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<title>Berkeley DB Reference Guide: Transaction limits</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Transaction Subsystem</dl></h3></td>
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<h1 align=center>Transaction limits</h1>
<h3>Transaction IDs</h3>
<p>Transactions are identified uniquely by 32-bit unsigned integers. The
high-order bit of the transaction ID is reserved (and defined to be 1)
resulting in just over two billion unique transaction IDs. Each time
that recovery is run, the beginning transaction ID is reset with new
transactions being numbered starting from 1. This means that recovery
must be run at least once every two billion transactions.
<p>It is possible that some environments may need to be aware of this
limitation. Consider an application performing 600 transactions a second
for 15 hours a day. The transaction ID space will run out in roughly 66
days:
<p><blockquote><pre>2^31 / (600 * 15 * 60 * 60) = 66</pre></blockquote>
<p>Doing only 100 transactions a second exhausts the transaction ID space
in roughly one year.
<p>The transaction ID name space is initialized each time
a database environment is created or recovered. If you
reach the end of the transaction ID name space, it must
be handled as if an application or system failure had
occurred. The most recently allocated transaction ID
is the <b>st_last_txnid</b> value in the transaction
statistics information, and is displayed by the
<a href="../../utility/db_stat.html">db_stat</a> utility.
<h3>Cursors</h3>
<p>When using transactions, cursors are localized to a single transaction.
That is, a cursor may not span transactions and must be opened and
closed within a single transaction. In addition, intermingling
transaction-protected cursor operations and non-transaction-protected
cursor operations on the same database in a single thread of control is
practically guaranteed to deadlock as the locks obtained for transactions
and non-transactions can conflict.
<h3>Multiple Threads of Control</h3>
<p>Since transactions must hold all their locks until commit, a single
transaction may accumulate a large number of long-term locks during its
lifetime. As a result, when two concurrently running transactions access
the same database, there is strong potential for conflict. While Berkeley
DB allows an application to have multiple outstanding transactions active
within a single thread of control, great care must be taken to ensure that
the transactions do not interfere with each other (e.g., attempt to obtain
conflicting locks on the same data). If two concurrently active
transactions in the same thread of control do encounter a lock conflict,
the thread of control will deadlock in such a manner that the deadlock
detector will be unable to resolve the problem. In this case, there is
no true deadlock, but because the transaction on which a transaction is
waiting is in the same thread of control, no forward progress can be made.
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