glog本身是非常高效的，google的大牛肯定知道大规模的写日志用glog的话肯定会影响业务线程的处理，带负荷的磁盘IO谁都桑不起。比如levelDB就是默认异步写，更不用说google的三驾马车都是分布式的。之前看过其论文，简直是引领时代。

在glog的issue里有人提出了异步写的问题，但是语焉不详，不过0.33版本已经有了接口，但是还不友好，但是完全可以实现磁盘日志的异步写。

今天算是花了点时间踩了点坑，算是基本可以搞了。稳定之后会把这个版本和glog，g2log，mudo logging一起测试下。mudo对buffer做了些trick，内部有两个bufferptr，做了双缓冲，据说效率很高，不过只有linux平台的，不过但把它的log抽离出来也不难，陈老师封装了mutex,thread，conditional等，在gcc4.8,clang3.3,VS2010都不是问题，已经没多大必要，而且之前为之乐道的linux下的threadsafe的initonce,现在C++11x也有了支持。

glog中可以让client定制接口是：

class GOOGLE_GLOG_DLL_DECL Logger {
 public:
  virtual ~Logger();

  // Writes "message[0,message_len-1]" corresponding to an event that
  // occurred at "timestamp".  If "force_flush" is true, the log file
  // is flushed immediately.
  //
  // The input message has already been formatted as deemed
  // appropriate by the higher level logging facility.  For example,
  // textual log messages already contain timestamps, and the
  // file:linenumber header.
  virtual void Write(bool force_flush,
                     time_t timestamp,
                     const char* message,
                     int message_len) = 0;

  // Flush any buffered messages
  virtual void Flush() = 0;

  // Get the current LOG file size.
  // The returned value is approximate since some
  // logged data may not have been flushed to disk yet.
  virtual uint32 LogSize() = 0;

  virtual void SetBasename(const char* basename) = 0;
  virtual void SetExtension(const char* ext) = 0 ;
  virtual void SetSymlinkBasename(const char* symlink_basename) = 0;

};

用Active object模式很好解决，就是我们通常所说的生产者消费者，在logmsg析构时就会fflush到磁盘，这次就会调用logger的write方法，此时就是我们接手的机会，把数据封装下，投递到业务线程，然后取出，实际写磁盘就好。

封装了简单的Active模式,Activer里封装了LogData用来封装打印实体，Buffer用来线程间传递数据，另外要显式设置Active的回调函数callBack.线程间传递数据用了C++11里的currentQueue,就不需要自己造轮子了：

/** ==========================================================================
* 2010 by KjellKod.cc. This is PUBLIC DOMAIN to use at your own risk and comes
* with no warranties. This code is yours to share, use and modify with no
* strings attached and no restrictions or obligations.
* ============================================================================
*
* Example of a Active Object, using C++11 std::thread mechanisms to make it
* safe for thread communication.
*
* This was originally published at http://sites.google.com/site/kjellhedstrom2/active-object-with-cpp0x
* and inspired from Herb Sutter‘s C++11 Active Object
* http://herbsutter.com/2010/07/12/effective-concurrency-prefer-using-active-objects-instead-of-naked-threads
*
* The code below uses JustSoftware Solutions Inc std::thread implementation
* http://www.justsoftwaresolutions.co.uk
*
* Last update 2012-10-10, by Kjell Hedstrom,
* e-mail: hedstrom at kjellkod dot cc
* linkedin: http://linkedin.com/se/kjellkod */

#ifndef ACTIVE_H_
#define ACTIVE_H_

#include <thread>
#include <functional>
#include <condition_variable>
#include <mutex>
#include <memory>
#include <concurrent_queue.h>
#include "shared_queue.h"

struct  Buffer
{
	Buffer():m_Len(0), m_pMsg(NULL){}
	~Buffer()
	{
		if (NULL != m_pMsg)
			delete []m_pMsg;
	}
	Buffer(int size):m_Len(size)
		, m_pMsg(new char[m_Len])
	{

	}
	int m_Len;
	char* m_pMsg;
};

typedef std::function<void(Buffer*)> Callback;

class Active {
private:
  Active(const Active&); // c++11 feature not yet in vs2010 = delete;
  Active& operator=(const Active&); // c++11 feature not yet in vs2010 = delete;
  Active();                         // Construction ONLY through factory createActive();
  void doDone(){done_ = true;}
  void run();
  void setCallBack(Callback aCallBack);


  Concurrency::concurrent_queue<Buffer*> mq_;
  std::thread thd_;
  bool done_;  // finished flag to be set through msg queue by ~Active
  Callback callBack_;

public:
  virtual ~Active();
  void send(Buffer* apBuffer);
  static std::unique_ptr<Active> createActive(Callback aCallBack); // Factory: safe construction & thread start
};




#endif
/** ==========================================================================
* 2010 by KjellKod.cc. This is PUBLIC DOMAIN to use at your own risk and comes
* with no warranties. This code is yours to share, use and modify with no
* strings attached and no restrictions or obligations.
* ============================================================================
*
* Example of a Active Object, using C++11 std::thread mechanisms to make it
* safe for thread communication.
*
* This was originally published at http://sites.google.com/site/kjellhedstrom2/active-object-with-cpp0x
* and inspired from Herb Sutter‘s C++11 Active Object
* http://herbsutter.com/2010/07/12/effective-concurrency-prefer-using-active-objects-instead-of-naked-threads
*
* The code below uses JustSoftware Solutions Inc std::thread implementation
* http://www.justsoftwaresolutions.co.uk
*
* Last update 2012-10-10, by Kjell Hedstrom,
* e-mail: hedstrom at kjellkod dot cc
* linkedin: http://linkedin.com/se/kjellkod */


#include "active.h"
#include <cassert>

Active::Active(): done_(false){}

Active::~Active() {
  Callback quit_token = std::bind(&Active::doDone, this);
  thd_.join();
}

// Add asynchronously a work-message to queue
void Active::send( Buffer* apBuffer )
{
	if (NULL != apBuffer)
	{
		mq_.push(apBuffer);
	}
}

void Active::run() {
  while (!done_) {
	if (!mq_.empty())
	{
		Buffer* pBuffer = NULL;
		mq_.try_pop(pBuffer);
		if (NULL != pBuffer)
		{
			callBack_(pBuffer);

			delete pBuffer;
		}
	}
  }
}

// Factory: safe construction of object before thread start
std::unique_ptr<Active> Active::createActive(Callback aCallBack){
  std::unique_ptr<Active> aPtr(new Active());
  aPtr->thd_ = std::thread(&Active::run, aPtr.get());
  aPtr->callBack_ = aCallBack;
  return aPtr;
}

void Active::setCallBack( Callback aCallBack )
{
	callBack_ = aCallBack;
}

重点是在threadlogger里，实现了Logger的接口。Write函数实现真正的写逻辑，几个set函数会在内部被调用。

#pragma once
#include <glog/logging.h>
#include <mutex>
#include "active.h"

using namespace std;

namespace google
{

class ThreadLog : public google::base::Logger
{
public:
	ThreadLog();
	~ThreadLog();
	virtual void Write(bool force_flush,
		time_t timestamp,
		const char* message,
		int message_len) ;
	virtual void Flush();
	virtual uint32 LogSize();

	// Configuration options
	void SetBasename(const char* basename);
	void SetExtension(const char* ext);
	void SetSymlinkBasename(const char* symlink_basename);
	void CallBack(Buffer* pBuffer);

private:
	static const uint32 kRolloverAttemptFrequency = 0x20;
	mutex lock_;
	bool base_filename_selected_;
	string base_filename_;
	string symlink_basename_;
	string filename_extension_;     // option users can specify (eg to add port#)
	FILE* file_;
	LogSeverity severity_;
	uint32 bytes_since_flush_;
	uint32 file_length_;
	unsigned int rollover_attempt_;
	int64 next_flush_time_;         // cycle count at which to flush log
	string hostname;
	bool stopWriting;
	std::unique_ptr<Active> m_pActive;
	bool CreateLogfile(const string& time_pid_string);
	void FlushUnlocked();
	void WriteInteral(bool force_flush, time_t timestamp, const char* message, int message_len);
};

}

#include "ThreadLog.h"
#include "port.h"
#include <fcntl.h>
#include <iomanip> 
#include "utilities.h"
#include <functional>

namespace google
{
	static int GetSize(bool& force_flush, time_t& timestamp, const char* message, int& message_len)
	{
		return sizeof(force_flush)+sizeof(timestamp)+sizeof(message_len)+message_len;
	}

	void ThreadLog::Write( bool force_flush, time_t timestamp, const char* message, int message_len )
	{
		Buffer* pBuffer = new Buffer(GetSize(force_flush, timestamp, message, message_len));
		char* curData = http://www.mamicode.com/pBuffer->m_pMsg;>


这样搞定之后，main函数可以这样使用，就可以把自己的ThreadLog类内嵌到glog里。
#define GLOG_NO_ABBREVIATED_SEVERITIES
#include <windows.h>
#include <glog/logging.h>
#include "ThreadLog.h"

using namespace google;
int main(int argc, char* argv[]) {
	google::InitGoogleLogging("test/testsss");
	google::base::Logger* mylogger = new google::ThreadLog;
	SetLogger(google::GLOG_INFO, mylogger);

	google::SetLogDestination(google::GLOG_INFO, "../Debug/logtestInfo"); 
	//google::SetLogDestination(google::GLOG_ERROR, "../Debug/logtestDebug");

	int num_cookies = 0;

	google::SetStderrLogging(google::GLOG_INFO);
	//google::SetStderrLogging(google::GLOG_ERROR);
	//google::LogToStderr();
	for (int i = 0; i < 1000; ++i){
		LOG(INFO) << "how are " << i << " cookies";
	}

	google::ShutdownGoogleLogging();
}

当然直接用这源码是无法编译成功的，我修改了glog内部的源码。
等下会把github工程地址：https://github.com/boyxiaolong/Proejcts/tree/master/asyn_glog-0.3.3
测试还有点问题，偶尔会有乱码，而且需要优化的是那个Buffer的动态申请。
不过都是后话了。