class ActiveRecord::ConnectionAdapters::ConnectionHandler

Parent:: Object

ConnectionHandler is a collection of ConnectionPool objects. It is used for keeping separate connection pools for Active Record models that connect to different databases.

For example, suppose that you have 5 models, with the following hierarchy:

class Author < ActiveRecord::Base
end

class BankAccount < ActiveRecord::Base
end

class Book < ActiveRecord::Base
  establish_connection "library_db"
end

class ScaryBook < Book
end

class GoodBook < Book
end

And a database.yml that looked like this:

development:
  database: my_application
  host: localhost

library_db:
  database: library
  host: some.library.org

Your primary database in the development environment is “my_application” but the Book model connects to a separate database called “library_db” (this can even be a database on a different machine).

Book, ScaryBook and GoodBook will all use the same connection pool to “library_db” while Author, BankAccount, and any other models you create will use the default connection pool to “my_application”.

The various connection pools are managed by a single instance of ConnectionHandler accessible via ActiveRecord::Core.connection_handler. All Active Record models use this handler to determine the connection pool that they should use.

Public Class Methods

new() Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 511
def initialize
  # These caches are keyed by klass.name, NOT klass. Keying them by klass
  # alone would lead to memory leaks in development mode as all previous
  # instances of the class would stay in memory.
  @owner_to_pool = ThreadSafe::Cache.new(:initial_capacity => 2) do |h,k|
    h[k] = ThreadSafe::Cache.new(:initial_capacity => 2)
  end
  @class_to_pool = ThreadSafe::Cache.new(:initial_capacity => 2) do |h,k|
    h[k] = ThreadSafe::Cache.new
  end
end

Public Instance Methods

active_connections?() Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 544
def active_connections?
  connection_pool_list.any?(&:active_connection?)
end

Returns true if there are any active connections among the connection pools that the ConnectionHandler is managing.

clear_active_connections!() Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 551
def clear_active_connections!
  connection_pool_list.each(&:release_connection)
end

Returns any connections in use by the current thread back to the pool, and also returns connections to the pool cached by threads that are no longer alive.

clear_all_connections!() Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 560
def clear_all_connections!
  connection_pool_list.each(&:disconnect!)
end

clear_reloadable_connections!() Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 556
def clear_reloadable_connections!
  connection_pool_list.each(&:clear_reloadable_connections!)
end

Clears the cache which maps classes.

connected?(klass) Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 578
def connected?(klass)
  conn = retrieve_connection_pool(klass)
  conn && conn.connected?
end

Returns true if a connection that's accessible to this class has already been opened.

connection_pool_list() Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 523
def connection_pool_list
  owner_to_pool.values.compact
end

connection_pools() Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 527
      def connection_pools
        ActiveSupport::Deprecation.warn("          In the next release, this will return the same as `#connection_pool_list`.
          (An array of pools, rather than a hash mapping specs to pools.)
".squish)

        Hash[connection_pool_list.map { |pool| [pool.spec, pool] }]
      end

establish_connection(owner, spec) Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 536
def establish_connection(owner, spec)
  @class_to_pool.clear
  raise RuntimeError, "Anonymous class is not allowed." unless owner.name
  owner_to_pool[owner.name] = ConnectionAdapters::ConnectionPool.new(spec)
end

remove_connection(owner) Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 587
def remove_connection(owner)
  if pool = owner_to_pool.delete(owner.name)
    @class_to_pool.clear
    pool.automatic_reconnect = false
    pool.disconnect!
    pool.spec.config
  end
end

Remove the connection for this class. This will close the active connection and the defined connection (if they exist). The result can be used as an argument for #establish_connection, for easily re-establishing the connection.

retrieve_connection_pool(klass) Show source

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 605
def retrieve_connection_pool(klass)
  class_to_pool[klass.name] ||= begin
    until pool = pool_for(klass)
      klass = klass.superclass
      break unless klass <= Base
    end

    class_to_pool[klass.name] = pool
  end
end

Retrieving the connection pool happens a lot so we cache it in @class_to_pool. This makes retrieving the connection pool O(1) once the process is warm. When a connection is established or removed, we invalidate the cache.

Ideally we would use fetch here, as class_to_pool may sometimes be nil. However, benchmarking (gist.github.com/jonleighton/3552829) showed that fetch is significantly slower than []. So in the nil case, no caching will take place, but that's ok since the nil case is not the common one that we wish to optimise for.