LevelDB被设计成跨平台数据库，支持在windows、linux、Android等多个操作系统。除了port文件夹中的AtomicPointer、Mutex等编程基础部件，和操作系统相关的其他操作统统被塞进了Env类中。
4.1 Env类
Env类从功能上划分包括如下几个部分：
- 文件相关操作。包括文件创建、删除文件目录，文件是否存在判定等。
- 线程相关操作。包括启动线程，执行临时任务、Sleep操作等。
- 其他操作。获取系统当前时间、创建logger文件等。
4.2 文件相关操作
文件相关操作接口定义如下：
class Env
{
......
//创建顺序读文件 -- 文件同时只能被一个线程访问
virtual Status NewSequentialFile(const std::string &fname,
SequentialFile **result) = 0;
//创建随机读文件 -- 文件同时只能被一个线程访问
virtual Status NewRandomAccessFile(const std::string &fname,
RandomAccessFile **result) = 0;
//创建写文件 -- 文件同时只能被一个线程访问
virtual Status NewWritableFile(const std::string &fname,
WritableFile **result) = 0;
//创建或打开写文件 -- 文件同时只能被一个线程访问
virtual Status NewAppendableFile(const std::string &fname,
WritableFile **result);
// Returns true iff the named file exists.
virtual bool FileExists(const std::string &fname) = 0;
// Store in *result the names of the children of the specified directory.
// The names are relative to "dir".
// Original contents of *results are dropped.
virtual Status GetChildren(const std::string &dir,
std::vector<std::string> *result) = 0;
// Delete the named file.
virtual Status DeleteFile(const std::string &fname) = 0;
// Create the specified directory.
virtual Status CreateDir(const std::string &dirname) = 0;
// Delete the specified directory.
virtual Status DeleteDir(const std::string &dirname) = 0;
// Store the size of fname in *file_size.
virtual Status GetFileSize(const std::string &fname, uint64_t *file_size) = 0;
// Rename file src to target.
virtual Status RenameFile(const std::string &src,
const std::string &target) = 0;
// Lock the specified file. Used to prevent concurrent access to
// the same db by multiple processes. On failure, stores NULL in
// *lock and returns non-OK.
//
// On success, stores a pointer to the object that represents the
// acquired lock in *lock and returns OK. The caller should call
// UnlockFile(*lock) to release the lock. If the process exits,
// the lock will be automatically released.
//
// If somebody else already holds the lock, finishes immediately
// with a failure. I.e., this call does not wait for existing locks
// to go away.
//
// May create the named file if it does not already exist.
virtual Status LockFile(const std::string &fname, FileLock **lock) = 0;
// Release the lock acquired by a previous successful call to LockFile.
// REQUIRES: lock was returned by a successful LockFile() call
// REQUIRES: lock has not already been unlocked.
virtual Status UnlockFile(FileLock *lock) = 0;
......
}
文件按操作方式分为顺序读文件(SequentialFile)、随机读文件(RandomAccessFile)及写文件(WritableFile)。
通用的文件操作包括删除文件、获取文件大小等；通用的目录操作包括创建、删除目录，获取目录的子目录或文件列表，逻辑简单此处不提。另外，有一组配对的文件锁操作LockFile、UnlockFile。
4.2.1 SequentialFile
顺序文件和随机文件实现都非常简单，二者最大的区别在于：顺序文件不提供基于offset的读操作接口。
顺序文件每次从当前位置读取指定大小的字节数，另外支持跳过中间部分内容，除此之外再无其他。
// A file abstraction for reading sequentially through a file
class SequentialFile
{
public:
SequentialFile() {}
virtual ~SequentialFile();
// Read up to "n" bytes from the file. "scratch[0..n-1]" may be
// written by this routine. Sets "*result" to the data that was
// read (including if fewer than "n" bytes were successfully read).
// May set "*result" to point at data in "scratch[0..n-1]", so
// "scratch[0..n-1]" must be live when "*result" is used.
// If an error was encountered, returns a non-OK status.
//
// REQUIRES: External synchronization
virtual Status Read(size_t n, Slice *result, char *scratch) = 0;
// Skip "n" bytes from the file. This is guaranteed to be no
// slower that reading the same data, but may be faster.
//
// If end of file is reached, skipping will stop at the end of the
// file, and Skip will return OK.
//
// REQUIRES: External synchronization
virtual Status Skip(uint64_t n) = 0;
private:
// No copying allowed
SequentialFile(const SequentialFile &);
void operator=(const SequentialFile &);
};
4.2.2 RandomAccessFile
随机读文件只有一个基于offset读取指定大小字节数的接口，定义如下：
// A file abstraction for randomly reading the contents of a file.
class RandomAccessFile
{
public:
RandomAccessFile() {}
virtual ~RandomAccessFile();
// Read up to "n" bytes from the file starting at "offset".
// "scratch[0..n-1]" may be written by this routine. Sets "*result"
// to the data that was read (including if fewer than "n" bytes were
// successfully read). May set "*result" to point at data in
// "scratch[0..n-1]", so "scratch[0..n-1]" must be live when
// "*result" is used. If an error was encountered, returns a non-OK
// status.
//
// Safe for concurrent use by multiple threads.
virtual Status Read(uint64_t offset, size_t n, Slice *result,
char *scratch) const = 0;
private:
// No copying allowed
RandomAccessFile(const RandomAccessFile &);
void operator=(const RandomAccessFile &);
};
4.2.3 WritableFile
写文件的接口定义相对“复杂”，除了追加操作外，还包括了关闭(close)、刷新(flush)、同步(sync)操作。接口定义如下：
// A file abstraction for sequential writing. The implementation
// must provide buffering since callers may append small fragments
// at a time to the file.
class WritableFile
{
public:
WritableFile() {}
virtual ~WritableFile();
virtual Status Append(const Slice &data) = 0;
virtual Status Close() = 0;
virtual Status Flush() = 0;
virtual Status Sync() = 0;
private:
// No copying allowed
WritableFile(const WritableFile &);
void operator=(const WritableFile &);
};
Flush和Sync的差异在于：Flush讲数据从用户缓冲区刷新到内核缓冲区，而Sync则会将内核缓冲区数据刷新到磁盘。
4.3 线程相关操作
接口定义如下：
// Arrange to run "(*function)(arg)" once in a background thread.
//
// "function" may run in an unspecified thread. Multiple functions
// added to the same Env may run concurrently in different threads.
// I.e., the caller may not assume that background work items are
// serialized.
virtual void Schedule(
void (*function)(void *arg),
void *arg) = 0;
// Start a new thread, invoking "function(arg)" within the new thread.
// When "function(arg)" returns, the thread will be destroyed.
virtual void StartThread(void (*function)(void *arg), void *arg) = 0;
// Sleep/delay the thread for the prescribed number of micro-seconds.
virtual void SleepForMicroseconds(int micros) = 0;
Schedule提供系统线程池，添加task后交由线程池调度，而无需用户创建线程池或者进行线程管理；StartThread负责创建单独线程。
4.4 其他操作
// *path is set to a temporary directory that can be used for testing. It may
// or many not have just been created. The directory may or may not differ
// between runs of the same process, but subsequent calls will return the
// same directory.
virtual Status GetTestDirectory(std::string *path) = 0;
// Create and return a log file for storing informational messages.
virtual Status NewLogger(const std::string &fname, Logger **result) = 0;
// Returns the number of micro-seconds since some fixed point in time. Only
// useful for computing deltas of time.
virtual uint64_t NowMicros() = 0;
4.5 总结
本节描述的是LevelDB系统构建基础，本身没有任何技术复杂度。作者设计遵循了单一职责、精简等基本设计原则，将接口做到最简，具有一定的借鉴意义。
转载请注明：【随安居士】http://www.jianshu.com/p/3c449af6fdac
