sghemawat

    bool CAS(db, key, oldvalue, newvalue) {
      lock some mutex;
      read key's value from db;
      bool result = (value == oldvalue);
      if (result) write key=>newvalue to db;
      unlock;
      return result;
    }

  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.  A DB write
    // with sync==true has similar crash semantics to a "write()"
    // system call followed by "fsync()".
    //
    // Default: false
    bool sync;