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<h1>Using the SQLite Online Backup API</h1>
<p>
Historically, backups (copies) of SQLite databases have been created
using the following method:
<ol>
<li> Establish a shared lock on the database file using the SQLite API (i.e.
the shell tool).
<li> Copy the database file using an external tool (for example the unix 'cp'
utility or the DOS 'copy' command).
<li> Relinquish the shared lock on the database file obtained in step 1.
</ol>
<p>
This procedure works well in many scenarios and is usually very
fast. However, this technique has the following shortcomings:
<ul>
<li> Any database clients wishing to write to the database file while a
backup is being created must wait until the shared lock is
relinquished.
<li> It cannot be used to copy data to or from in-memory databases.
<li> If a power failure or operating system failure occurs while copying
the database file the backup database may be corrupted following
system recovery.
</ul>
<p>
The <a href="c3ref/backup_finish.html#sqlite3backupinit">Online Backup API</a> was created to
address these concerns. The online backup API allows the contents of
one database to be copied into another database, overwriting the
original contents of the target database. The copy operation may be
done incrementally, in which case the source database does not need
to be locked for the duration of the copy, only for the brief periods
of time when it is actually being read from. This allows other database
users to continue uninterrupted while a backup of an online database is
made.
<p>
The online backup API is <a href="c3ref/backup_finish.html#sqlite3backupinit">documented here</a>.
The remainder of this page contains two C language examples illustrating
common uses of the API and discussions thereof. Reading these examples
is no substitute for reading the API documentation!
<h2>Example 1: Loading and Saving In-Memory Databases</h2>
<pre>
<span style="color:blue;font-style:italic">/*</span>
<span style="color:blue;font-style:italic">** This function is used to load the contents of a database file on disk </span>
<span style="color:blue;font-style:italic">** into the "main" database of open database connection pInMemory, or</span>
<span style="color:blue;font-style:italic">** to save the current contents of the database opened by pInMemory into</span>
<span style="color:blue;font-style:italic">** a database file on disk. pInMemory is probably an in-memory database, </span>
<span style="color:blue;font-style:italic">** but this function will also work fine if it is not.</span>
<span style="color:blue;font-style:italic">**</span>
<span style="color:blue;font-style:italic">** Parameter zFilename points to a nul-terminated string containing the</span>
<span style="color:blue;font-style:italic">** name of the database file on disk to load from or save to. If parameter</span>
<span style="color:blue;font-style:italic">** isSave is non-zero, then the contents of the file zFilename are </span>
<span style="color:blue;font-style:italic">** overwritten with the contents of the database opened by pInMemory. If</span>
<span style="color:blue;font-style:italic">** parameter isSave is zero, then the contents of the database opened by</span>
<span style="color:blue;font-style:italic">** pInMemory are replaced by data loaded from the file zFilename.</span>
<span style="color:blue;font-style:italic">**</span>
<span style="color:blue;font-style:italic">** If the operation is successful, SQLITE_OK is returned. Otherwise, if</span>
<span style="color:blue;font-style:italic">** an error occurs, an SQLite error code is returned.</span>
<span style="color:blue;font-style:italic">*/</span>
int loadOrSaveDb(<a href="c3ref/sqlite3.html">sqlite3</a> *pInMemory, const char *zFilename, int isSave){
int rc; <span style="color:blue;font-style:italic">/* Function return code */</span>
<a href="c3ref/sqlite3.html">sqlite3</a> *pFile; <span style="color:blue;font-style:italic">/* Database connection opened on zFilename */</span>
<a href="c3ref/backup.html">sqlite3_backup</a> *pBackup; <span style="color:blue;font-style:italic">/* Backup object used to copy data */</span>
<a href="c3ref/sqlite3.html">sqlite3</a> *pTo; <span style="color:blue;font-style:italic">/* Database to copy to (pFile or pInMemory) */</span>
<a href="c3ref/sqlite3.html">sqlite3</a> *pFrom; <span style="color:blue;font-style:italic">/* Database to copy from (pFile or pInMemory) */</span>
<span style="color:blue;font-style:italic">/* Open the database file identified by zFilename. Exit early if this fails</span>
<span style="color:blue;font-style:italic">** for any reason. */</span>
rc = <a href="c3ref/open.html">sqlite3_open</a>(zFilename, &pFile);
if( rc==SQLITE_OK ){
<span style="color:blue;font-style:italic">/* If this is a 'load' operation (isSave==0), then data is copied</span>
<span style="color:blue;font-style:italic">** from the database file just opened to database pInMemory. </span>
<span style="color:blue;font-style:italic">** Otherwise, if this is a 'save' operation (isSave==1), then data</span>
<span style="color:blue;font-style:italic">** is copied from pInMemory to pFile. Set the variables pFrom and</span>
<span style="color:blue;font-style:italic">** pTo accordingly. */</span>
pFrom = (isSave ? pInMemory : pFile);
pTo = (isSave ? pFile : pInMemory);
<span style="color:blue;font-style:italic">/* Set up the backup procedure to copy from the "main" database of </span>
<span style="color:blue;font-style:italic">** connection pFile to the main database of connection pInMemory.</span>
<span style="color:blue;font-style:italic">** If something goes wrong, pBackup will be set to NULL and an error</span>
<span style="color:blue;font-style:italic">** code and message left in connection pTo.</span>
<span style="color:blue;font-style:italic">**</span>
<span style="color:blue;font-style:italic">** If the backup object is successfully created, call backup_step()</span>
<span style="color:blue;font-style:italic">** to copy data from pFile to pInMemory. Then call backup_finish()</span>
<span style="color:blue;font-style:italic">** to release resources associated with the pBackup object. If an</span>
<span style="color:blue;font-style:italic">** error occurred, then an error code and message will be left in</span>
<span style="color:blue;font-style:italic">** connection pTo. If no error occurred, then the error code belonging</span>
<span style="color:blue;font-style:italic">** to pTo is set to SQLITE_OK.</span>
<span style="color:blue;font-style:italic">*/</span>
pBackup = <a href="c3ref/backup_finish.html#sqlite3backupinit">sqlite3_backup_init</a>(pTo, "main", pFrom, "main");
if( pBackup ){
(void)<a href="c3ref/backup_finish.html#sqlite3backupstep">sqlite3_backup_step</a>(pBackup, -1);
(void)<a href="c3ref/backup_finish.html#sqlite3backupfinish">sqlite3_backup_finish</a>(pBackup);
}
rc = <a href="c3ref/errcode.html">sqlite3_errcode</a>(pTo);
}
<span style="color:blue;font-style:italic">/* Close the database connection opened on database file zFilename</span>
<span style="color:blue;font-style:italic">** and return the result of this function. */</span>
(void)<a href="c3ref/close.html">sqlite3_close</a>(pFile);
return rc;
}
</pre>
<p>
The C function to the right demonstrates of one of the simplest,
and most common, uses of the backup API: loading and saving the contents
of an in-memory database to a file on disk. The backup API is used as
follows in this example:
<ol>
<li>Function <a href="c3ref/backup_finish.html#sqlite3backupinit">sqlite3_backup_init()</a> is called to create an <a href="c3ref/backup.html">sqlite3_backup</a>
object to copy data between the two databases (either from a file and
into the in-memory database, or vice-versa).
<li>Function <a href="c3ref/backup_finish.html#sqlite3backupstep">sqlite3_backup_step()</a> is called with a parameter of
<tt>-1</tt> to copy the entire source database to the destination.
<li>Function <a href="c3ref/backup_finish.html#sqlite3backupfinish">sqlite3_backup_finish()</a> is called to clean up resources
allocated by <a href="c3ref/backup_finish.html#sqlite3backupinit">sqlite3_backup_init()</a>.
</ol>
<p><b>Error handling</b>
<p>
If an error occurs in any of the three main backup API routines
then the <a href="c3ref/c_abort.html">error code</a> and <a href="c3ref/errcode.html">message</a> are attached to
the destination <a href="c3ref/sqlite3.html">database connection</a>.
Additionally, if
<a href="c3ref/backup_finish.html#sqlite3backupstep">sqlite3_backup_step()</a> encounters an error, then the <a href="c3ref/c_abort.html">error code</a> is returned
by both the <a href="c3ref/backup_finish.html#sqlite3backupstep">sqlite3_backup_step()</a> call itself, and by the subsequent call
to <a href="c3ref/backup_finish.html#sqlite3backupfinish">sqlite3_backup_finish()</a>. So a call to <a href="c3ref/backup_finish.html#sqlite3backupfinish">sqlite3_backup_finish()</a>
does not overwrite an <a href="c3ref/c_abort.html">error code</a> stored in the destination
<a href="c3ref/sqlite3.html">database connection</a> by <a href="c3ref/backup_finish.html#sqlite3backupstep">sqlite3_backup_step()</a>. This feature
is used in the example code to reduce amount of error handling required.
The return values of the <a href="c3ref/backup_finish.html#sqlite3backupstep">sqlite3_backup_step()</a> and <a href="c3ref/backup_finish.html#sqlite3backupfinish">sqlite3_backup_finish()</a>
calls are ignored and the error code indicating the success or failure of
the copy operation collected from the destination <a href="c3ref/sqlite3.html">database connection</a>
afterward.
<p><b>Possible Enhancements</b>
<p>
The implementation of this function could be enhanced in at least two ways:
<ol>
<li> Failing to obtain the lock on database file zFilename (an <a href="c3ref/c_abort.html">SQLITE_BUSY</a>
error) could be handled, and
<li> Cases where the page-sizes of database pInMemory and zFilename are
different could be handled better.
</ol>
<p>
Since database zFilename is a file on disk, then it may be accessed
externally by another process. This means that when the call to
sqlite3_backup_step() attempts to read from or write data to it, it may
fail to obtain the required file lock. If this happens, this implementation
will fail, returning SQLITE_BUSY immediately. The solution would be to
register a busy-handler callback or
timeout with <a href="c3ref/sqlite3.html">database connection</a> pFile
using <a href="c3ref/busy_handler.html">sqlite3_busy_handler()</a> or <a href="c3ref/busy_timeout.html">sqlite3_busy_timeout()</a>
as soon as it is opened. If it fails to obtain a required lock immediately,
<a href="c3ref/backup_finish.html#sqlite3backupstep">sqlite3_backup_step()</a> uses any registered busy-handler callback or timeout
in the same way as <a href="c3ref/step.html">sqlite3_step()</a> or <a href="c3ref/exec.html">sqlite3_exec()</a> does.
<p>
Usually, it does not matter if the page-sizes of the source database and the
destination database are different before the contents of the destination
are overwritten. The page-size of the destination database is simply changed
as part of the backup operation. The exception is if the destination database
happens to be an in-memory database. In this case, if the page sizes
are not the same at the start of the backup operation, then the operation
fails with an SQLITE_READONLY error. Unfortunately, this could occur when
loading a database image from a file into an in-memory database using
function loadOrSaveDb().
<p>
However, if in-memory database pInMemory has just been opened (and is
therefore completely empty) before being passed to function loadOrSaveDb(),
then it is still possible to change its page size using an SQLite "PRAGMA
page_size" command. Function loadOrSaveDb() could detect this case, and
attempt to set the page-size of the in-memory database to the page-size
of database zFilename before invoking the online backup API functions.
<h2>Example 2: Online Backup of a Running Database</h2>
<pre>
<span style="color:blue;font-style:italic">/*</span>
<span style="color:blue;font-style:italic">** Perform an online backup of database pDb to the database file named</span>
<span style="color:blue;font-style:italic">** by zFilename. This function copies 5 database pages from pDb to</span>
<span style="color:blue;font-style:italic">** zFilename, then unlocks pDb and sleeps for 250 ms, then repeats the</span>
<span style="color:blue;font-style:italic">** process until the entire database is backed up.</span>
<span style="color:blue;font-style:italic">** </span>
<span style="color:blue;font-style:italic">** The third argument passed to this function must be a pointer to a progress</span>
<span style="color:blue;font-style:italic">** function. After each set of 5 pages is backed up, the progress function</span>
<span style="color:blue;font-style:italic">** is invoked with two integer parameters: the number of pages left to</span>
<span style="color:blue;font-style:italic">** copy, and the total number of pages in the source file. This information</span>
<span style="color:blue;font-style:italic">** may be used, for example, to update a GUI progress bar.</span>
<span style="color:blue;font-style:italic">**</span>
<span style="color:blue;font-style:italic">** While this function is running, another thread may use the database pDb, or</span>
<span style="color:blue;font-style:italic">** another process may access the underlying database file via a separate </span>
<span style="color:blue;font-style:italic">** connection.</span>
<span style="color:blue;font-style:italic">**</span>
<span style="color:blue;font-style:italic">** If the backup process is successfully completed, SQLITE_OK is returned.</span>
<span style="color:blue;font-style:italic">** Otherwise, if an error occurs, an SQLite error code is returned.</span>
<span style="color:blue;font-style:italic">*/</span>
int backupDb(
<a href="c3ref/sqlite3.html">sqlite3</a> *pDb, <span style="color:blue;font-style:italic">/* Database to back up */</span>
const char *zFilename, <span style="color:blue;font-style:italic">/* Name of file to back up to */</span>
void(*xProgress)(int, int) <span style="color:blue;font-style:italic">/* Progress function to invoke */ </span>
){
int rc; <span style="color:blue;font-style:italic">/* Function return code */</span>
<a href="c3ref/sqlite3.html">sqlite3</a> *pFile; <span style="color:blue;font-style:italic">/* Database connection opened on zFilename */</span>
<a href="c3ref/backup.html">sqlite3_backup</a> *pBackup; <span style="color:blue;font-style:italic">/* Backup handle used to copy data */</span>
<span style="color:blue;font-style:italic">/* Open the database file identified by zFilename. */</span>
rc = <a href="c3ref/open.html">sqlite3_open</a>(zFilename, &pFile);
if( rc==SQLITE_OK ){
<span style="color:blue;font-style:italic">/* Open the <a href="c3ref/backup.html">sqlite3_backup</a> object used to accomplish the transfer */</span>
pBackup = <a href="c3ref/backup_finish.html#sqlite3backupinit">sqlite3_backup_init</a>(pFile, "main", pDb, "main");
if( pBackup ){
<span style="color:blue;font-style:italic">/* Each iteration of this loop copies 5 database pages from database</span>
<span style="color:blue;font-style:italic">** pDb to the backup database. If the return value of backup_step()</span>
<span style="color:blue;font-style:italic">** indicates that there are still further pages to copy, sleep for</span>
<span style="color:blue;font-style:italic">** 250 ms before repeating. */</span>
do {
rc = <a href="c3ref/backup_finish.html#sqlite3backupstep">sqlite3_backup_step</a>(pBackup, 5);
xProgress(
<a href="c3ref/backup_finish.html">sqlite3_backup_remaining</a>(pBackup),
<a href="c3ref/backup_finish.html#sqlite3backuppagecount">sqlite3_backup_pagecount</a>(pBackup)
);
if( rc==SQLITE_OK || rc==SQLITE_BUSY || rc==SQLITE_LOCKED ){
<a href="c3ref/sleep.html">sqlite3_sleep</a>(250);
}
} while( rc==SQLITE_OK || rc==SQLITE_BUSY || rc==SQLITE_LOCKED );
<span style="color:blue;font-style:italic">/* Release resources allocated by backup_init(). */</span>
(void)<a href="c3ref/backup_finish.html#sqlite3backupfinish">sqlite3_backup_finish</a>(pBackup);
}
rc = <a href="c3ref/errcode.html">sqlite3_errcode</a>(pFile);
}
<span style="color:blue;font-style:italic">/* Close the database connection opened on database file zFilename</span>
<span style="color:blue;font-style:italic">** and return the result of this function. */</span>
(void)<a href="c3ref/close.html">sqlite3_close</a>(pFile);
return rc;
}
</pre>
<p>
The function presented in the previous example copies the entire source
database in one call to <a href="c3ref/backup_finish.html#sqlite3backupstep">sqlite3_backup_step()</a>. This requires holding a
read-lock on the source database file for the duration of the operation,
preventing any other database user from writing to the database. It also
holds the mutex associated with database pInMemory throughout the copy,
preventing any other thread from using it. The C function in this section,
designed to be called by a background thread or process for creating a
backup of an online database, avoids these problems using the following
approach:
<ol>
<li>Function <a href="c3ref/backup_finish.html#sqlite3backupinit">sqlite3_backup_init()</a> is called to create an <a href="c3ref/backup.html">sqlite3_backup</a>
object to copy data from database pDb to the backup database file
identified by zFilename.
<li>Function <a href="c3ref/backup_finish.html#sqlite3backupstep">sqlite3_backup_step()</a> is called with a parameter of 5 to
copy 5 pages of database pDb to the backup database (file zFilename).
<li>If there are still more pages to copy from database pDb, then the
function sleeps for 250 milliseconds (using the <a href="c3ref/sleep.html">sqlite3_sleep()</a>
utility) and then returns to step 2.
<li>Function <a href="c3ref/backup_finish.html#sqlite3backupfinish">sqlite3_backup_finish()</a> is called to clean up resources
allocated by <a href="c3ref/backup_finish.html#sqlite3backupinit">sqlite3_backup_init()</a>.
</ol>
<p><b>File and Database Connection Locking</b>
<p>
During the 250 ms sleep in step 3 above, no read-lock is held on the database
file and the mutex associated with pDb is not held. This allows other threads
to use <a href="c3ref/sqlite3.html">database connection</a> pDb and other connections to write to the
underlying database file.
<p>
If another thread or process writes to the source database while this
function is sleeping, then SQLite detects this and usually restarts the
backup process when sqlite3_backup_step() is next called. There is one
exception to this rule: If the source database is not an in-memory database,
and the write is performed from within the same process as the backup
operation and uses the same database handle (pDb), then the destination
database (the one opened using connection pFile) is automatically updated
along with the source. The backup process may then be continued after the
xSleep() call returns as if nothing had happened.
<p>
Whether or not the backup process is restarted as a result of writes to
the source database mid-backup, the user can be sure that when the backup
operation is completed the backup database contains a consistent and
up-to-date snapshot of the original. However:
<ul>
<li> Writes to an in-memory source database, or writes to a file-based
source database by an external process or thread using a
database connection other than pDb are significantly more expensive
than writes made to a file-based source database using pDb (as the
entire backup operation must be restarted in the former two cases).
<li> If the backup process is restarted frequently enough it may never
run to completion and the backupDb() function may never return.
</ul>
<p><b>backup_remaining() and backup_pagecount()</b>
<p>
The backupDb() function uses the sqlite3_backup_remaining() and
sqlite3_backup_pagecount() functions to report its progress via the
user-supplied xProgress() callback. Function sqlite3_backup_remaining()
returns the number of pages left to copy and sqlite3_backup_pagecount()
returns the total number of pages in the source database (in this case
the database opened by pDb). So the percentage completion of the process
may be calculated as:
<p style="font-family:fixed;margin-left:5em">
Completion = 100% * (pagecount() - remaining()) / pagecount()
<p>
The sqlite3_backup_remaining() and sqlite3_backup_pagecount() APIs report
values stored by the previous call to sqlite3_backup_step(), they do not
actually inspect the source database file. This means that if the source
database is written to by another thread or process after the call to
sqlite3_backup_step() returns but before the values returned by
sqlite3_backup_remaining() and sqlite3_backup_pagecount() are used, the
values may be technically incorrect. This is not usually a problem.
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