question

I am trying to implement a multithreaded method in rails such that I can create/update multiple records very quickly.

This is the outline of my program.

ActiveRecord::Base.transaction do
  (1..10).each do |i|
    arr[i] = Thread.new {
       <some logic on obj>
       ...
       ...
       obj.save! 
    }
  end
  arr.each {|t| t.join}
end

This gives me warnings on my log.

DEPRECATION WARNING: Database connections will not be closed automatically, 
please close your database connection at the end of the thread by calling `close`
on your connection.

And it gives me an error

could not obtain a database connection within 5 seconds (waited 5.059358 seconds). 
The max pool size is currently 5; consider increasing it.

I tried: - changing database.yaml and increasing the poolsize and timeout there. - modified the existing code in the following way.

   ActiveRecord::Base.connection_pool.clear_stale_cached_connections!
   begin
     ActiveRecord::Base.transaction do
        (1..10).each do |i|
          arr[i] = Thread.new {
             <some logic on obj>
             ...
             ...
             obj.save!
             ActiveRecord::Base.connection.close 
          }
        end
        arr.each {|t| t.join}
     end
   ensure
     ActiveRecord::Base.connection.close if ActiveRecord::Base.connection
     ActiveRecord::Base.clear_active_connections!
   end

I am still getting the same warning and error. I am obviously missing the concept here. How do I proceed with this?

Answer 1

To prevent connection leak in multi threads, you have to manage the connection manually. You can try:

Thread.new do
  ActiveRecord::Base.connection_pool.with_connection do
    # Do whatever
  end
end

One problem of ActiveRecord::Base.connection_pool.with_connection is that it always prepare a connection even if the code inside does not need it.

We can improve it a little bit by using ActiveRecord::Base.connection_pool.release_connection:

Thread.new do
  begin
    # Do whatever
  ensure
    ActiveRecord::Base.connection_pool.release_connection
  end
end

Answer 2

if you want to run in N threads, make sure you have N connections in the pool

#  ActiveRecord::Base.connection_config
#  # => {pool: 5, timeout: 5000, database: "db/development.sqlite3", adapter: "sqlite3"}

otherwise you will be able to fetch a connection from the pool.