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

config file as shown in Listing 14-3 to ensure that verbose debugging information is generated for your F# code Otherwise, errors reported by ASPNET will be difficult to diagnose..

In fact, other sessions just trying to insert a new record into this table would be blocked as well, as they would be attempting to lock the same bitmap key entry In short, the developers had created a table that at most one person would be able to insert or update against at a time! We can see this easily using a simple scenario..

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The ANSI/ISO SQL standard defines four levels of transaction isolation, with different possible outcomes for the same transaction scenario. That is, the same work performed in the same fashion with the same inputs may result in different answers, depending on your isolation level. These isolation levels are defined in terms of three phenomena that are either permitted or not at a given isolation level:

A number of application settings are relevant to web applications, but most of these are beyond the scope of this book. The easiest way to get some guidance is by looking at the web.config.comments file. For Microsoft .NET, you can find this file in your Windows directory and the Microsoft.Net\Framework\ {your .NET version}\CONFIG folder within. This same directory also contains another configuration file called machine.config, which has settings that apply to all web applications on the host machine. Many of the common, default settings are stored in this file.

Dirty read: The meaning of this term is as bad as it sounds. You are permitted to read uncommitted, or dirty, data. You would achieve this effect by just opening an OS file that someone else is writing and reading whatever data happens to be there. Data integrity is compromised, foreign keys are violated, and unique constraints are ignored. Non-repeatable read: This simply means that if you read a row at time T1 and attempt to reread that row at time T2, the row may have changed, it may have disappeared, it may have been updated, and so on. Phantom read: This means that if you execute a query at time T1 and re-execute it at time T2, additional rows may have been added to the database, which will affect your results. This differs from the non-repeatable read in that with a phantom read, data you already read has not been changed, but rather that more data satisfies your query criteria than before.

Note The ANSI/ISO SQL standard defines transaction-level characteristics, not just individual statement-bystatement level characteristics. In the following pages, we ll examine transaction-level isolation, not just statement-level isolation.

Listing 14-3. web.config: An ASP.NET Website Configuration File for Listing 14-2 < xml version="1.0" > <configuration> <system.web> <compilation debug="true" /> </system.web> <system.codedom> <compilers> <compiler language="F#;f#;fs;fsharp" extension=".fs" type="Microsoft.FSharp.Compiler.CodeDom.FSharpAspNetCodeProvider, FSharp.Compiler.CodeDom, Version=1.9.2.9, Culture=neutral, PublicKeyToken=a19089b1c74d0809"/> </compilers> </system.codedom> </configuration>

The SQL isolation levels are defined based on whether or not they allow each of the preceding phenomena. I find it interesting to note that the SQL standard does not impose a specific locking scheme or mandate particular behaviors, but rather describes these isolation levels in terms of these phenomena, allowing for many different locking/concurrency mechanisms to exist (see Table 7-1). Table 7-1. ANSI Isolation Levels

Embedded scripts in web applications tend to create spaghetti code that confuses details of the presentation layer with the underlying logic of the application. However, embedding scripts is a useful way of trying new features. The normal practice for ASP.NET website development is for each Page.aspx file to have backing code in a particular language, such as Page.aspx.fs for F# code. Listing 14-4 shows the code from Listing 14-2 but without the embedded script and with ASP.NET directives at the top of the file indicating that a code-behind file is being used. Listing 14-4. Time2.aspx: A Simple ASP.NET Web Form with F# Code-Behind <%@ Page Language="F#" AutoEventWireup="true" CodeFile="Time2.aspx.fs" Inherits="FSharpWeb.Time2" %>

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