bool MemTable::Add(SequenceNumber s, ValueType type,
                   const Slice& key, /* user key */
                   const Slice& value, bool allow_concurrent,
                   MemTablePostProcessInfo* post_process_info) {
  // 一條key-value Entry的數(shù)據(jù)格式
  //  key_size     : varint32 of internal_key.size()
  //  key bytes    : char[internal_key.size()]
  //  value_size   : varint32 of value.size()
  //  value bytes  : char[value.size()]
  uint32_t key_size = static_cast<uint32_t>(key.size());
  uint32_t val_size = static_cast<uint32_t>(value.size());
  uint32_t internal_key_size = key_size + 8;
  const uint32_t encoded_len = VarintLength(internal_key_size) +
                               internal_key_size + VarintLength(val_size) +
                               val_size;
  char* buf = nullptr;
  // 通過判斷key-value的類型來選擇memtable, 范圍刪除的kv插入range_del_table_
  std::unique_ptr<MemTableRep>& table =
      type == kTypeRangeDeletion ? range_del_table_ : table_;
  KeyHandle handle = table->Allocate(encoded_len, &buf);
  //...
  // 是否允許并發(fā)插入
  if (!allow_concurrent) {
    // 是否制定了函數(shù)提取key的前綴
    if (insert_with_hint_prefix_extractor_ != nullptr &&
        insert_with_hint_prefix_extractor_->InDomain(key_slice)) {
      // ...
      bool res = table->InsertWithHint(handle, &insert_hints_[prefix]);
    } else {
      // 插入key-value pair
      bool res = table->Insert(handle);
      if (UNLIKELY(!res)) {
        return res;
      }
    }
  } else {
    // 插入key-value pair
bool res = table->InsertConcurrently(handle);
    if (UNLIKELY(!res)) {
      return res;
    }
  }
  return true;
}

bool MemTable::Get(const LookupKey& key, std::string* value, Status* s,
                   MergeContext* merge_context,
                   RangeDelAggregator* range_del_agg, SequenceNumber* seq,
                   const ReadOptions& read_opts, ReadCallback* callback,
                   bool* is_blob_index) {
  // 在range_del_table_上初始化一個迭代器
  std::unique_ptr<InternalIterator> range_del_iter(
      NewRangeTombstoneIterator(read_opts));
  Status status = range_del_agg->AddTombstones(std::move(range_del_iter));
  if (!status.ok()) {
    *s = status;
    return false;
  }
  Slice user_key = key.user_key();
  // 利用前綴提取過濾判斷key是否存在
  bool const may_contain =
      nullptr == prefix_bloom_

class InlineSkipList {
 private:
  struct Node;
  struct Splice;
 public:
  using DecodedKey = \
    typename std::remove_reference<Comparator>::type::DecodedType;
…
  Allocator* const allocator_; 
  Comparator const compare_;
  Node* const head_;
  std::atomic<int> max_height_;  
  Splice* seq_splice_;
};

template <class Comparator>
struct InlineSkipList<Comparator>::Node {
 private:
  // 存放該節(jié)點的next_節(jié)點的數(shù)組
  // 數(shù)組大小為該節(jié)點的height，當調(diào)用NewNode()分配內(nèi)存初始化整個數(shù)組
  std::atomic<Node*> next_[1];
};

bool InlineSkipList<Comparator>::Insert(const char* key, Splice* splice,
                                        bool allow_partial_splice_fix) {
  Node* x = reinterpret_cast<Node*>(const_cast<char*>(key)) - 1; // x即為next_[0]
  const DecodedKey key_decoded = compare_.decode_key(key);
  int height = x->UnstashHeight();
  assert(height >= 1 && height <= kMaxHeight_);
  int max_height = max_height_.load(std::memory_order_relaxed);
  // 更新max_height
  while (height > max_height) {
    if (max_height_.compare_exchange_weak(max_height, height)) {
      // successfully updated it
      max_height = height;
      break;
    }
    // 否則重試，可能因為其他人增加了它而退出循環(huán)
  }
  assert(max_height <= kMaxPossibleHeight);
  // 插入節(jié)點的時候，需要借助一個Splice對象，該對象主要保存著最近一次插入的節(jié)點快照
  // 它保存著一個prev和next的節(jié)點指針數(shù)組，由Level可以索引到對應Level的節(jié)點
  int recompute_height = 0;
  if (splice->height_ < max_height) {
    // 當重置splice
    splice->prev_[max_height] = head_;
    splice->next_[max_height] = nullptr;
    splice->height_ = max_height;
    recompute_height = max_height;
  } else {
    while (recompute_height < max_height) {
      if (splice->prev_[recompute_height]->Next(recompute_height) !=
          splice->next_[recompute_height]) { //判斷該層的splice是否緊密，即prev_->Next是否等于next_
        ++recompute_height;
      } else if (splice->prev_[recompute_height] != head_ &&
                 !KeyIsAfterNode(key_decoded,
                                 splice->prev_[recompute_height])) { //小于splice當前層的prev_
        // ...
      } else if (KeyIsAfterNode(key_decoded,
                                splice->next_[recompute_height])) { //大于splice當前層的prev_
        // ...
      } else {
        // 找到了合適的level
        break;
      }
    }
  }
  assert(recompute_height <= max_height);
  if (recompute_height > 0) {//計算splice
    RecomputeSpliceLevels(key_decoded, splice, recompute_height); // 找到要插入的key合適的splice
  }
  bool splice_is_valid = true;
  if (UseCAS) {//CAS無鎖機制
    //...
  } else {
    for (int i = 0; i < height; ++i) {
      if (i >= recompute_height &&
          splice->prev_[i]->Next(i) != splice->next_[i]) { // 確保splice此Level有效，如果無效的話再查找一次
        FindSpliceForLevel<false>(key_decoded, splice->prev_[i], nullptr, i,
                                  &splice->prev_[i], &splice->next_[i]);
      }
      // Checking for duplicate keys on the level 0 is sufficient
      if (UNLIKELY(i == 0 && splice->next_[i] != nullptr &&
                   compare_(x->Key(), splice->next_[i]->Key()) >= 0)) {
        // duplicate key
        return false;
      }
      if (UNLIKELY(i == 0 && splice->prev_[i] != head_ &&
                   compare_(splice->prev_[i]->Key(), x->Key()) >= 0)) {
        // duplicate key
        return false;
      }
      //…
      x->NoBarrier_SetNext(i, splice->next_[i]); //將新節(jié)點next指向?qū)膎ext節(jié)點
      splice->prev_[i]->SetNext(i, x); //將splice的prev節(jié)點的next指向新節(jié)點
    }
  }
  if (splice_is_valid) {//將新節(jié)點Height下的prev節(jié)點都設置為該節(jié)點，因為原先的prev和next之間已經(jīng)不連續(xù)了。
    for (int i = 0; i < height; ++i) {
      splice->prev_[i] = x;
    }
    //...
  } else {
    splice->height_ = 0;
  }
  return true;
}

InlineSkipList<Comparator>::FindGreaterOrEqual(const char* key) const {
  Node* x = head_;
  int level = GetMaxHeight() - 1;//從最高層開始查找
  Node* last_bigger = nullptr;
  const DecodedKey key_decoded = compare_.decode_key(key);
  while (true) {
    Node* next = x->Next(level); 
    if (next != nullptr) {
      PREFETCH(next->Next(level), 0, 1);
    }
    // Make sure the lists are sorted
    assert(x == head_ || next == nullptr || KeyIsAfterNode(next->Key(), x));
    // Make sure we haven't overshot during our search
    assert(x == head_ || KeyIsAfterNode(key_decoded, x));
    int cmp = (next == nullptr || next == last_bigger)
                  ? 1
                  : compare_(next->Key(), key_decoded);
    if (cmp == 0 || (cmp > 0 && level == 0)) { // 找到相等的key或者查找的key不在此范圍內(nèi)
      return next;
    } else if (cmp < 0) { //待查找 key 比 next 大，則在該層繼續(xù)查找
      x = next;
    } else { // 待查找 key 不大于 next，且沒到底，則繼續(xù)往下層查找
      // Switch to next list, reuse compare_() result
      last_bigger = next;
      level--;
    }
  }
}