Get
LevelDB提供了Get接口用于给定key的查找：
Status DBImpl::Get(const ReadOptions &options,
const Slice &key,
std::string *value)Get操作可以指定在某个snapshot的情况下进行，如果指定了snapshot，则获取该snapshot的sequencenumber，如果没有指定snapshot，就取当前最新的sequencenumber：
Status s;
MutexLock l(&mutex_);
SequenceNumber snapshot;
if (options.snapshot != nullptr)
{
snapshot =
static_cast<const SnapshotImpl *>(options.snapshot)->sequence_number();
}
else
{
snapshot = versions_->LastSequence();
}
MemTable *mem = mem_;
MemTable *imm = imm_;
Version *current = versions_->current();
mem->Ref();
if (imm != nullptr)
imm->Ref();
current->Ref();首先在memtable里找，如果找到了就结束查找，然后再immutable memtable里找（如果immutable memtable存在），如果找到了就结束查找，在这两个地方查找使用的都是MemTable类提供的Get接口函数（在这里有分析https://www.cnblogs.com/YuNanlong/p/9426795.html）。最后使用Version类提供的Get接口函数在sstable中查找：
// Unlock while reading from files and memtables
{
mutex_.Unlock();
// First look in the memtable, then in the immutable memtable (if any).
LookupKey lkey(key, snapshot);
if (mem->Get(lkey, value, &s))
{
// Done
}
else if (imm != nullptr && imm->Get(lkey, value, &s))
{
// Done
}
else
{
s = current->Get(options, lkey, value, &stats);
have_stat_update = true;
}
mutex_.Lock();
}如果在sstable中查找了，会更新查找涉及到的sstable的seek次数，可能会触发compact条件，因此需要调用MaybeScheduleCompaction函数进行可能的compact操作（在这里有分析https://www.cnblogs.com/YuNanlong/p/9440548.html）：
if (have_stat_update && current->UpdateStats(stats))
{
MaybeScheduleCompaction();
}
mem->Unref();
if (imm != nullptr)
imm->Unref();
current->Unref();
return s;接下来分析Version类封装的Get函数：
Status Version::Get(const ReadOptions &options,
const LookupKey &k,
std::string *value,
GetStats *stats)首先是一些变量必要的初始化：
Slice ikey = k.internal_key();
Slice user_key = k.user_key();
const Comparator *ucmp = vset_->icmp_.user_comparator();
Status s;
stats->seek_file = nullptr;
stats->seek_file_level = -1;
FileMetaData *last_file_read = nullptr;
int last_file_read_level = -1;
// We can search level-by-level since entries never hop across
// levels. Therefore we are guaranteed that if we find data
// in an smaller level, later levels are irrelevant.
std::vector<FileMetaData *> tmp;
FileMetaData *tmp2;在每一层中搜索：
for (int level = 0; level < config::kNumLevels; level++)
{如果该level没有文件则直接跳过：
size_t num_files = files_[level].size();
if (num_files == 0)
continue;如果当前位于level0，将所有可能包含key的文件都加入files中：
// Get the list of files to search in this level
FileMetaData *const *files = &files_[level][0];
if (level == 0)
{
// Level-0 files may overlap each other. Find all files that
// overlap user_key and process them in order from newest to oldest.
tmp.reserve(num_files);
for (uint32_t i = 0; i < num_files; i++)
{
FileMetaData *f = files[i];
if (ucmp->Compare(user_key, f->smallest.user_key()) >= 0 &&
ucmp->Compare(user_key, f->largest.user_key()) <= 0)
{
tmp.push_back(f);
}
}
if (tmp.empty())
continue;
std::sort(tmp.begin(), tmp.end(), NewestFirst);
files = &tmp[0];
num_files = tmp.size();
}如果当前不是level0，则调用FindFile进行二分查找，找到file后验证要找的key是不是在file中，如果是，加入files：
else
{
// Binary search to find earliest index whose largest key >= ikey.
uint32_t index = FindFile(vset_->icmp_, files_[level], ikey);
if (index >= num_files)
{
files = nullptr;
num_files = 0;
}
else
{
tmp2 = files[index];
if (ucmp->Compare(user_key, tmp2->smallest.user_key()) < 0)
{
// All of "tmp2" is past any data for user_key
files = nullptr;
num_files = 0;
}
else
{
files = &tmp2;
num_files = 1;
}
}
}遍历找到的files，如果seek的文件不止一个，则记录下第一个seek的文件，之后要将这个文件的seek减一（调用UpdateStats函数）：
扫描二维码关注公众号，回复：
2624689 查看本文章
for (uint32_t i = 0; i < num_files; ++i)
{
if (last_file_read != nullptr && stats->seek_file == nullptr)
{
// We have had more than one seek for this read. Charge the 1st file.
stats->seek_file = last_file_read;
stats->seek_file_level = last_file_read_level;
}
FileMetaData *f = files[i];
last_file_read = f;
last_file_read_level = level;调用table_cache_->Get函数在文件中搜索key值，如果没有找到，则继续搜索下一个file，如果找到了，不论是删除的还是过期的，都返回（因为之后就算找到了key，也比现在的key旧，被现在的key覆盖）：
Saver saver;
saver.state = kNotFound;
saver.ucmp = ucmp;
saver.user_key = user_key;
saver.value = value;
s = vset_->table_cache_->Get(options, f->number, f->file_size,
ikey, &saver, SaveValue);
if (!s.ok())
{
return s;
}
switch (saver.state)
{
case kNotFound:
break; // Keep searching in other files
case kFound:
return s;
case kDeleted:
s = Status::NotFound(Slice()); // Use empty error message for speed
return s;
case kCorrupt:
s = Status::Corruption("corrupted key for ", user_key);
return s;
}
}230 Love u