The intent of this white paper is to convey information regarding database locks as they apply to transactions in general and the more specific case of how they are implemented by the Progress SQL-92 server.

We'll begin with a general overview discussing why locks are needed and how they affect transactions. Transactions and locking are outlined in the SQL standard so no introduction would be complete without discussing the guidelines set forth here. Once we have a grasp on the general concepts of locking we'll dive into lock modes, such as table and record locks and their effect on different types of database operations. Next, the subject of timing will be introduced, when locks are obtained and when they are released. From here we'll get into lock contention and deadlocks, which are multiple operations or transactions all attempting to get locks on the same resource at the same time. And to conclude our discussion on locking we?ll take a look at how we can see locks in our application so we know which transactions obtain which types of locks. Finally, this white paper describes differences in locking behavior between previous and current versions of Progress and differences in locking behavior when both 4GL and SQL92 clients are accessing the same resources.