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                # SQLite學習手冊(在線備份) 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個步驟可以應用于大多數場景,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并釋放文件共享鎖。 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中提供了一組用于在線數據庫備份的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,至于使用細節可以參考SQLite官方網站"APIs Reference"([http://www.sqlite.org/c3ref/backup_finish.html](http://www.sqlite.org/c3ref/backup_finish.html))。 1). 函數sqlite3_backup_init()用于創建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()用于數據拷貝,如果該函數的第二個參數為-1,那么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()用于釋放sqlite3_backup_init()函數申請的資源,以避免資源泄露。 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,由于sqlite3_backup_finish()函數并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): ``` 1 /* 2 This function is used to load the contents of a database file on disk 3 into the "main" database of open database connection pInMemory, or 4 to save the current contents of the database opened by pInMemory into 5 a database file on disk. pInMemory is probably an in-memory database, 6 but this function will also work fine if it is not. 7 8 Parameter zFilename points to a nul-terminated string containing the 9 name of the database file on disk to load from or save to. If parameter 10 isSave is non-zero, then the contents of the file zFilename are 11 overwritten with the contents of the database opened by pInMemory. If 12 parameter isSave is zero, then the contents of the database opened by 13 pInMemory are replaced by data loaded from the file zFilename. 14 15 If the operation is successful, SQLITE_OK is returned. Otherwise, if 16 an error occurs, an SQLite error code is returned. 17 */ 18 int loadOrSaveDb(sqlite3 *pInMemory, const char *zFilename, int isSave){ 19 int rc; /* Function return code */ 20 sqlite3 *pFile; /* Database connection opened on zFilename */ 21 sqlite3_backup *pBackup; /* Backup object used to copy data */ 22 sqlite3 *pTo; /* Database to copy to (pFile or pInMemory) */ 23 sqlite3 *pFrom; /* Database to copy from (pFile or pInMemory) */ 24 25 /* Open the database file identified by zFilename. Exit early if this fails 26 for any reason. */ 27 rc = sqlite3_open(zFilename, &pFile); 28 if( rc==SQLITE_OK ){ 29 30 /* If this is a 'load' operation (isSave==0), then data is copied 31 from the database file just opened to database pInMemory. 32 Otherwise, if this is a 'save' operation (isSave==1), then data 33 is copied from pInMemory to pFile. Set the variables pFrom and 34 pTo accordingly. */ 35 pFrom = (isSave ? pInMemory : pFile); 36 pTo = (isSave ? pFile : pInMemory); 37 38 /* Set up the backup procedure to copy from the "main" database of 39 connection pFile to the main database of connection pInMemory. 40 If something goes wrong, pBackup will be set to NULL and an error 41 code and message left in connection pTo. 42 43 If the backup object is successfully created, call backup_step() 44 to copy data from pFile to pInMemory. Then call backup_finish() 45 to release resources associated with the pBackup object. If an 46 error occurred, then an error code and message will be left in 47 connection pTo. If no error occurred, then the error code belonging 48 to pTo is set to SQLITE_OK. 49 */ 50 pBackup = sqlite3_backup_init(pTo, "main", pFrom, "main"); 51 if( pBackup ){ 52 (void)sqlite3_backup_step(pBackup, -1); 53 (void)sqlite3_backup_finish(pBackup); 54 } 55 rc = sqlite3_errcode(pTo); 56 } 57 58 /* Close the database connection opened on database file zFilename 59 and return the result of this function. */ 60 (void)sqlite3_close(pFile); 61 return rc; 62 } ``` 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,這里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()用于創建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()被調用用于拷貝數據,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,然后再重復步驟2). 4). 函數sqlite3_backup_finish()用于釋放sqlite3_backup_init()函數申請的資源,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,并且在操作時使用的也是同一個數據庫連接句柄,那么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,而后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): ``` 1 /* 2 Perform an online backup of database pDb to the database file named 3 by zFilename. This function copies 5 database pages from pDb to 4 zFilename, then unlocks pDb and sleeps for 250 ms, then repeats the 5 process until the entire database is backed up. 6 7 The third argument passed to this function must be a pointer to a progress 8 function. After each set of 5 pages is backed up, the progress function 9 is invoked with two integer parameters: the number of pages left to 10 copy, and the total number of pages in the source file. This information 11 may be used, for example, to update a GUI progress bar. 12 13 While this function is running, another thread may use the database pDb, or 14 another process may access the underlying database file via a separate 15 connection. 16 17 If the backup process is successfully completed, SQLITE_OK is returned. 18 Otherwise, if an error occurs, an SQLite error code is returned. 19 */ 20 int backupDb( 21 sqlite3 *pDb, /* Database to back up */ 22 const char *zFilename, /* Name of file to back up to */ 23 void(*xProgress)(int, int) /* Progress function to invoke */ 24 ){ 25 int rc; /* Function return code */ 26 sqlite3 *pFile; /* Database connection opened on zFilename */ 27 sqlite3_backup *pBackup; /* Backup handle used to copy data */ 28 29 /* Open the database file identified by zFilename. */ 30 rc = sqlite3_open(zFilename, &pFile); 31 if( rc==SQLITE_OK ){ 32 33 /* Open the sqlite3_backup object used to accomplish the transfer */ 34 pBackup = sqlite3_backup_init(pFile, "main", pDb, "main"); 35 if( pBackup ){ 36 37 /* Each iteration of this loop copies 5 database pages from database 38 pDb to the backup database. If the return value of backup_step() 39 indicates that there are still further pages to copy, sleep for 40 250 ms before repeating. */ 41 do { 42 rc = sqlite3_backup_step(pBackup, 5); 43 xProgress( 44 sqlite3_backup_remaining(pBackup), 45 sqlite3_backup_pagecount(pBackup) 46 ); 47 if( rc==SQLITE_OK || rc==SQLITE_BUSY || rc==SQLITE_LOCKED ){ 48 sqlite3_sleep(250); 49 } 50 } while( rc==SQLITE_OK || rc==SQLITE_BUSY || rc==SQLITE_LOCKED ); 51 52 /* Release resources allocated by backup_init(). */ 53 (void)sqlite3_backup_finish(pBackup); 54 } 55 rc = sqlite3_errcode(pFile); 56 } 57 58 /* Close the database connection opened on database file zFilename 59 and return the result of this function. */ 60 (void)sqlite3_close(pFile); 61 return rc; 62 } ```
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