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<title>Berkeley DB Reference Guide: Deadlock detection using timers</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Locking Subsystem</dl></h3></td>
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<p>
<h3 align=center>Deadlock detection using timers</h3>
<p>Lock and transaction timeouts may be used in place of, or in addition
to, regular deadlock detection. If lock timeouts are set, lock requests
will return <a href="../../ref/program/errorret.html#DB_LOCK_NOTGRANTED">DB_LOCK_NOTGRANTED</a> from a lock call when it is
detected that the lock's timeout has expired, that is, the lock request
has blocked, waiting, longer than the specified timeout. If transaction
timeouts are set, lock requests will return <a href="../../ref/program/errorret.html#DB_LOCK_NOTGRANTED">DB_LOCK_NOTGRANTED</a>
from a lock call when it has been detected that the transaction has been
active longer than the specified timeout.</p>
<p>If lock or transaction timeouts have been set, database operations will
return <a href="../../ref/program/errorret.html#DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a> when the lock timeout has expired or the
transaction has been active longer than the specified timeout.
Applications wanting to distinguish between true deadlock and timeout
can use the <a href="../../api_c/env_set_flags.html#DB_TIME_NOTGRANTED">DB_TIME_NOTGRANTED</a> configuration flag, which causes
database operations to instead return <a href="../../ref/program/errorret.html#DB_LOCK_NOTGRANTED">DB_LOCK_NOTGRANTED</a> in the
case of timeout.</p>
<p>As lock and transaction timeouts are only checked when lock requests
first block or when deadlock detection is performed, the accuracy of
the timeout depends on how often deadlock detection is performed. More
specifically, transactions will continue to run after their timeout has
expired if they do not block on a lock request after that time.
A separate deadlock detection thread (or process) should always
be used if the application depends on timeouts; otherwise, if
there are no new blocked lock requests a pending timeout will
never trigger.</p>
<p>If the database environment deadlock detector has been configured with
the <a href="../../api_c/env_set_lk_detect.html#DB_LOCK_EXPIRE">DB_LOCK_EXPIRE</a> option, timeouts are the only mechanism by
which deadlocks will be broken. If the deadlock detector has been
configured with a different option, then regular deadlock detection will
be performed, and in addition, if timeouts have also been specified,
lock requests and transactions will time out as well.</p>
<p>Lock and transaction timeouts may be specified on a database environment
wide basis using the <a href="../../api_c/env_set_timeout.html">DB_ENV->set_timeout</a> method. Lock timeouts may be
specified on a per-lock request basis using the <a href="../../api_c/lock_vec.html">DB_ENV->lock_vec</a> method. Lock
and transaction timeouts may be specified on a per-transaction basis
using the <a href="../../api_c/txn_set_timeout.html">DB_TXN->set_timeout</a> method. Per-lock and per-transaction
timeouts supersede environment wide timeouts.</p>
<p>For example, consider that the environment wide transaction timeout has
been set to 20ms, the environment wide lock timeout has been set to
10ms, a transaction has been created in this environment and its timeout
value set to 8ms, and a specific lock request has been made on behalf
of this transaction where the lock timeout was set to 4ms. By default,
transactions in this environment will be timed out if they block waiting
for a lock after 20ms. The specific transaction described will be timed
out if it blocks waiting for a lock after 8ms. By default, any lock
request in this system will be timed out if it blocks longer than 10ms,
and the specific lock described will be timed out if it blocks longer
than 4ms.</p>
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