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SQLite教程（九）：在線備份

更新時間：2015年05月04日 16:45:09 投稿：junjie

這篇文章主要介紹了SQLite教程（九）：在線備份,本文講解了常用備份方法、在線備份APIs簡介、高級應(yīng)用技巧等內(nèi)容,需要的朋友可以參考下

一、常用備份：

    下面的方法是比較簡單且常用的SQLite數(shù)據(jù)庫備份方式，見如下步驟：
    1). 使用SQLite API或Shell工具在源數(shù)據(jù)庫文件上加共享鎖。
    2). 使用Shell工具(cp或copy)拷貝數(shù)據(jù)庫文件到備份目錄。
    3). 解除數(shù)據(jù)庫文件上的共享鎖。
    以上3個步驟可以應(yīng)用于大多數(shù)場景，而且速度也比較快，然而卻存在一定的剛性缺陷，如：
    1). 所有打算在源數(shù)據(jù)庫上執(zhí)行寫操作的連接都不得不被掛起，直到整個拷貝過程結(jié)束并釋放文件共享鎖。
    2). 不能拷貝數(shù)據(jù)到in-memory數(shù)據(jù)庫。
    3). 在拷貝過程中，一旦備份數(shù)據(jù)庫所在的主機出現(xiàn)任何突發(fā)故障，備份數(shù)據(jù)庫可能會被破壞。
    在SQLite中提供了一組用于在線數(shù)據(jù)庫備份的APIs函數(shù)(C接口)，可以很好的解決上述方法存在的不足。通過該組函數(shù)，可以將源數(shù)據(jù)庫中的內(nèi)容拷貝到另一個數(shù)據(jù)庫，同時覆蓋目標(biāo)數(shù)據(jù)庫中的數(shù)據(jù)。整個拷貝過程可以以增量的方式完成，在此情況下，源數(shù)據(jù)庫也不需要在整個拷貝過程中都被加鎖，而只是在真正讀取數(shù)據(jù)時加共享鎖。這樣，其它的用戶在訪問源數(shù)據(jù)庫時就不會被掛起。

二、在線備份APIs簡介：

    SQLite提供了以下3個APIs函數(shù)用于完成此操作，這里僅僅給出它們的基本用法，至于使用細節(jié)可以參考SQLite官方網(wǎng)站"APIs Reference"(http://www.sqlite.org/c3ref/backup_finish.html)。
    1). 函數(shù)sqlite3_backup_init()用于創(chuàng)建sqlite3_backup對象，該對象將作為本次拷貝操作的句柄傳給其余兩個函數(shù)。
    2). 函數(shù)sqlite3_backup_step()用于數(shù)據(jù)拷貝，如果該函數(shù)的第二個參數(shù)為-1，那么整個拷貝過程都將在該函數(shù)的一次調(diào)用中完成。
    3). 函數(shù)sqlite3_backup_finish()用于釋放sqlite3_backup_init()函數(shù)申請的資源，以避免資源泄露。
    在整個拷貝過程中如果出現(xiàn)任何錯誤，我們都可以通過調(diào)用目的數(shù)據(jù)庫連接的sqlite3_errcode()函數(shù)來獲取具體的錯誤碼。此外，如果sqlite3_backup_step()調(diào)用失敗，由于sqlite3_backup_finish()函數(shù)并不會修改當(dāng)前連接的錯誤碼，因此我們可以在調(diào)用sqlite3_backup_finish()之后再獲取錯誤碼，從而在代碼中減少了一次錯誤處理。見如下代碼示例(來自SQLite官網(wǎng))：

復(fù)制代碼代碼如下:

/*
** 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、高級應(yīng)用技巧：

    在上面的例子中，我們是通過sqlite3_backup_step()函數(shù)的一次調(diào)用完成了整個拷貝過程。該實現(xiàn)方式仍然存在之前說過的掛起其它寫訪問連接的問題，為了解決該問題，這里我們將繼續(xù)介紹另外一種更高級的實現(xiàn)方式--分片拷貝，其實現(xiàn)步驟如下：
    1). 函數(shù)sqlite3_backup_init()用于創(chuàng)建sqlite3_backup對象，該對象將作為本次拷貝操作的句柄傳給其余兩個函數(shù)。
    2). 函數(shù)sqlite3_backup_step()被調(diào)用用于拷貝數(shù)據(jù)，和之前方法不同的是，該函數(shù)的第二個參數(shù)不再是-1，而是一個普通的正整數(shù)，表示每次調(diào)用將會拷貝的頁面數(shù)量，如5。
    3). 如果在函數(shù)sqlite3_backup_step()調(diào)用結(jié)束后，仍然有更多的頁面需要被拷貝，那么我們將主動休眠250ms，然后再重復(fù)步驟2).
    4). 函數(shù)sqlite3_backup_finish()用于釋放sqlite3_backup_init()函數(shù)申請的資源，以避免資源泄露。
    在上述步驟3)中我們主動休眠250ms，此期間，該拷貝操作不會在源數(shù)據(jù)庫上持有任何讀鎖，這樣其它的數(shù)據(jù)庫連接在進行寫操作時亦將不會被掛起。然而在休眠期間，如果另外一個線程或進程對源數(shù)據(jù)庫進行了寫操作，SQLite將會檢測到該事件的發(fā)生，從而在下一次調(diào)用sqlite3_backup_step()函數(shù)時重新開始整個拷貝過程。唯一的例外是，如果源數(shù)據(jù)庫不是in-memory數(shù)據(jù)庫，同時寫操作是在與拷貝操作同一個進程內(nèi)完成，并且在操作時使用的也是同一個數(shù)據(jù)庫連接句柄，那么目的數(shù)據(jù)庫中數(shù)據(jù)也將被此操作同時自動修改。在下一次調(diào)用sqlite3_backup_step()時，也將不會有任何影響發(fā)生?！　?br />     事實上，在SQLite中仍然提供了另外兩個輔助性函數(shù)backup_remaining()和backup_pagecount()，其中前者將返回在當(dāng)前備份操作中還有多少頁面需要被拷貝，而后者將返回本次操作總共需要拷貝的頁面數(shù)量。顯而易見的是，通過這兩個函數(shù)的返回結(jié)果，我們可以實時顯示本次備份操作的整體進度，計算公式如下：
    Completion = 100% * (pagecount() - remaining()) / pagecount()
    見以下代碼示例(來自SQLite官網(wǎng))：

復(fù)制代碼代碼如下:

/*
** 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;
}

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