[thrift]thrift中的对象序列化

本文炒冷饭.说实话,一直挺看好Thrift,支持的语言又多,代码写的有很清晰,效率又不低,为啥研究Protocol Buffer的人那么多.不管那么多了....

Thrift中的对象序列化是我很看好的东西,他用compiler+类库,让你高效的完成任务,而且可以少犯错误.试想,有谁可以保证自己设计的对象,不会再改变呢?数据库的schema改了,你可以改改查询语句,但是如果你对象改了,之前序列化好的东西,有时候就很难搞回来了.(哎.....)

废话不说,看Thrift里面怎么搞的.

1. Thrift支持的数据类型

Thrift支持的数据类型定义在TProtocol.h这个头文件中,有一个TType的枚举:

enum TType {
  T_STOP       = 0,
  T_VOID       = 1,
  T_BOOL       = 2,
  T_BYTE       = 3,
  T_I08        = 3,
  T_I16        = 6,
  T_I32        = 8,
  T_U64        = 9,
  T_I64        = 10,
  T_DOUBLE     = 4,
  T_STRING     = 11,
  T_UTF7       = 11,
  T_STRUCT     = 12,
  T_MAP        = 13,
  T_SET        = 14,
  T_LIST       = 15,
  T_UTF8       = 16,
  T_UTF16      = 17
};

而每一种Protocol都不一定全部支持这么多数据格式,T_LIST之前的都是被支持的.T_STRING是c string,可以和utf-8兼容.

2. Thrift对各种数据类型的读写

Thrift把对象序列化抽象成TProtocol这样一个抽象类,这个类的成员非常多,但是思路很明显,就是对各种数据类型的读写操作:

class TProtocol {
 public:
  virtual ~TProtocol() {}
  uint32_t writeMessageBegin(const std::string& name,
                             const TMessageType messageType,
                             const int32_t seqid);
  uint32_t writeMessageEnd();
  uint32_t writeFieldBegin(const char* name,
                           const TType fieldType,
                           const int16_t fieldId) ;
  uint32_t writeFieldEnd();
  uint32_t writeFieldStop();
  //写各种类型的数据
  uint32_t writeBool(const bool value);
  uint32_t writeByte(const int8_t byte);
  uint32_t writeI16(const int16_t i16);
  uint32_t writeStructBegin(const char* name);
  uint32_t writeStructEnd();  
  //此处省略若干行

  uint32_t readMessageBegin(std::string& name,
                            TMessageType& messageType,
                            int32_t& seqid);
  uint32_t readMessageEnd();
  uint32_t readFieldBegin(std::string& name,
                          TType& fieldType,
                          int16_t& fieldId);
  uint32_t readFieldEnd() ;
  //读各种类型的数据
  uint32_t readBool(bool& value);
  uint32_t readI16(int16_t& i16) ;
  uint32_t readStructBegin(std::string& name) ;
  uint32_t readStructEnd(); 
  //此处省略若干行

};

每一种数据类型都一对read/write方法,另外Message,Field也有read/write方法.

网友可能很奇怪,为啥抽象类的方法不是虚的.......其实我这里代码省略了很多,之前0.5.0版本的thrift里面,这些方法都是虚的,对TProtocol的实现都重写了这些方法;0.6.0里面,直接的read/write方法都不是虚的,但是添加了额外的虚函数:

  //比如说对list的读
  virtual uint32_t readListBegin_virt(TType& elemType,
                                      uint32_t& size) = 0;
  virtual uint32_t readListEnd_virt() = 0;
  //另外有
  uint32_t readListBegin(TType& elemType, uint32_t& size) {
    T_VIRTUAL_CALL();
    return readListBegin_virt(elemType, size);
  }
  uint32_t readListEnd() {
    T_VIRTUAL_CALL();
    return readListEnd_virt();
  }

其实和直接使用虚函数是一样的.

OK,接口看完了,这就去看对接口的实现,我们来看TBinaryProtocolT是怎么实现的.这里多说几句,Thrift里面实现了好多种序列化,如果你觉得这种序列化不好,可以去重新实现一个上面说的那个接口,就可以工作了:-D

TBinaryProtocolT里面我们看一两个具有代表性的,int32_t/map的读写:

template <class Transport_>
uint32_t TBinaryProtocolT<Transport_>::writeI32(const int32_t i32) {
  //把数字转化成网络字节序
  int32_t net = (int32_t)htonl(i32);
  //然后写入到transport
  this->trans_->write((uint8_t*)&net, 4);
  //返回写入数据的大小
  return 4;
}
template <class Transport_>
uint32_t TBinaryProtocolT<Transport_>::readI32(int32_t& i32) {
  uint8_t b[4];
  this->trans_->readAll(b, 4);
  i32 = *(int32_t*)b;
  //读取四个字节,转为本地字节序
  i32 = (int32_t)ntohl(i32);
  //返回读出数据的大小
  return 4;
}
template <class Transport_>
uint32_t TBinaryProtocolT<Transport_>::writeMapBegin(const TType keyType,
                                                     const TType valType,
                                                     const uint32_t size) {
  uint32_t wsize = 0;
  //写入一个byte的key类型TType
  wsize += writeByte((int8_t)keyType);
  //写入一个byte的value类型TType
  wsize += writeByte((int8_t)valType);
  //再写入元素的个数,int32_t的
  wsize += writeI32((int32_t)size);
  return wsize;
}
template <class Transport_>
uint32_t TBinaryProtocolT<Transport_>::readMapBegin(TType& keyType,
                                                    TType& valType,
                                                    uint32_t& size) {
  int8_t k, v;
  uint32_t result = 0;
  int32_t sizei;
  //读的时候也是类似,读取key的类型,value的类型,还有元素的个数
  result += readByte(k);
  keyType = (TType)k;
  result += readByte(v);
  valType = (TType)v;
  result += readI32(sizei);
  if (sizei < 0) {
    throw TProtocolException(TProtocolException::NEGATIVE_SIZE);
  } else if (this->container_limit_ && sizei > this->container_limit_) {
    throw TProtocolException(TProtocolException::SIZE_LIMIT);
  }
  size = (uint32_t)sizei;
  return result;
}

可以看到,代码内聚很强,也很易懂.float/double的序列化是通过强转成uint32_t/uint64_t来实现的,string么,先去写一个大小,然后才是内容.list和set都是类似的~~

对于field的read/write都是直接写该filed的类型信息,而field那么就忽略掉了,因为二进制序列化用不到那些东西,只有json这样的文本序列化才能用到:-D,有兴趣的可以去看看JSON Protocol的实现

3. 代码生成

如果让你手写对象的序列化,反序列化,你肯定要抱怨了,因为那样出错的机会非常大.Thrift和Protocol Buffer都给你提供了编译器,写好IDL之后,可以用编译器生成好代码~~这样可以保证不会出错.以UserProfile为例:

struct UserProfile {
  1: i32 uid,
  2: string name,
  3: string blurb
}

生成代码: thrift-0.6.0.exe --gen cpp UserProfile.thrift

这样,thrift会在gen-cpp文件夹内生成好UserProfile的代码,我们只想看UserProfile类的read和write是怎么实现的:

uint32_t UserProfile::write(::apache::thrift::protocol::TProtocol* oprot) const {
  uint32_t xfer = 0;
  //write的代码比较简单,就是按照顺序
  //把field的类型和fieldid和value写进去
  xfer += oprot->writeStructBegin("UserProfile");
  xfer += oprot->writeFieldBegin("uid", ::apache::thrift::protocol::T_I32, 1);
  xfer += oprot->writeI32(this->uid);
  xfer += oprot->writeFieldEnd();
  xfer += oprot->writeFieldBegin("name", ::apache::thrift::protocol::T_STRING, 2);
  xfer += oprot->writeString(this->name);
  xfer += oprot->writeFieldEnd();
  xfer += oprot->writeFieldBegin("blurb", ::apache::thrift::protocol::T_STRING, 3);
  xfer += oprot->writeString(this->blurb);
  xfer += oprot->writeFieldEnd();
  xfer += oprot->writeFieldStop();
  xfer += oprot->writeStructEnd();
  return xfer;
}


uint32_t UserProfile::read(::apache::thrift::protocol::TProtocol* iprot) {

  uint32_t xfer = 0;
  std::string fname;
  ::apache::thrift::protocol::TType ftype;
  int16_t fid;

  xfer += iprot->readStructBegin(fname);

  using ::apache::thrift::protocol::TProtocolException;


  while (true)
  {
  //read的时候,每次都是先读出来field的类型和fieldid
    xfer += iprot->readFieldBegin(fname, ftype, fid);
    if (ftype == ::apache::thrift::protocol::T_STOP) {
      break;
    }
    switch (fid)
    {
      //然后查看当前反序列化的fieldid和读出来的field是不是同一个类型的
      //如果是就反序列化
      //不是就skip....
      //类型就是靠之前说的TType
      case 1:
        if (ftype == ::apache::thrift::protocol::T_I32) {
          xfer += iprot->readI32(this->uid);
          this->__isset.uid = true;
        } else {
          xfer += iprot->skip(ftype);
        }
        break;
      case 2:
        if (ftype == ::apache::thrift::protocol::T_STRING) {
          xfer += iprot->readString(this->name);
          this->__isset.name = true;
        } else {
          xfer += iprot->skip(ftype);
        }
        break;
      case 3:
        if (ftype == ::apache::thrift::protocol::T_STRING) {
          xfer += iprot->readString(this->blurb);
          this->__isset.blurb = true;
        } else {
          xfer += iprot->skip(ftype);
        }
        break;
      default:
        xfer += iprot->skip(ftype);
        break;
    }
    xfer += iprot->readFieldEnd();
  }

  xfer += iprot->readStructEnd();

  return xfer;
}

使用这个类也是比较简单的,Thrift的transport对读写操作做了一定的抽象,你可以读写网络端口,文件,内存等,我们这里用内存:

typedef unsigned long    uint32_t;
typedef unsigned char    uint8_t;
uint32_t bufferSize = 64*1024;
uint8_t *buffer = (uint8_t*)malloc(bufferSize);
boost::shared_ptr<TMemoryBuffer> _write(new TMemoryBuffer(buffer,bufferSize,TMemoryBuffer::TAKE_OWNERSHIP));
TProtocol *protowrite = new TBinaryProtocol(_write);

UserProfile _userProfile;
//在这里修改_userProfile的属性
//....
//序列化
uint32_t writeSize = _userProfile.write(protowrite);

boost::shared_ptr<TMemoryBuffer> _read(new TMemoryBuffer(buffer,bufferSize));
TProtocol *protoread = new TBinaryProtocol(_read);
UserProfile _userProfile2;
_userProfile2.read(protoread);

asser(_userProfile == _userProfile2);

很ez吧

4. Thrift向后兼容性的实现

看生成好的read代码可以看到有一个skip的方法(当他类型不一样的时候),这个就是向后兼容性实现的关键.一个对象,难免被改来改去,改不要紧,field id要变化,不要一个field id用到死..当然你继续用也没问题,比如你之前field id 1 type int32_t,之后还是这样,读是读出来了,有可能逻辑不对了.....来看看skip的实现.

//策略就是,读到什么抛弃什么
template <class Protocol_>
uint32_t skip(Protocol_& prot, TType type) 
{
  switch (type) {
  case T_BOOL:
    {
      bool boolv;
      return prot.readBool(boolv);
    }
  case T_BYTE:
    {
      int8_t bytev;
      return prot.readByte(bytev);
    }
   //此处省略若干行
   //因为代码都是类似的

    //对结构体,map/list/set之类的skip操作是比较复杂的
    case T_STRUCT:
    {
      uint32_t result = 0;
      std::string name;
      int16_t fid;
      TType ftype;
      result += prot.readStructBegin(name);
      while (true) {
        result += prot.readFieldBegin(name, ftype, fid);
        if (ftype == T_STOP) {
          break;
        }
        result += skip(prot, ftype);
        result += prot.readFieldEnd();
      }
      result += prot.readStructEnd();
      return result;
    }
}

OK,至此,Thrift对象序列化的代码基本上就看的差不多了.因为我们不会去用Thrift的Service,所以那部分代码没看过....

Thrift千好万好,但是如果你数据写好了,schema丢失了.....那就不好玩了

PS:很早之前看过Thrift,但是没写篇文章总结一下.这篇也算是了了心愿.