SQLite学习手册(SQLite在线备份)

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 */

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 sqlite3_backup object used to accomplish the transfer */
pBackup = sqlite3_backup_init(pFile, “main”, pDb, “main”); if( pBackup
){

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):

一、常用备份:

/* 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;}

/* Open the database file identified by zFilename. Exit early if this
fails ** for any reason. */ rc = sqlite3_open(zFilename, &pFile);
if( rc==SQLITE_OK ){

/* Close the database connection opened on database file zFilename **
and return the result of this function. */ (void)sqlite3_close(pFile);
return rc;}

/* 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); }

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:

/* 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);

/* Open the database file identified by zFilename. */ rc =
sqlite3_open(zFilename, &pFile); if( rc==SQLITE_OK ){

/* 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 );

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”(_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):

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