Accessing extra-database information: concurrency control and correctness

Traditional concurrency control theory views transactions in terms of read and write operations on database items. Thus, the effects of accessing non-database entities, such as the system clock or the log, on a transaction's behavior are not explicitly considered. In this paper, we are motivated by a desire to include accesses to such extra-data items within the purview of transaction and database correctness. We provide a formal treatment of concurrency control when transactions are allowed access to extra-data by discussing the inter-transaction dependencies that are induced when transactions access extra-data. We also develop a spectrum of correctness criteria that apply when such transactions are considered and outline mechanisms to enforce these criteria. Furthermore, we show that allowing databases to view data which has been traditionally kept hidden from users increases the database functionality and in many cases can lead to improved performance