establish_connection

# File activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb, line 115
      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