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<!--$Id: deadlock.so,v 10.15 2000/08/10 17:54:49 bostic Exp $-->
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<title>Berkeley DB Reference Guide: Deadlock detection</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Transaction Protected Applications</dl></h3></td>
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<p>
<h1 align=center>Deadlock detection</h1>
<p>The first component of the infrastructure, deadlock detection, is not
so much a requirement specific to transaction protected applications,
but rather is necessary for almost all applications where more than a
single thread of control will be accessing the database at one time.
While Berkeley DB automatically handles database locking, it is normally
possible for deadlock to occur. It is not required by all transactional
applications, but exceptions are rare.
<p>When the deadlock occurs, two (or more) threads of control each request
additional locks which can never be granted because one of the threads
of control waiting holds the requested resource.
<p>For example, consider two processes A and B. Let's say that A obtains
an exclusive lock on item X, and B obtains an exclusive lock on item Y.
Then, A requests a lock on Y and B requests a lock on X. A will wait
until resource Y becomes available and B will wait until resource X
becomes available. Unfortunately, since both A and B are waiting,
neither will release the locks they hold and neither will ever obtain
the resource on which it is waiting. In order to detect that deadlock
has happened, a separate process or thread must review the locks
currently held in the database. If deadlock has occurred, a victim must
be selected, and that victim will then return the error
<a href="../../ref/program/errorret.html#DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a> from whatever Berkeley DB call it was making.
<p>Berkeley DB provides a separate UNIX-style utility which can be used to
perform this deadlock detection, named <a href="../../utility/db_deadlock.html">db_deadlock</a>.
Alternatively, applications can create their own deadlock utility or
thread using the underlying <a href="../../api_c/lock_detect.html">lock_detect</a> function, or specify
that Berkeley DB run the deadlock detector internally whenever there is a
conflict over a lock (see <a href="../../api_c/env_set_lk_detect.html">DBENV->set_lk_detect</a> for more
information). The following code fragment does the latter:
<p><blockquote><pre>void
env_open(DB_ENV **dbenvp)
{
DB_ENV *dbenv;
int ret;
<p>
/* Create the environment handle. */
if ((ret = db_env_create(&dbenv, 0)) != 0) {
fprintf(stderr,
"txnapp: db_env_create: %s\n", db_strerror(ret));
exit (1);
}
<p>
/* Set up error handling. */
dbenv->set_errpfx(dbenv, "txnapp");
<p>
<b> /* Do deadlock detection internally. */
if ((ret = dbenv->set_lk_detect(dbenv, DB_LOCK_DEFAULT)) != 0) {
dbenv->err(dbenv, ret, "set_lk_detect: DB_LOCK_DEFAULT");
exit (1);
}</b>
<p>
/*
* Open a transactional environment:
* create if it doesn't exist
* free-threaded handle
* run recovery
* read/write owner only
*/
if ((ret = dbenv->open(dbenv, ENV_DIRECTORY,
DB_CREATE | DB_INIT_LOCK | DB_INIT_LOG |
DB_INIT_MPOOL | DB_INIT_TXN | DB_RECOVER | DB_THREAD,
S_IRUSR | S_IWUSR)) != 0) {
dbenv->err(dbenv, ret, "dbenv->open: %s", ENV_DIRECTORY);
exit (1);
}
<p>
*dbenvp = dbenv;
}</pre></blockquote>
<p>Deciding how often to run the deadlock detector and which of the
deadlocked transactions will be forced to abort when the deadlock is
detected is a common tuning parameter for Berkeley DB applications.
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