class ActiveRecord::ConnectionAdapters::ConnectionHandler

in order to look up the correct connection pool.
about the model. The model needs to pass a connection specification name to the handler,
The ConnectionHandler class is not coupled with the Active models, as it has no knowledge
should use.
All Active Record models use this handler to determine the connection pool that they
ConnectionHandler accessible via ActiveRecord::Base.connection_handler.
The various connection pools are managed by a single instance of
will use the default connection pool to “my_application”.
“library_db” while Author, BankAccount, and any other models you create
Book, ScaryBook, and GoodBook will all use the same connection pool to
(this can even be a database on a different machine).
but the Book model connects to a separate database called “library_db”
Your primary database in the development environment is “my_application”
host: some.library.org
database: library
library_db:
host: localhost
database: my_application
development:
And a database.yml that looked like this:
end
class GoodBook < Book
end
class ScaryBook < Book
end
establish_connection :library_db
class Book < ActiveRecord::Base
end
class BankAccount < ActiveRecord::Base
end
class Author < ActiveRecord::Base
For example, suppose that you have 5 models, with the following hierarchy:
for keeping separate connection pools that connect to different databases.
ConnectionHandler is a collection of ConnectionPool objects. It is used
= Active Record Connection Handler

def active_connections?(role = nil)

pools that the ConnectionHandler is managing.
Returns true if there are any active connections among the connection 
def active_connections?(role = nil)
  each_connection_pool(role).any?(&:active_connection?)
end

def clear_active_connections!(role = nil)

longer alive.
and also returns connections to the pool cached by threads that are no
Returns any connections in use by the current thread back to the pool, 
def clear_active_connections!(role = nil)
  each_connection_pool(role).each do |pool|
    pool.release_connection
    pool.disable_query_cache!
  end
end

def clear_all_connections!(role = nil) 

def clear_all_connections!(role = nil)
  each_connection_pool(role).each(&:disconnect!)
end

def clear_reloadable_connections!(role = nil)

See ConnectionPool#clear_reloadable_connections! for details.

Clears the cache which maps classes. 
def clear_reloadable_connections!(role = nil)
  each_connection_pool(role).each(&:clear_reloadable_connections!)
end

def connected?(connection_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)

already been opened.
Returns true if a connection that's accessible to this class has 
def connected?(connection_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)
  pool = retrieve_connection_pool(connection_name, role: role, shard: shard)
  pool && pool.connected?
end

def connection_pool_list(role = nil)

all pools belonging to the connection handler will be returned.
Returns the pools for a connection handler and given role. If +:all+ is passed, 
def connection_pool_list(role = nil)
  if role.nil? || role == :all
    connection_name_to_pool_manager.values.flat_map { |m| m.pool_configs.map(&:pool) }
  else
    connection_name_to_pool_manager.values.flat_map { |m| m.pool_configs(role).map(&:pool) }
  end
end

def connection_pool_names # :nodoc:

:nodoc: 
def connection_pool_names # :nodoc:
  connection_name_to_pool_manager.keys
end

def determine_owner_name(owner_name, config) 

def determine_owner_name(owner_name, config)
  if owner_name.is_a?(String) || owner_name.is_a?(Symbol)
    ConnectionDescriptor.new(owner_name.to_s)
  elsif config.is_a?(Symbol)
    ConnectionDescriptor.new(config.to_s)
  else
    owner_name
  end
end

def disconnect_pool_from_pool_manager(pool_manager, role, shard) 

def disconnect_pool_from_pool_manager(pool_manager, role, shard)
  pool_config = pool_manager.remove_pool_config(role, shard)
  if pool_config
    pool_config.disconnect!
    pool_config.db_config
  end
end

def each_connection_pool(role = nil, &block) # :nodoc:

:nodoc: 
def each_connection_pool(role = nil, &block) # :nodoc:
  role = nil if role == :all
  return enum_for(__method__, role) unless block_given?
  connection_name_to_pool_manager.each_value do |manager|
    manager.each_pool_config(role) do |pool_config|
      yield pool_config.pool
    end
  end
end

def establish_connection(config, owner_name: Base, role: Base.current_role, shard: Base.current_shard, clobber: false) 

def establish_connection(config, owner_name: Base, role: Base.current_role, shard: Base.current_shard, clobber: false)
  owner_name = determine_owner_name(owner_name, config)
  pool_config = resolve_pool_config(config, owner_name, role, shard)
  db_config = pool_config.db_config
  pool_manager = set_pool_manager(pool_config.connection_descriptor)
  # If there is an existing pool with the same values as the pool_config
  # don't remove the connection. Connections should only be removed if we are
  # establishing a connection on a class that is already connected to a different
  # configuration.
  existing_pool_config = pool_manager.get_pool_config(role, shard)
  if !clobber && existing_pool_config && existing_pool_config.db_config == db_config
    # Update the pool_config's connection class if it differs. This is used
    # for ensuring that ActiveRecord::Base and the primary_abstract_class use
    # the same pool. Without this granular swapping will not work correctly.
    if owner_name.primary_class? && (existing_pool_config.connection_descriptor != owner_name)
      existing_pool_config.connection_descriptor = owner_name
    end
    existing_pool_config.pool
  else
    disconnect_pool_from_pool_manager(pool_manager, role, shard)
    pool_manager.set_pool_config(role, shard, pool_config)
    payload = {
      connection_name: pool_config.connection_descriptor.name,
      role: role,
      shard: shard,
      config: db_config.configuration_hash
    }
    ActiveSupport::Notifications.instrumenter.instrument("!connection.active_record", payload) do
      pool_config.pool
    end
  end
end

def flush_idle_connections!(role = nil)

See ConnectionPool#flush! for details.

Disconnects all currently idle connections. 
def flush_idle_connections!(role = nil)
  each_connection_pool(role).each(&:flush!)
end

def get_pool_manager(connection_name)

Returns the pool manager for a connection name / identifier. 
def get_pool_manager(connection_name)
  connection_name_to_pool_manager[connection_name]
end

def initialize 

def initialize
  # These caches are keyed by pool_config.connection_name (PoolConfig#connection_name).
  @connection_name_to_pool_manager = Concurrent::Map.new(initial_capacity: 2)
end

def pool_managers 

def pool_managers
  connection_name_to_pool_manager.values
end

def prevent_writes # :nodoc:

:nodoc: 
def prevent_writes # :nodoc:
  ActiveSupport::IsolatedExecutionState[:active_record_prevent_writes]
end

def prevent_writes=(prevent_writes) # :nodoc:

:nodoc: 
def prevent_writes=(prevent_writes) # :nodoc:
  ActiveSupport::IsolatedExecutionState[:active_record_prevent_writes] = prevent_writes
end

def remove_connection_pool(connection_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard) 

def remove_connection_pool(connection_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard)
  if pool_manager = get_pool_manager(connection_name)
    disconnect_pool_from_pool_manager(pool_manager, role, shard)
  end
end

def resolve_pool_config(config, connection_name, role, shard)


# => { host: "localhost", database: "foo", adapter: "sqlite3" }
pool_config.db_config.configuration_hash
pool_config = Base.configurations.resolve_pool_config(:production)
config = { "production" => { "host" => "localhost", "database" => "foo", "adapter" => "sqlite3" } }

== Example

Accepts a hash one layer deep that contains all connection information.
Returns an instance of PoolConfig for a given adapter. 
def resolve_pool_config(config, connection_name, role, shard)
  db_config = Base.configurations.resolve(config)
  db_config.validate!
  raise(AdapterNotSpecified, "database configuration does not specify adapter") unless db_config.adapter
  ConnectionAdapters::PoolConfig.new(connection_name, db_config, role, shard)
end

def retrieve_connection(connection_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard) # :nodoc:

:nodoc:
for (not necessarily the current class).
opened and set as the active connection for the class it was defined
active or defined connection: if it is the latter, it will be
Locate the connection of the nearest super class. This can be an 
def retrieve_connection(connection_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard) # :nodoc:
  pool = retrieve_connection_pool(connection_name, role: role, shard: shard, strict: true)
  pool.lease_connection
end

def retrieve_connection_pool(connection_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard, strict: false)

When a connection is established or removed, we invalidate the cache.
This makes retrieving the connection pool O(1) once the process is warm.
Retrieving the connection pool happens a lot, so we cache it in @connection_name_to_pool_manager. 
def retrieve_connection_pool(connection_name, role: ActiveRecord::Base.current_role, shard: ActiveRecord::Base.current_shard, strict: false)
  pool_manager = get_pool_manager(connection_name)
  pool = pool_manager&.get_pool_config(role, shard)&.pool
  if strict && !pool
    selector = [
      ("'#{shard}' shard" unless shard == ActiveRecord::Base.default_shard),
      ("'#{role}' role" unless role == ActiveRecord::Base.default_role),
    ].compact.join(" and ")
    selector = [
      (connection_name unless connection_name == "ActiveRecord::Base"),
      selector.presence,
    ].compact.join(" with ")
    selector = " for #{selector}" if selector.present?
    message = "No database connection defined#{selector}."
    raise ConnectionNotDefined.new(message, connection_name: connection_name, shard: shard, role: role)
  end
  pool
end

def set_pool_manager(connection_descriptor)

Get the existing pool manager or initialize and assign a new one. 
def set_pool_manager(connection_descriptor)
  connection_name_to_pool_manager[connection_descriptor.name] ||= PoolManager.new
end