每次我们进行Compation的时候，可能会新增sstable文件，也有可能会删除老的sstable文件，无论删除还是新增，这些变化都会生成VersionEdit对象，通过这个对象来描述sstable文件的变化。VersionEdit需要Apply到Version中，通过Version我们就可以知道当前版本下有多少sstable文件。因为Leveldb可能会存在多个版本，因此就有了VersionSet，通过链表的方式来管理多个Version。下面是一个VersionEdit如何Apply到Version的过程。

Status VersionSet::LogAndApply(VersionEdit* edit, port::Mutex* mu) {
  if (edit->has_log_number_) {
    assert(edit->log_number_ >= log_number_);
    assert(edit->log_number_ < next_file_number_);
  } else {
    edit->SetLogNumber(log_number_);
  }

  if (!edit->has_prev_log_number_) {
    edit->SetPrevLogNumber(prev_log_number_);
  }

  edit->SetNextFile(next_file_number_);
  edit->SetLastSequence(last_sequence_);

  // 1. 创建一个新的Version
  Version* v = new Version(this);
  {
    // 2. 将VersionEdit做Apply当前的Version，然后产生一个新的Version
    Builder builder(this, current_);
    builder.Apply(edit);
    builder.SaveTo(v);
  }
  // 3. 提前计算出下一次Compation需要合并的level和sstable文件
  Finalize(v);
  
  // 4. 最后将Version信息存入到Manifest文件中。
  // Initialize new descriptor log file if necessary by creating
  // a temporary file that contains a snapshot of the current version.
  std::string new_manifest_file;
  Status s;
  if (descriptor_log_ == nullptr) {
    // No reason to unlock *mu here since we only hit this path in the
    // first call to LogAndApply (when opening the database).
    assert(descriptor_file_ == nullptr);
    new_manifest_file = DescriptorFileName(dbname_, manifest_file_number_);
    edit->SetNextFile(next_file_number_);
    s = env_->NewWritableFile(new_manifest_file, &descriptor_file_);
    if (s.ok()) {
      descriptor_log_ = new log::Writer(descriptor_file_);
      s = WriteSnapshot(descriptor_log_);
    }
  }

  // Unlock during expensive MANIFEST log write
  {
    mu->Unlock();

    // Write new record to MANIFEST log
    if (s.ok()) {
      std::string record;
      edit->EncodeTo(&record);
      s = descriptor_log_->AddRecord(record);
      if (s.ok()) {
        s = descriptor_file_->Sync();
      }
      if (!s.ok()) {
        Log(options_->info_log, "MANIFEST write: %s\\n", s.ToString().c_str());
      }
    }

    // If we just created a new descriptor file, install it by writing a
    // new CURRENT file that points to it.
    if (s.ok() && !new_manifest_file.empty()) {
      s = SetCurrentFile(env_, dbname_, manifest_file_number_);
    }

    mu->Lock();
  }

  // Install the new version
  // 5. 重新设置当前Version为最新产生的Version
  if (s.ok()) {
    AppendVersion(v);
    log_number_ = edit->log_number_;
    prev_log_number_ = edit->prev_log_number_;
  } else {
    delete v;
    if (!new_manifest_file.empty()) {
      delete descriptor_log_;
      delete descriptor_file_;
      descriptor_log_ = nullptr;
      descriptor_file_ = nullptr;
      env_->RemoveFile(new_manifest_file);
    }
  }

  return s;
}