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<th width="60%" align="center">Chapter 11.
Berkeley DB Transactional Data Store Applications
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<h2 class="title" style="clear: both"><a id="transapp_faq"></a>Transaction FAQ</h2>
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<li>
<p>
<span class="bold"><strong>What should a transactional program do when an error occurs?</strong></span>
</p>
<p>
Any time an error occurs, such that a transactionally protected
set of operations cannot complete successfully, the transaction
must be aborted. While deadlock is by far the most common of
these errors, there are other possibilities; for example,
running out of disk space for the filesystem. In Berkeley DB
transactional applications, there are three classes of error
returns: "expected" errors, "unexpected but recoverable"
errors, and a single "unrecoverable" error. Expected errors
are errors like
<a class="link" href="program_errorret.html#program_errorret.DB_NOTFOUND">DB_NOTFOUND</a>,
which indicates that a searched-for key item is not present in
the database. Applications may want to explicitly test for and
handle this error, or, in the case where the absence of a key
implies the enclosing transaction should fail, simply call
<a href="../api_reference/C/txnabort.html" class="olink">DB_TXN->abort()</a>. Unexpected but recoverable errors are errors like
<a class="link" href="program_errorret.html#program_errorret.DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a>,
which indicates that an operation has been selected to resolve
a deadlock, or a system error such as EIO, which likely
indicates that the filesystem has no available disk space.
Applications must immediately call <a href="../api_reference/C/txnabort.html" class="olink">DB_TXN->abort()</a> when these
returns occur, as it is not possible to proceed otherwise. The
only unrecoverable error is
<a class="link" href="program_errorret.html#program_errorret.DB_RUNRECOVERY">DB_RUNRECOVERY</a>,
which indicates that the system must stop and recovery must be
run.
</p>
</li>
<li>
<p>
<span class="bold"><strong>How can hot backups work? Can't you get an
inconsistent picture of the database when you copy
it?</strong></span>
</p>
<p>
First, Berkeley DB is based on the technique of "write-ahead
logging", which means that before any change is made to a
database, a log record is written that describes the change.
Further, Berkeley DB guarantees that the log record that
describes the change will always be written to stable storage
(that is, disk) before the database page where the change was
made is written to stable storage. Because of this guarantee,
we know that any change made to a database will appear either
in just a log file, or both the database and a log file, but
never in just the database.
</p>
<p>
Second, you can always create a consistent and correct database
based on the log files and the databases from a database
environment. So, during a hot backup, we first make a copy of
the databases and then a copy of the log files. The tricky
part is that there may be pages in the database that are
related for which we won't get a consistent picture during this
copy. For example, let's say that we copy pages 1-4 of the
database, and then are swapped out. For whatever reason
(perhaps because we needed to flush pages from the cache, or
because of a checkpoint), the database pages 1 and 5 are
written. Then, the hot backup process is re-scheduled, and it
copies page 5. Obviously, we have an inconsistent database
snapshot, because we have a copy of page 1 from before it was
written by the other thread of control, and a copy of page 5
after it was written by the other thread. What makes this work
is the order of operations in a hot backup. Because of the
write-ahead logging guarantees, we know that any page written
to the database will first be referenced in the log. If we
copy the database first, then we can also know that any
inconsistency in the database will be described in the log
files, and so we know that we can fix everything up during
recovery.
</p>
</li>
<li>
<p>
<span class="bold"><strong>My application has <a class="link" href="program_errorret.html#program_errorret.DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a>
errors. Is the normal, and what should I do?</strong></span>
</p>
<p>
It is quite rare for a transactional application to be deadlock
free. All applications should be prepared to handle deadlock
returns, because even if the application is deadlock free when
deployed, future changes to the application or the Berkeley DB
implementation might introduce deadlocks.
</p>
<p>
Practices which reduce the chance of deadlock include:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>Not using cursors which move backwards through the database (<a href="../api_reference/C/dbcget.html#dbcget_DB_PREV" class="olink">DB_PREV</a>),
as backward scanning cursors can deadlock with page splits;</li>
<li>Configuring <a href="../api_reference/C/dbset_flags.html#dbset_flags_DB_REVSPLITOFF" class="olink">DB_REVSPLITOFF</a> to turn off reverse splits in
applications which repeatedly delete and re-insert the same keys, to
minimize the number of page splits as keys are re-inserted;</li>
<li>Not configuring <a href="../api_reference/C/dbopen.html#dbopen_DB_READ_UNCOMMITTED" class="olink">DB_READ_UNCOMMITTED</a> as that flag requires write
transactions upgrade their locks when aborted, which can lead to deadlock.
Generally, <a href="../api_reference/C/dbcget.html#dbcget_DB_READ_COMMITTED" class="olink">DB_READ_COMMITTED</a> or non-transactional read operations
are less prone to deadlock than <a href="../api_reference/C/dbopen.html#dbopen_DB_READ_UNCOMMITTED" class="olink">DB_READ_UNCOMMITTED</a>.</li>
</ul>
</div>
<p>
</p>
</li>
<li>
<p>
<span class="bold"><strong>How can I move a database from one
transactional environment into another?</strong></span>
</p>
<p>
Because database pages contain references to log records,
databases cannot be simply moved into different database
environments. To move a database into a different environment,
dump and reload the database before moving it. If the database
is too large to dump and reload, the database may be prepared
in place using the <a href="../api_reference/C/envlsn_reset.html" class="olink">DB_ENV->lsn_reset()</a> method or the <span class="bold"><strong>-r</strong></span> argument to the <a href="../api_reference/C/db_load.html" class="olink">db_load</a> utility.
</p>
</li>
<li>
<p>
<span class="bold"><strong>I'm seeing the error "log_flush: LSN past
current end-of-log", what does that mean?</strong></span>
</p>
<p>
The most common cause of this error is that a system
administrator has removed all of the log files from a database
environment. You should shut down your database environment as
gracefully as possible, first flushing the database environment
cache to disk, if that's possible. Then, dump and reload your
databases. If the database is too large to dump and reload,
the database may be reset in place using the <a href="../api_reference/C/envlsn_reset.html" class="olink">DB_ENV->lsn_reset()</a>
method or the <span class="bold"><strong>-r</strong></span> argument to
the <a href="../api_reference/C/db_load.html" class="olink">db_load</a> utility. However, if you reset the database in place,
you should verify your databases before using them again. (It
is possible for the databases to be corrupted by running after
all of the log files have been removed, and the longer the
application runs, the worse it can get.)
</p>
</li>
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Berkeley DB Replication
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