ASP.NET PDF Viewer using C#, VB/NET

Oracle actually exposes to developers the enqueue lock mechanism that it uses internally, via the DBMS_LOCK package. You might be wondering why you would want to create your own locks. The answer is typically application specific. For example, you might use this package to serialize access to some resource external to Oracle. Say you are using the UTL_FILE routine that allows you to write to a file on the server s file system. You might have developed a common message routine that every application calls to record messages. Since the file is external, Oracle won t coordinate the many users trying to modify it simultaneously. In comes the DBMS_LOCK package. Now, before you open, write, and close the file, you will request a lock named after the file in exclusive mode, and after you close the file, you will manually release the lock. In this fashion, only one person at a time will be able to write a message to this file. Everyone else will queue up. The DBMS_LOCK package allows you to manually release a lock when you are done with it, or to give it up automatically when you commit, or even to keep it as long as you are logged in.

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ASP.NET directives. See Table 14-4 for a list of commonly used ASP.NET directives. HTML comment. HTML meta-information. Usually generated when the file is first created and can be ignored. Inserts F# or other scripting code as part of the generated code for the page. The F# code should be a portion of an object type definition. Indicates that the given HTML element is treated as an ASP.NET control and generated dynamically on the server as part of the page object (with the ID as its name). Inserts F# or other scripting code into a rendering of the page as an expression. For example, <%= let a = 40+2 in a.ToString() %>. Data-binding expression: inserts F# code as the current item of an ASP.NET data control. A convenient shortcut for referring to a given data field is via the Eval method. For example, <%# this.Eval("Rgb") %>.

Summary

This chapter covered a lot of material that, at times, may have made you scratch your head. While locking is rather straightforward, some of its side effects are not. However, it is vital that you understand these issues. For example, if you were not aware of the table lock Oracle uses to enforce a foreign key relationship when the foreign key is not indexed, then your application would suffer from poor performance. If you did not understand how to review the data dictionary to see who was locking whom, you might never figure that one out. You would just assume that the database hangs sometimes. I sometimes wish I had a dollar for every time I was able to solve the insolvable hanging issue by simply running the query to detect unindexed foreign keys and suggesting that we index the one causing the problem. I would be very rich.

Beyond the HTML and ASP.NET controls, the key code in Listing 14-2 is the embedded F# script beginning with <script language="F#" runat="server">. F# scripts embedded in .aspx pages must be a set of member declarations, in other words, fragments of an F# type definition. ASP.NET inserts these into the code generated for the page object. The embedded F# script contains two members. The first is as follows: member this.Form1_Load(sender: obj, e: EventArgs) = if not this.Page.IsPostBack then this.Time.Text <- DateTime.Now.ToString() This member is associated with the Form1 control and is invoked on the server when the page is loaded, in other words, as part of the process of generating the final HTML for the page. We can see this because the event callback for the control mentions this event handler:

As stated in the last chapter, one of the key challenges in developing multiuser, database-driven applications is to maximize concurrent access but, at the same time, ensure that each user is able to read and modify the data in a consistent fashion. In this chapter, we re going to take a detailed look at how Oracle achieves multi-version read consistency and what that means to you, the developer. I will also introduce a new term, write consistency, and use it to describe how Oracle works not only in a read environment with read consistency, but also in a mixed read and write environment.