12.3. Locks and Concurrency

Concurrency is a major issue for any database system. It addresses the notion of two or more users each trying to interact with the same object at the same time. The nature of that interaction may be different for each user (updating, deleting, reading, inserting), and the ideal way to handle the competition for control of the object changes depending on just what all the users in question are doing and just how important their actions are. The more users — more specifically, the more transactions — that you can run with reasonable success at the same time, the higher your concurrency is said to be.

In the Online Transaction Processing (OLTP) environment, concurrency is usually the first thing we deal with in data, and it is the focus of most of the database notions put forward in this book. (Online Analytical Processing (OLAP) is usually something of an afterthought — it shouldn't necessarily be that way, but it is.) Dealing with the issue of concurrency can be critical to the performance of your system. At the foundation of dealing with concurrency in databases is a process called locking.

Locks are a mechanism for preventing a process from performing an action on an object that conflicts with something already being done on that object. That is, you can't do some things to an object if someone else got there first. What you can and cannot do depends on what the other user is doing. It is also a means of describing what is being done, so the system ...

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