hideto

再谈ActiveRecord、MySQL和transaction

第一篇：ActiveRecord如何与MySQL交互

我们自定义的MyModel继承于ActiveRecord::Base类

MyModel < ActiveRecord::Base
MyModel.find_xxx -> find_by_sql -> connection.select_all(sql)
MyModel.create_xxx   ->  connection.insert(sql)
MyModel.update_xxx   ->  connection.update(sql)
MyModel.destroy      ->  connection.delete(sql)

这个connection是什么东西？让我们一步步往下看

可以看到MyModel的一些方法都在base.rb里实现为connection对象相关的方法
如果我们使用的是MySQL数据库，则这里的connection为ActiveRecord::ConnectionAdapters::MysqlAdapter对象
这是在Rails初始化时做的：

environment.rb的Rails::Initializer.run -> initializer.rb的process方法 -> initializer.rb的initialize_database方法
-> ActiveRecord::Base.establish_connection（connenction_specification.rb里对ActiveRecord::Base添加该方法和connection方法）
-> mysql_adapter.rb的mysql_connection方法 -> mysql.rb的real_connect方法 -> TCPSocket::new/UNIXSocket::new

而MysqlAdapter继承于AbstractAdapter，AbstractAdapter又include了DatabaseStatements所以：

MyModel < ActiveRecord::Base
MyModel.find_xxx -> find_by_sql -> DatabaseStatements.select_all(sql) -> MysqlAdapter.select(sql) -> MysqlAdapter.execute(sql) -> MysqlAdapter.@connection.query(sql)
MyModel.create_xxx   ->  MysqlAdapter.insert(sql) -> MysqlAdapter.execute(sql) -> MysqlAdapter.@connection.query(sql)
MyModel.update_xxx   ->  MysqlAdapter.update(sql) -> MysqlAdapter.execute(sql) -> MysqlAdapter.@connection.query(sql)
MyModel.destroy      ->  DatabaseStatements.delete(sql) -> MysqlAdapter.execute(sql) -> MysqlAdapter.@connection.query(sql)

这里的MysqlAdapter.@connection在mysql_adapter.rb的mysql_connection方法里初始化了：

require_mysql
mysql = Mysql.init
mysql.ssl_set(config[:sslkey], config[:sslcert], config[:sslca], config[:sslcapath], config[:sslcipher]) if config[:sslkey]

ConnectionAdapters::MysqlAdapter.new(mysql, logger, [host, username, password, database, port, socket], config)

继续追踪：

@connection.query(sql) -> real_query(sql) -> command(sql) -> Net.read/write -> socket.read/write

至此，Rails自带的MySQL Adapter了解清楚了。

第二篇：ActiveRecord对MySQL的事务封装
首先看看MySQL的事务控制(摘自MySQL5.1参考手册)：


开始追踪:

# transactions.rb

def self.included(base)
  base.extend(ClassMethods)

  base.class_eval do
    [:destroy, :save, :save!].each do |method|
      alias_method_chain method, :transactions
    end
  end
end

这就是Rails里所谓的“隐式事务”，给你安排好了
这表示ActiveRecord对save，save!和destroy方法都包上了事务，而create、update、delete等方法最终都是调用save、save!和destroy方法

追踪:

transactions.rb的transaction -> connection.transaction -> MysqlAdapter.transaction(没有) -> AbstractAdapter.transaciton(没有) -> DatabaseStatements.transaction

看下database_statements.rb里的transaciton方法：

def transaction(start_db_transaction = true)
  transaction_open = false
  begin
    if block_given?
      if start_db_transaction
        begin_db_transaction 
        transaction_open = true
      end
      yield
    end
  rescue Exception => database_transaction_rollback
    if transaction_open
      transaction_open = false
      rollback_db_transaction
    end
    raise
  end
ensure
  commit_db_transaction if transaction_open
end

马上又回到mysql_adapter.rb里对transaction打开、提交、回滚的实现：

def begin_db_transaction #:nodoc:
  execute "BEGIN"
rescue Exception
  # Transactions aren't supported
end

def commit_db_transaction #:nodoc:
  execute "COMMIT"
rescue Exception
  # Transactions aren't supported
end

def rollback_db_transaction #:nodoc:
  execute "ROLLBACK"
rescue Exception
  # Transactions aren't supported
end

execute即Mysql.query，上面提到了。

那么为啥ActiveRecord的transaction不支持嵌套呢？
还是看transaction.rb:

def transaction(*objects, &block)
  previous_handler = trap('TERM') { raise TransactionError, "Transaction aborted" }
  increment_open_transactions

  begin
    unless objects.empty?
      ActiveSupport::Deprecation.warn "Object transactions are deprecated and will be removed from Rails 2.0.  See http://www.rubyonrails.org/deprecation for details.", caller
      objects.each { |o| o.extend(Transaction::Simple) }
      objects.each { |o| o.start_transaction }
    end

    result = connection.transaction(Thread.current['start_db_transaction'], &block)

    objects.each { |o| o.commit_transaction }
    return result
  rescue Exception => object_transaction_rollback
    objects.each { |o| o.abort_transaction }
    raise
  ensure
    decrement_open_transactions
    trap('TERM', previous_handler)
  end
end

private
  def increment_open_transactions #:nodoc:
    open = Thread.current['open_transactions'] ||= 0
    Thread.current['start_db_transaction'] = open.zero?
    Thread.current['open_transactions'] = open + 1
  end

  def decrement_open_transactions #:nodoc:
    Thread.current['open_transactions'] -= 1
  end

原来，Rails每次遇到调用transaction方法时，看看当前线程里open transaction的次数:
1, 如果有多次，则调用connection.transaction(false, &block)
2, 如果为零次，则调用connection.transaction(true, &block)
而database_statements.rb里的transaciton方法根据true or false参数来判断是否重复open transaction
这样，Rails遇到最外层的transaction打开后，就不会再打开内部的transaction了，这也是手动写多层嵌套transaction时只有最外层的起作用的原因

至此，Rails对transaction的封装了解清楚。