SQLite教程(九):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):
/*
** 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):
/*
** 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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