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Concurrency controls are the collection of functions that the database provides to allow many people to access and modify data simultaneously. As noted in the previous chapter, the lock is one of the core mechanisms by which Oracle regulates concurrent access to shared database resources and prevents interference between concurrent database transactions. To briefly summarize, Oracle uses a variety of locks, including the following: TX (Transaction) locks: These locks are acquired for the duration of a datamodifying transaction. TM (DML Enqueue) and DDL locks: These locks ensure that the structure of an object is not altered while you are modifying its contents (TM lock) or the object itself (DDL lock). Latches and Mutexes: These are internal locks that Oracle employs to mediate access to its shared data structures. We ll refer to both as Latches in this chapter, although they might be implemented by a Mutex on your operating system, depending on the Oracle Version.

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<form id="Form1" runat="server" onload="Form1_Load"> The implementation of Form1_Load uses the IsPostBack property of the page object to detect whether the page is loaded for the first time or whether it is loaded in reaction to some event invoked by a control. The reaction to an event that involves executing server-side code is called a postback, and it is triggered by sending data from the client back to the current page. When the page is loaded for the first time, this event handler sets the content of this.Time. This is the label element of the page being presented to the user (note the close correspondence between ASP.NET elements and HTML elements). The second part of the script is the following member: member this.Reload_Click(sender: obj, e: EventArgs) = this.Time.Text <- "(R) " + DateTime.Now.ToString() This member is associated with the Reload button control and is invoked when the button is clicked. Because the event handler associated with the button has to be executed on the server, the page will be reloaded, and the event will be triggered. Note that the entire page is reloaded, so the Form1_Load handler will be called as well, but for this and subsequent reloads, the IsPostBack property is set to true and the label will not be updated by that event handler.

In each case, there is minimal overhead associated with lock acquisition. TX transaction locks are extremely scalable both in terms of performance and cardinality. TM and DDL locks are applied in the least restrictive mode whenever possible. Latches and enqueues are both very lightweight and fast (enqueues are slightly the heavier of the two, though more feature-rich). Problems only arise from poorly designed applications that hold locks for longer than necessary and cause blocking in the database. If you design your code well, Oracle s locking mechanisms will allow for scalable, highly concurrent applications.

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Note I used the phrase longer than necessary. That does not mean you should attempt to commit (end your

transaction) as soon as possible. Transactions should be exactly as long as they need to be and no longer than that. That is, your transaction is your unit of work; it is all or nothing. You should commit when your unit of work is complete and not before and not any later either!

The text of embedded script fragments are added verbatim to the generated code, indented to an appropriate position. The generated code may appear in error messages including syntax errors, so you may occasionally need to look at the details of the generated ASP.NET code.

But Oracle s support for concurrency goes beyond efficient locking. It implements a multiversioning architecture (introduced in 1 Developing Successful Oracle Applications ) that provides controlled yet highly concurrent access to data. Multi-versioning describes Oracle s ability to simultaneously materialize multiple versions of the data and is the mechanism by which Oracle provides read-consistent views of data (i.e., consistent results with respect to a point in time). A rather pleasant side effect of multi-versioning is that a reader of data will never be blocked by a writer of data. In other words, writes do not block reads. This is one of the fundamental differences between Oracle and other databases. A query that only reads information in Oracle will never be blocked; it will never deadlock with another session, and it will never get an answer that didn t exist in the database.

Note There is a short period of time during the processing of a distributed Two Phase Commit where Oracle will