[leveldb] 2.open操作介绍

 1 DBImpl::DBImpl(const Options& raw_options, const std::string& dbname)
 2     : env_(raw_options.env), // Env* const
 3       internal_comparator_(raw_options.comparator), // const InternalKeyComparator
 4       internal_filter_policy_(raw_options.filter_policy), // const InternalFilterPolicy
 5       options_(SanitizeOptions(dbname, &internal_comparator_, // const Options
 6                                &internal_filter_policy_, raw_options)),
 7       owns_info_log_(options_.info_log != raw_options.info_log), // bool
 8       owns_cache_(options_.block_cache != raw_options.block_cache), // bool
 9       dbname_(dbname), // const std::string
10       db_lock_(NULL), // FileLock*
11       shutting_down_(NULL), // port::AtomicPointer
12       bg_cv_(&mutex_), // port::CondVar
13       mem_(NULL), // MemTable*
14       imm_(NULL), // MemTable*
15       logfile_(NULL), // WritableFile*
16       logfile_number_(0), // uint64_t
17       log_(NULL), // log::Writer*
18       seed_(0), // uint32_t
19       tmp_batch_(new WriteBatch), // WriteBatch*
20       bg_compaction_scheduled_(false), // bool
21       manual_compaction_(NULL) { // ManualCompaction*
22   has_imm_.Release_Store(NULL);
23 
24   // Reserve ten files or so for other uses and give the rest to TableCache.
25   const int table_cache_size = options_.max_open_files - kNumNonTableCacheFiles;
26   table_cache_ = new TableCache(dbname_, &options_, table_cache_size);
27 
28   versions_ = new VersionSet(dbname_, &options_, table_cache_,
29                              &internal_comparator_);
30 }

 1 class AtomicPointer {
 2  private:
 3   void* rep_;
 4  public:
 5   AtomicPointer() { }
 6   explicit AtomicPointer(void* p) : rep_(p) {}
 7   inline void* NoBarrier_Load() const { return rep_; }
 8   inline void NoBarrier_Store(void* v) { rep_ = v; }
 9   inline void* Acquire_Load() const {
10     void* result = rep_;
11     MemoryBarrier();
12     return result;
13   }
14   inline void Release_Store(void* v) {
15     MemoryBarrier();
16     rep_ = v;
17   }
18 }; 

 1 DB::Open(const Options& options, const std::string& dbname,
 2                 DB** dbptr) { // 工厂函数
 3   *dbptr = NULL; // 设置结果默认值, 指针传值
 4 
 5   DBImpl* impl = new DBImpl(options, dbname);
 6   impl->mutex_.Lock(); // 数据恢复时上锁, 禁止所有可能的后台任务
 7   VersionEdit edit;
 8   // Recover handles create_if_missing, error_if_exists
 9   bool save_manifest = false;
10   Status s = impl->Recover(&edit, &save_manifest); // 读log恢复状态
11   if (s.ok() && impl->mem_ == NULL) {
12     // Create new log and a corresponding memtable. 复位
13     uint64_t new_log_number = impl->versions_->NewFileNumber();
14     WritableFile* lfile;
15     s = options.env->NewWritableFile(LogFileName(dbname, new_log_number),
16                                      &lfile);
17     if (s.ok()) {
18       edit.SetLogNumber(new_log_number);
19       impl->logfile_ = lfile;
20       impl->logfile_number_ = new_log_number;
21       impl->log_ = new log::Writer(lfile);
22       impl->mem_ = new MemTable(impl->internal_comparator_);
23       impl->mem_->Ref();
24     }
25   }
26   if (s.ok() && save_manifest) {
27     edit.SetPrevLogNumber(0);  // No older logs needed after recovery.
28     edit.SetLogNumber(impl->logfile_number_);
29     s = impl->versions_->LogAndApply(&edit, &impl->mutex_); // 同步VersionEdit到MANIFEST文件
30   }
31   if (s.ok()) {
32     impl->DeleteObsoleteFiles(); // 清理无用文件
33     impl->MaybeScheduleCompaction(); // 有写入就有可能要compact
34   }
35   impl->mutex_.Unlock(); // 初始化完毕
36   if (s.ok()) {
37     assert(impl->mem_ != NULL);
38     *dbptr = impl;
39   } else {
40     delete impl;
41   }
42   return s;
43 }