leveldb源码分析8

Options options;              // data block的选项
Options index_block_options;  // index block的选项
WritableFile* file;           // sstable文件
uint64_t offset; 
// 要写入data block在sstable文件中的偏移，初始0
Status status;                //当前状态-初始ok
BlockBuilder data_block;      //当前操作的data block
BlockBuilder index_block;     // sstable的index block
std::string last_key;         //当前data block最后的k/v对的key
int64_t num_entries;          //当前data block的个数，初始0
bool closed;                  //调用了Finish() or Abandon()，初始false
FilterBlockBuilder*filter_block; 
//根据filter数据快速定位key是否在block中
bool pending_index_entry;     //见下面的Add函数，初始false
BlockHandle pending_handle;   //添加到index block的data block的信息
std::string compressed_output;//压缩后的data block，临时存储，写入后即被清空

Rep* r = rep_;
assert(!r->closed);
if (!ok()) return;
if (r->num_entries > 0) 
{
    assert(r->options.comparator->Compare(key, Slice(r->last_key))> 0);
}

if (r->pending_index_entry) 
{
     assert(r->data_block.empty());
     r->options.comparator->FindShortestSeparator(&r->last_key,key);
     std::string handle_encoding;
     r->pending_handle.EncodeTo(&handle_encoding);
     r->index_block.Add(r->last_key, Slice(handle_encoding));
     r->pending_index_entry =false;
}

if (r->filter_block != NULL) 
{
    r->filter_block->AddKey(key);
}

r->last_key.assign(key.data(), key.size());
r->num_entries++;
r->data_block.Add(key,value);

const size_testimated_block_size = r->data_block.CurrentSizeEstimate();
if (estimated_block_size >=r->options.block_size)  Flush();

Rep* r = rep_;
assert(!r->closed);               // 首先保证未关闭，且状态ok
if (!ok()) return;
if (r->data_block.empty())return; // data block是空的
// 保证pending_index_entry为false，即data block的Add已经完成
assert(!r->pending_index_entry);
// 写入data block，并设置其index entry信息—BlockHandle对象
WriteBlock(&r->data_block, &r->pending_handle);
//写入成功，则Flush文件，并设置r->pending_index_entry为true，
//以根据下一个data block的first key调整index entry的key—即r->last_key
if (ok()) 
{
     r->pending_index_entry =true;
     r->status =r->file->Flush();
}
if (r->filter_block != NULL)
{ 
     //将data block在sstable中的便宜加入到filter block中
     r->filter_block->StartBlock(r->offset); 
     // 并指明开始新的data block
}

void WriteBlock(BlockBuilder* block, BlockHandle* handle)

Rep* r = rep_;
Slice raw = block->Finish(); 
// 获得data block的序列化字符串
Slice block_contents;
CompressionType type =r->options.compression;
switch (type)
{
     case kNoCompression: block_contents= raw; break; // 不压缩
     case kSnappyCompression: 
     { 
          // snappy压缩格式
          std::string* compressed =&r->compressed_output;
          if(port::Snappy_Compress(raw.data(), raw.size(), compressed) &&
             compressed->size()< raw.size() - (raw.size() / 8u)) 
          {
                block_contents =*compressed;
          } 
          else 
          { 
                 // 如果不支持Snappy，或者压缩率低于12.5%，依然当作不压缩存储
                 block_contents = raw;
                 type = kNoCompression;
          }
          break;
     }
}  

WriteRawBlock(block_contents,type, handle);  
r->compressed_output.clear();  
block->Reset();

void WriteRawBlock(const Slice& data, CompressionType, BlockHandle*handle);

Rep* r = rep_;
handle->set_offset(r->offset); 
// 为index设置data block的handle信息
handle->set_size(block_contents.size());
r->status =r->file->Append(block_contents); // 写入data block内容
if (r->status.ok()) 
{
     // 写入1byte的type和4bytes的crc32
     chartrailer[kBlockTrailerSize];
     trailer[0] = type;
     uint32_t crc = crc32c::Value(block_contents.data(),
                                  block_contents.size());
     crc = crc32c::Extend(crc, trailer, 1);  // Extend crc tocover block type
     EncodeFixed32(trailer+1, crc32c::Mask(crc));
     r->status =r->file->Append(Slice(trailer, kBlockTrailerSize));
     if (r->status.ok()) 
     { 
          // 写入成功更新offset-下一个data block的写入偏移
          r->offset +=block_contents.size() + kBlockTrailerSize;
      }
}

1  Rep* r = rep_;
2  Flush();
3  assert(!r->closed);
4  r->closed = true;
5  BlockHandle filter_block_handle,metaindex_block_handle, index_block_handle;

if (ok() &&r->filter_block != NULL) 
{
     WriteRawBlock(r->filter_block->Finish(), kNoCompression,&filter_block_handle);
}

if (ok()) 
{
     BlockBuildermeta_index_block(&r->options);
     if (r->filter_block !=NULL) 
     {
          //加入从"filter.Name"到filter data位置的映射
          std::string key ="filter.";
          key.append(r->options.filter_policy->Name());
          std::string handle_encoding;
          filter_block_handle.EncodeTo(&handle_encoding);
          meta_index_block.Add(key,handle_encoding);
     }
     // TODO(postrelease): Add stats and other metablocks
     WriteBlock(&meta_index_block, &metaindex_block_handle);
}

if (ok()) 
{
     if (r->pending_index_entry)
     { 
          // Flush时会被设置为true
          r->options.comparator->FindShortSuccessor(&r->last_key);
          std::string handle_encoding;
          r->pending_handle.EncodeTo(&handle_encoding);
          r->index_block.Add(r->last_key, Slice(handle_encoding)); 
          // 加入到index block中
          r->pending_index_entry =false;
      }
      WriteBlock(&r->index_block, &index_block_handle);
}

if (ok()) 
{
     Footer footer;
     footer.set_metaindex_handle(metaindex_block_handle);
     footer.set_index_handle(index_block_handle);
     std::string footer_encoding;
     footer.EncodeTo(&footer_encoding);
     r->status =r->file->Append(footer_encoding);
     if (r->status.ok()) 
     {
          r->offset +=footer_encoding.size();
     }
}