Transaction mask

A transaction mask is an array of two bit pairs that represents the state of all transactions starting with the oldest interesting and ending with the next transaction. The oldest interesting transaction is the first transaction in the database after transaction zero) whose state is not committed. Transaction zero is the system transaction and is always active. The next transaction is the transaction after the one that started most recently.

In the Classic architecture, each connection maintains its own copy of the transaction mask. In shared server architectures, each server maintains a single copy of the transaction mask. In Classic, and in particular on machines with memory sizes that were typical in the early 90's, you could eat up a lot of memory describing a system that had a few hundred thousand transactions between the oldest interesting and the next transaction, even if you only use two bits per transaction.