LevelDB源码分析之从Put说起

之前分享的文章leveldb实现原理分析详细描述了LevelDB的实现原理，本文从Put接口来看下leveldb数据写流程的源码实现。

LevelDB的读写接口在类DB中定义（leveldb/db.h），接口声明如下：

// Set the database entry for "key" to "value".  Returns OK on success,
  // and a non-OK status on error.
  // Note: consider setting options.sync = true.
  virtual Status Put(const WriteOptions& options,
                     const Slice& key,
                     const Slice& value) = 0;

  // Remove the database entry (if any) for "key".  Returns OK on
  // success, and a non-OK status on error.  It is not an error if "key"
  // did not exist in the database.
  // Note: consider setting options.sync = true.
  virtual Status Delete(const WriteOptions& options, const Slice& key) = 0;

  // Apply the specified updates to the database.
  // Returns OK on success, non-OK on failure.
  // Note: consider setting options.sync = true.
  virtual Status Write(const WriteOptions& options, WriteBatch* updates) = 0;

  // If the database contains an entry for "key" store the
  // corresponding value in *value and return OK.
  //
  // If there is no entry for "key" leave *value unchanged and return
  // a status for which Status::IsNotFound() returns true.
  //
  // May return some other Status on an error.
  virtual Status Get(const ReadOptions& options,
                     const Slice& key, std::string* value) = 0;​

而接口的实现则在DBImpl类中（leveldb/db_impl.h）。这篇文章跟随Put接口，去了解leveldb的内部实现。

DBImpl类中的Put方法实现如下：

Status DBImpl::Put(const WriteOptions& o, const Slice& key, const Slice& val) {
  return DB::Put(o, key, val);
}

它什么也没有做只是调用了父类DB的Put实现，DB::Put的实现如下：

Status DB::Put(const WriteOptions& opt, const Slice& key, const Slice& value) {
  WriteBatch batch;
  batch.Put(key, value);
  return Write(opt, &batch);
}

父类的实现也很简单，就是将待写入的key-value放入WriteBatch对象中，然后调用了Write方法。在继续看Write方法实现之前，我们先看下这个方法用到的几个类：WriteOption，WriteBatch和Slice.它们的定义如下：

struct WriteOptions {
  // If true, the write will be flushed from the operating system
  // buffer cache (by calling WritableFile::Sync()) before the write
  // is considered complete.  If this flag is true, writes will be
  // slower.
  //
  // If this flag is false, and the machine crashes, some recent
  // writes may be lost.  Note that if it is just the process that
  // crashes (i.e., the machine does not reboot), no writes will be
  // lost even if sync==false.
  //
  // In other words, a DB write with sync==false has similar
  // crash semantics as the "write()" system call.