The transaction ID space in Berkeley DB is 2^31, or 2 billion entries. 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:
2^31 / (600 * 15 * 60 * 60) = 66
Doing only 100 transactions a second exhausts the transaction ID space in roughly one year.
The transaction ID space is reset each time recovery is run. If you reach the end of your transaction ID space, shut down your applications and restart them after running recovery. The most recently allocated transaction ID is the st_last_txnid value in the transaction statistics information, and is displayed by the db_stat utility.