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目前项目已快截止，编码任务也基本完成，现在主要是性能测试。
项目是按照“Tair LDB基于Prefixkey的范围查找性能优化项目提议方案”的步骤一步步完成的，首先先介绍第一个关键问题是如何解决的。在提案中有以下描述：
由于getrange接口的数据是从prefixput/prefixincr接口进来的，那么prefix的长度信息就可以从它们的pkey参数得到，pkey的数据类型是dataentry，有属性prefixsize，那么我们在客户端将pkey和skey合并为mkey（已经设置mkey的prefixsize为pkey的size）后与value一起传送到服务器端。
在客户端与服务器端的连接过程中，将key的类型封装成LdbKey类，value的类型封装成LdbItem类，LdbItem里面含有key的prefixsize信息，然后两者都转化为Slice类型发送到leveldb底层进行存储操作。注意此时value里面包含了prefixszie信息（序列化信息，不能直接提取），因此我们在生成filter block时可以从value中提取出prefix_size信息（按LdbItem的格式进行分析提取）以生成我们所需要的prefix bloomfilter。提取的具体实现可以放在leveldb层的外面，在leveldb里面进行调用即可（分离操作）。
这里面提到一个关键信息：key的prefix_size信息在db中是存储在value中而不是在key中.
那么value的格式是什么样的呢？
首先value的内容是由LdbItem的数据得到的，知道了LdbItem里数据的存储格式也就知道了value的数据存储格式。LdbItem里data_的数据构成是由下面这个set函数完成的：
// meta_ MUST already be set correctly
void set(const char* value_data, const int32_t value_size)
{
if (value_data != NULL && value_size > 0)
{
char *metap = reinterpret_cast<char *>(&meta_);
int real_meta_size = LDB_ITEM_META_BASE_SIZE;
LdbItemMetaBase *metabp = reinterpret_cast<LdbItemMetaBase *>(&meta_);
free();
if (metabp->flag_ & TAIR_ITEM_FLAG_NEWMETA)
{
if (META_VER_PREFIX == metabp->meta_version_)
real_meta_size = LDB_ITEM_META_SIZE;
else if (META_VER_BASE == metabp->meta_version_)
real_meta_size = LDB_ITEM_META_BASE_SIZE;
}
data_size_ = value_size + real_meta_size;
data_ = new char[data_size_];
memcpy(data_, metap, real_meta_size);
memcpy(data_ + real_meta_size, value_data, value_size);
alloc_ = true;
}
可以知道data_的内容由两部分构成：
| LdbItemMeta数据 | 真实的value内容 |
或者
| LdbItemMetaBase数据 | 真实的value内容 |
两者的区别在于LdbItemMeta数据包含prefix_size信息而LdbItemMetaBase数据不包含，这通过这两个简单的数据结构组成部分就知道了。
struct LdbItemMetaBase
{
LdbItemMetaBase() : meta_version_(0), flag_(0), version_(0), cdate_(0), mdate_(0), edate_(0){}
uint8_t meta_version_; // meta data version
uint8_t flag_; // flag
uint16_t version_; // version
uint32_t cdate_; // create time
uint32_t mdate_; // modify time
uint32_t edate_; // expired time(for meta when get value. dummy with key)
};
struct LdbItemMeta // change value() and set() ,if you want to add new metadata
{
LdbItemMeta(): prefix_size_(0) {}
struct LdbItemMetaBase base_;
uint16_t prefix_size_; // prefix key size(for getRange conflict detect)
uint16_t reserved; //
};
因此如果value的第一部分是LdbItemMeta数据，就说明它包含prefixsize信息，我们就可以将value内容按LdbItemMeta的格式进行解析，从而提取出其中的prefixsize信息。
下面是具体的解析提取程序：
// get prefix size from slice value content
int get_prefix_size(const leveldb::Slice &value) {
// parse LdbItemMetaBase from value
char *val = const_cast<char*>(value.data());
LdbItemMeta *metap = reinterpret_cast<LdbItemMeta*>(val);
// check if prefix is set
if (metap->base_.flag_ & TAIR_ITEM_FLAG_NEWMETA) {
// if prefix is set, parse it from LdbItemMeta. if not, return 0
if (META_VER_PREFIX == metap->base_.meta_version_) {
return metap->prefix_size_;
} else {
return 0;
}
}
return 0;
}