一、常用備份:
下面的方法是比較簡單且常用的SQLite數據庫備份方式,見如下步驟:
1). 使用SQLite API或Shell工具在源數據庫文件上加共享鎖。
2). 使用Shell工具(cp或copy)拷貝數據庫文件到備份目錄。
3). 解除數據庫文件上的共享鎖。
以上3個步驟可以應用於大多數場景,而且速度也比較快,然而卻存在一定的剛性缺陷,如:
1). 所有打算在源數據庫上執行寫操作的連接都不得不被掛起,直到整個拷貝過程結束並釋放文件共享鎖。
2). 不能拷貝數據到in-memory數據庫。
3). 在拷貝過程中,一旦備份數據庫所在的主機出現任何突發故障,備份數據庫可能會被破壞。
在SQLite中提供了一組用於在線數據庫備份的APIs函數(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,可以將源數據庫中的內容拷貝到另一個數據庫,同時覆蓋目標數據庫中的數據。整個拷貝過程可以以增量的方式完成,在此情況下,源數據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據時加共享鎖。這樣,其它的用戶在訪問源數據庫時就不會被掛起。
二、在線備份APIs簡介:
SQLite提供了以下3個APIs函數用於完成此操作,這裡僅僅給出它們的基本用法,至於使用細節可以參考SQLite官方網站"APIs Reference"(http://www.sqlite.org/c3ref/backup_finish.html)。
1). 函數sqlite3_backup_init()用於創建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。
2). 函數sqlite3_backup_step()用於數據拷貝,如果該函數的第二個參數為-1,那麼整個拷貝過程都將在該函數的一次調用中完成。
3). 函數sqlite3_backup_finish()用於釋放sqlite3_backup_init()函數申請的資源,以避免資源洩露。
在整個拷貝過程中如果出現任何錯誤,我們都可以通過調用目的數據庫連接的sqlite3_errcode()函數來獲取具體的錯誤碼。此外,如果sqlite3_backup_step()調用失敗,由於sqlite3_backup_finish()函數並不會修改當前連接的錯誤碼,因此我們可以在調用sqlite3_backup_finish()之後再獲取錯誤碼,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):
復制代碼 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、高級應用技巧:
在上面的例子中,我們是通過sqlite3_backup_step()函數的一次調用完成了整個拷貝過程。該實現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,這裡我們將繼續介紹另外一種更高級的實現方式--分片拷貝,其實現步驟如下:
1). 函數sqlite3_backup_init()用於創建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。
2). 函數sqlite3_backup_step()被調用用於拷貝數據,和之前方法不同的是,該函數的第二個參數不再是-1,而是一個普通的正整數,表示每次調用將會拷貝的頁面數量,如5。
3). 如果在函數sqlite3_backup_step()調用結束後,仍然有更多的頁面需要被拷貝,那麼我們將主動休眠250ms,然後再重復步驟2).
4). 函數sqlite3_backup_finish()用於釋放sqlite3_backup_init()函數申請的資源,以避免資源洩露。
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據庫上持有任何讀鎖,這樣其它的數據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據庫進行了寫操作,SQLite將會檢測到該事件的發生,從而在下一次調用sqlite3_backup_step()函數時重新開始整個拷貝過程。唯一的例外是,如果源數據庫不是in-memory數據庫,同時寫操作是在與拷貝操作同一個進程內完成,並且在操作時使用的也是同一個數據庫連接句柄,那麼目的數據庫中數據也將被此操作同時自動修改。在下一次調用sqlite3_backup_step()時,也將不會有任何影響發生。
事實上,在SQLite中仍然提供了另外兩個輔助性函數backup_remaining()和backup_pagecount(),其中前者將返回在當前備份操作中還有多少頁面需要被拷貝,而後者將返回本次操作總共需要拷貝的頁面數量。顯而易見的是,通過這兩個函數的返回結果,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):
復制代碼 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;
}