Custom mysql2 adapter for connecting Rails via ProxySQL

database.yml

database: &default
  host: 127.0.0.1
  adapter: proxy_mysql2
  ...

proxy_mysql2_adapter.rb

require "active_record/connection_adapters/mysql2_adapter"
require "activerecord-import/adapters/mysql2_adapter"

module ActiveRecord
  module ConnectionHandling
    # Provide an extensible connection adapter based on mysql2_connection implemented in
    #
    # In the configuration of shard.yml or database.yml, one may declare to use this custom adapter:
    #
    #   adapter: "proxy_mysql2"
    #
    # Note that we can't just call it "proxysql" because database tasks looks for the pattern /mysql/ in the name.
    def proxy_mysql2_connection(config)
      config = config.symbolize_keys

      config[:username] ||= "root"
      config[:flags] ||= 0

      if Mysql2::Client.const_defined?(:FOUND_ROWS)
        if config[:flags].kind_of?(Array)
          config[:flags].push("FOUND_ROWS".freeze)
        else
          config[:flags] |= Mysql2::Client::FOUND_ROWS
        end
      end

      client = Mysql2::Client.new(config)
      ConnectionAdapters::ProxyMysql2Adapter.new(client, logger, nil, config)
    rescue Mysql2::Error => error
      if error.message.include?("Unknown database")
        raise ActiveRecord::NoDatabaseError
      else
        raise
      end
    end
  end
end

# When acquiring a new connection in Rails5, it sets the `sql_mode` in the beginning of the query. ProxySQL ignores all the variables after setting the `sql_mode`.
# ```
# # rails5 - ProxySQL ignores the subsequent variables after sql_mode.
# SET @@SESSION.sql_mode = CONCAT(REPLACE(REPLACE(REPLACE(@@sql_mode, 'STRICT_TRANS_TABLES', ''), 'STRICT_ALL_TABLES', ''), 'TRADITIONAL', ''), ',NO_AUTO_VALUE_ON_ZERO'), @@SESSION.max_statement_time = 300000,
#     @@SESSION.sql_auto_is_null = 0, @@SESSION.wait_timeout = 2147483"
# ```
#
# As a result, the connection isn't set up properly in Rails5.
#
# Except the `sql_mode`, the rest of the variables are the same. In practice, `sql_mode = ''` in Rails4 falls back to the default of MySQL server. `sql_model = 'NO_AUTO_VALUE_ON_ZERO,NO_ENGINE_SUBSTITUTION'` in Rails5, practically, has the same effect.
#
# The adapter ensures that the session variables are set properly and keeps multiplexing enabled in ProxySQL.
class ActiveRecord::ConnectionAdapters::ProxyMysql2Adapter < ActiveRecord::ConnectionAdapters::Mysql2Adapter
  include ActiveRecord::Import::MysqlAdapter

  # Copied from https://github.com/rails/rails/blob/4-2-stable/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb#L866-L906
  # and https://github.com/rails/rails/blob/4-2-stable/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb#L255
  def configure_connection
    @connection.query_options.merge!(as: :array)

    variables = @config.fetch(:variables, {}).stringify_keys

    # By default, MySQL 'where id is null' selects the last inserted id.
    # Turn this off. http://dev.rubyonrails.org/ticket/6778
    variables["sql_auto_is_null"] = 0

    # Increase timeout so the server doesn't disconnect us.
    wait_timeout = self.class.type_cast_config_to_integer(@config[:wait_timeout])
    wait_timeout = 2147483 unless wait_timeout.is_a?(Integer)
    variables["wait_timeout"] = wait_timeout

    # Make MySQL reject illegal values rather than truncating or blanking them, see
    # http://dev.mysql.com/doc/refman/5.0/en/server-sql-mode.html#sqlmode_strict_all_tables
    # If the user has provided another value for sql_mode, don't replace it.
    unless variables.has_key?("sql_mode")
      variables["sql_mode"] = strict_mode? ? "STRICT_ALL_TABLES" : ""
    end

    # NAMES does not have an equals sign, see
    # http://dev.mysql.com/doc/refman/5.0/en/set-statement.html#id944430
    # (trailing comma because variable_assignments will always have content)
    if @config[:encoding]
      encoding = "NAMES #{@config[:encoding]}"
      encoding << " COLLATE #{@config[:collation]}" if @config[:collation]
      encoding << ", "
    end

    # Gather up all of the SET variables...
    variable_assignments = variables.map do |k, v|
      if v == ":default" || v == :default
        "@@SESSION.#{k} = DEFAULT" # Sets the value to the global or compile default
      elsif !v.nil?
        "@@SESSION.#{k} = #{quote(v)}"
      end
      # or else nil; compact to clear nils out
    end.compact.join(", ")

    # The ProxySQL config at /etc/proxysql.cnf keeps multiplexing enabled on queries tagged with /* keep_multiplexing_enabled */
    # Otherwise multiplexing is disabled because it sets the session variables, and the db connections are not shared between processes.
    # https://github.com/sysown/proxysql/wiki/Multiplexing#ad-hoc-enabledisable-of-multiplexing
    @connection.query("SET #{encoding} #{variable_assignments} /* keep_multiplexing_enabled */")
  end
end

Thanks @arthurchui, by the way, did you nagios with ProxySQL ?

We configured Nagios with the MySQL database but not ProxySQL. @hopewise

When I require require "activerecord-import/adapters/mysql2_adapter" I am getting error

/Users/samir/.rvm/gems/ruby-2.3.3@backend/gems/activerecord-import-0.13.0/lib/activerecord-import/adapters/mysql_adapter.rb:1:in `<top (required)>': uninitialized constant ActiveRecord::Import (NameError)

@arthurchui shouldn't we not add comment keep_multiplexing_enabled while doing a transaction?

For example, having a cluster that has writer and reader, while doing a transaction: When storing data to DB (writer connection), then querying the DB again for updated data (reader connection), before the replication is done for that record.

What do you think?

Making this into a gem would be gold