To demonstrate a thread safe implementation of a connection pool, we will use a class instance variable, a Mutex, and the connection_pool gem.

Class Instance Variables

Class instance variables work like regular class variables except for two main differences.

1. Class instance variables are available only to class methods and not to instance methods.
2. Class instance variables are not shared with sub-classes. They belong exclusively to the class itself.

We will use class instance variables (mainly for reason 1) to share a connection pool which all threads will be able to access. Here is a quick example of what class instance variables can do for us.

class Example
  @my_value = 1

  class << self
    # class methods
    def my_value
      @my_value
    end
    def my_value=(value)
      @my_value = value
    end
  end
  # instance methods
  def my_value
    @my_value
  end
  def my_value=(value)
    @my_value = value
  end
end
# class method
puts Example.my_value # my_value = 1
Example.my_value = 2
puts Example.my_value # my_value = 2
# instance method
a = Example.new
puts a.my_value # my_value is not initialized at the instance level
a.my_value = 3
puts a.my_value # my_value = 3
# class method
puts Example.my_value # my_value is still 2 at the class level

When my_value was accessed by class methods, its value was only accessed and updated at the class level. When accessed as an instance method, it was accessed and updated in a separate memory space at the instance level.

If you want to learn more about class instance variables, take a look at this article and this post.

Now that we have a way to access the _same _connection pool across threads, we need to actually create the pool so threads can use it.

Connection Pool

The connection_pool gem will allow us to create a pool of connections which we can use to grab a connection and return it after we’re done using it. This allows us to specify a static number of connections and forces our threads to share those connections.

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