SQLite学习手册(SQLite在线备份)


    一、常用备份:
        下面的方法是比较简单且常用的SQLite数据库备份方式,见如下步骤:
        1). 使用SQLite API或Shell工具在源数据库文件上加共享锁。
        2). 使用Shell工具(cp或copy)拷贝数据库文件到备份目录。
        3). 解除数据库文件上的共享锁。
        以上3个步骤可以应用于大多数场景,而且速度也比较快,然而却存在一定的刚性缺陷,如:
        1). 所有打算在源数据库上执行写操作的连接都不得不被挂起,直到整个拷贝过程结束并释放文件共享锁。
        2). 不能拷贝数据到in-memory数据库。
        3). 在拷贝过程中,一旦备份数据库所在的主机出现任何突发故障,备份数据库可能会被破坏。
        在SQLite中提供了一组用于在线数据库备份的APIs函数(C接口),可以很好的解决上述方法存在的不足。通过该组函数,可以将源数据库中的内容拷贝到另一个数据库,同时覆盖目标数据库中的数据。整个拷贝过程可以以增量的方式完成,在此情况下,源数据库也不需要在整个拷贝过程中都被加锁,而只是在真正读取数据时加共享锁。这样,其它的用户在访问源数据库时就不会被挂起。
    二、在线备份APIs简介:
        SQLite提供了以下3个APIs函数用于完成此操作,这里仅仅给出它们的基本用法,至于使用细节可以参考SQLite官方网站"APIs Reference"(http://www.sqlite.org/c3ref/backup_finish.html)。
        1). 函数sqlite3_backup_init()用于创建sqlite3_backup对象,该对象将作为本次拷贝操作的句柄传给其余两个函数。
        2). 函数sqlite3_backup_step()用于数据拷贝,如果该函数的第二个参数为-1,那么整个拷贝过程都将在该函数的一次调用中完成。
        3). 函数sqlite3_backup_finish()用于释放sqlite3_backup_init()函数申请的资源,以避免资源泄露。
        在整个拷贝过程中如果出现任何错误,我们都可以通过调用目的数据库连接的sqlite3_errcode()函数来获取具体的错误码。此外,如果sqlite3_backup_step()调用失败,由于sqlite3_backup_finish()函数并不会修改当前连接的错误码,因此我们可以在调用sqlite3_backup_finish()之后再获取错误码,从而在代码中减少了一次错误处理。见如下代码示例(来自SQLite官网):
    复制代码 代码如下:
    /*
    ** This function is used to load the contents of a database file on disk
    ** into the "main" database of open database connection pInMemory, or
    ** to save the current contents of the database opened by pInMemory into
    ** a database file on disk. pInMemory is probably an in-memory database,
    ** but this function will also work fine if it is not.
    **
    ** Parameter zFilename points to a nul-terminated string containing the
    ** name of the database file on disk to load from or save to. If parameter
    ** isSave is non-zero, then the contents of the file zFilename are
    ** overwritten with the contents of the database opened by pInMemory. If
    ** parameter isSave is zero, then the contents of the database opened by
    ** pInMemory are replaced by data loaded from the file zFilename.
    **
    ** If the operation is successful, SQLITE_OK is returned. Otherwise, if
    ** an error occurs, an SQLite error code is returned.
    */
    int loadOrSaveDb(sqlite3 *pInMemory, const char *zFilename, int isSave){
      int rc;                   /* Function return code */
      sqlite3 *pFile;           /* Database connection opened on zFilename */
      sqlite3_backup *pBackup;  /* Backup object used to copy data */
      sqlite3 *pTo;             /* Database to copy to (pFile or pInMemory) */
      sqlite3 *pFrom;           /* Database to copy from (pFile or pInMemory) */
      /* Open the database file identified by zFilename. Exit early if this fails
      ** for any reason. */
      rc = sqlite3_open(zFilename, &pFile);
      if( rc==SQLITE_OK ){
        /* If this is a 'load' operation (isSave==0), then data is copied
        ** from the database file just opened to database pInMemory.
        ** Otherwise, if this is a 'save' operation (isSave==1), then data
        ** is copied from pInMemory to pFile.  Set the variables pFrom and
        ** pTo accordingly. */
        pFrom = (isSave ? pInMemory : pFile);
        pTo   = (isSave ? pFile     : pInMemory);
        /* Set up the backup procedure to copy from the "main" database of
        ** connection pFile to the main database of connection pInMemory.
        ** If something goes wrong, pBackup will be set to NULL and an error
        ** code and  message left in connection pTo.
        **
        ** If the backup object is successfully created, call backup_step()
        ** to copy data from pFile to pInMemory. Then call backup_finish()
        ** to release resources associated with the pBackup object.  If an
        ** error occurred, then  an error code and message will be left in
        ** connection pTo. If no error occurred, then the error code belonging
        ** to pTo is set to SQLITE_OK.
    */
        pBackup = sqlite3_backup_init(pTo, "main", pFrom, "main");
        if( pBackup ){
          (void)sqlite3_backup_step(pBackup, -1);
          (void)sqlite3_backup_finish(pBackup);
        }
        rc = sqlite3_errcode(pTo);
      }
      /* Close the database connection opened on database file zFilename
      ** and return the result of this function. */
      (void)sqlite3_close(pFile);
      return rc;
    }
    三、高级应用技巧:
        在上面的例子中,我们是通过sqlite3_backup_step()函数的一次调用完成了整个拷贝过程。该实现方式仍然存在之前说过的挂起其它写访问连接的问题,为了解决该问题,这里我们将继续介绍另外一种更高级的实现方式--分片拷贝,其实现步骤如下:
        1). 函数sqlite3_backup_init()用于创建sqlite3_backup对象,该对象将作为本次拷贝操作的句柄传给其余两个函数。
        2). 函数sqlite3_backup_step()被调用用于拷贝数据,和之前方法不同的是,该函数的第二个参数不再是-1,而是一个普通的正整数,表示每次调用将会拷贝的页面数量,如5。
        3). 如果在函数sqlite3_backup_step()调用结束后,仍然有更多的页面需要被拷贝,那么我们将主动休眠250ms,然后再重复步骤2).
        4). 函数sqlite3_backup_finish()用于释放sqlite3_backup_init()函数申请的资源,以避免资源泄露。
        在上述步骤3)中我们主动休眠250ms,此期间,该拷贝操作不会在源数据库上持有任何读锁,这样其它的数据库连接在进行写操作时亦将不会被挂起。然而在休眠期间,如果另外一个线程或进程对源数据库进行了写操作,SQLite将会检测到该事件的发生,从而在下一次调用sqlite3_backup_step()函数时重新开始整个拷贝过程。唯一的例外是,如果源数据库不是in-memory数据库,同时写操作是在与拷贝操作同一个进程内完成,并且在操作时使用的也是同一个数据库连接句柄,那么目的数据库中数据也将被此操作同时自动修改。在下一次调用sqlite3_backup_step()时,也将不会有任何影响发生。  
        事实上,在SQLite中仍然提供了另外两个辅助性函数backup_remaining()和backup_pagecount(),其中前者将返回在当前备份操作中还有多少页面需要被拷贝,而后者将返回本次操作总共需要拷贝的页面数量。显而易见的是,通过这两个函数的返回结果,我们可以实时显示本次备份操作的整体进度,计算公式如下:
        Completion = 100% * (pagecount() - remaining()) / pagecount()
        见以下代码示例(来自SQLite官网):
    复制代码 代码如下:
    /*
    ** Perform an online backup of database pDb to the database file named
    ** by zFilename. This function copies 5 database pages from pDb to
    ** zFilename, then unlocks pDb and sleeps for 250 ms, then repeats the
    ** process until the entire database is backed up.
    **
    ** The third argument passed to this function must be a pointer to a progress
    ** function. After each set of 5 pages is backed up, the progress function
    ** is invoked with two integer parameters: the number of pages left to
    ** copy, and the total number of pages in the source file. This information
    ** may be used, for example, to update a GUI progress bar.
    **
    ** While this function is running, another thread may use the database pDb, or
    ** another process may access the underlying database file via a separate
    ** connection.
    **
    ** If the backup process is successfully completed, SQLITE_OK is returned.
    ** Otherwise, if an error occurs, an SQLite error code is returned.
    */
    int backupDb(
      sqlite3 *pDb,               /* Database to back up */
      const char *zFilename,      /* Name of file to back up to */
      void(*xProgress)(int, int)  /* Progress function to invoke */    
    ){
      int rc;                     /* Function return code */
      sqlite3 *pFile;             /* Database connection opened on zFilename */
      sqlite3_backup *pBackup;    /* Backup handle used to copy data */
      /* Open the database file identified by zFilename. */
      rc = sqlite3_open(zFilename, &pFile);
      if( rc==SQLITE_OK ){
        /* Open the sqlite3_backup object used to accomplish the transfer */
        pBackup = sqlite3_backup_init(pFile, "main", pDb, "main");
        if( pBackup ){
          /* Each iteration of this loop copies 5 database pages from database
          ** pDb to the backup database. If the return value of backup_step()
          ** indicates that there are still further pages to copy, sleep for
          ** 250 ms before repeating. */
          do {
            rc = sqlite3_backup_step(pBackup, 5);
            xProgress(
                sqlite3_backup_remaining(pBackup),
                sqlite3_backup_pagecount(pBackup)
            );
            if( rc==SQLITE_OK || rc==SQLITE_BUSY || rc==SQLITE_LOCKED ){
              sqlite3_sleep(250);
            }
          } while( rc==SQLITE_OK || rc==SQLITE_BUSY || rc==SQLITE_LOCKED );
          /* Release resources allocated by backup_init(). */
          (void)sqlite3_backup_finish(pBackup);
        }
        rc = sqlite3_errcode(pFile);
      }
      /* Close the database connection opened on database file zFilename
      ** and return the result of this function. */
      (void)sqlite3_close(pFile);
      return rc;
    }