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<h2 class="title" style="clear: both"><a id="txn_limits"></a>Transaction limits</h2>
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<span class="sect2">
<a href="txn_limits.html#idp2936928">Transaction IDs</a>
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<p>
Transactions are identified by 31-bit unsigned integers,
which means there are just over two billion unique
transaction IDs. When a database environment is initially
created or recovery is run, the transaction ID name space
is reset, and new transactions are numbered starting from
0x80000000 (2,147,483,648). The IDs will wrap if the
maximum transaction ID is reached, starting again from
0x80000000. The most recently allocated transaction ID is
the <span class="bold"><strong>st_last_txnid</strong></span> value
in the transaction statistics information, and can be
displayed by the <a href="../api_reference/C/db_stat.html" class="olink">db_stat</a> utility.
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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 because the locks obtained for
transactions and non-transactions can conflict.
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Because 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. Although
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 block each other (for example, 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 so that the deadlock detector cannot
detect 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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