.NET/Java PDF, Tiff, Barcode SDK Library

Normally, the columns referenced in the WHERE clause of the UPDATE or DELETE statement are used to determine whether or not the modification needs to restart Oracle will perform a consistent read using these columns and, upon retrieving the block in current mode, it will restart the statement if it detects that any of them have changed Normally, the other columns in the row are not inspected For example, let s simply rerun the previous example and use WHERE Y>0 to find the rows in both sessions, the output we ll see in the first session (the one that gets blocked) would be: ops$tkyte%ORA11GR2> update t set x = x+1 where y > 0; oldx = 1, oldy = 1 newx = 2, newy = 1 oldx = 2, oldy = 1 newx = 3, newy = 1 1 row updated.

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So why did Oracle fire the trigger twice when it was looking at the Y value Does it examine the whole row As you can see from the output, the update was, in fact, restarted and the trigger again fired twice, even though we were searching on Y>0 and did not modify Y at all But, if we recreate the trigger to simply print out the fact that it fired, rather than reference the :OLD and :NEW values ops$tkyte%ORA11GR2> create or replace trigger t_bufer 2 before update on t for each row 3 begin 4 dbms_outputput_line( 'fired' ); 5 end;.

So far in this chapter we have looked at server-side web applications. In recent years a new class of rich-client web applications has emerged, leading to what is commonly called the Ajax development paradigm. This is a general term for any web application that incorporates substantial amounts of code executed on the client side of the application by running JavaScript in the web browser.

Note There are many variants of the fork() and exec() calls, such as vfork() and execve(). The call used

Developing Ajax applications can be done in two ways when using F#: You can manually write and serve additional JavaScript files as part of your web application. You can use the F# Web Tools to write both client and server code purely in F#. Developing Ajax applications by the first technique follows a fairly standard path mostly independent of F#. You can find more details on this technique at http://www.expert-fsharp.com/ Topics/WebProgramming.

by Oracle may vary by operating system and implementation, but the net effect is the same. fork() creates a new process that is a clone of the parent process; on UNIX, this is the only way to create a new process. exec() loads a new program image over the existing program image in memory, thus starting a new program. So, SQL*Plus can fork (copy itself) and then exec the Oracle binary, the dedicated server, overlaying the copy of itself with this new program.

We can see this parent/child process creation clearly on UNIX when we run the client and server on the same machine: $ sqlplus / SQL*Plus: Release 11.2.0.1.0 Production on Sun Jan 24 07:28:13 2010 Copyright (c) 1982, 2009, Oracle. All rights reserved.

F# Web Tools is an open source project at http://www.codeplex.com/fsharpwebtools. These tools represent an extremely powerful way of writing robust, efficient, integrated client/server applications in a single, type-checked framework. In particular, they draw on several advanced features of F#, the combination of which offers a unique programming experience for web applications: Client-side and server-side code is authored in one project, and code to handle clientside events can be written in the same way as server-side event handlers that we used in many places in this chapter. The program runs initially as a server-side application. The client-side code is written as an F# workflow and is automatically translated to JavaScript using F# quotations and reflection and is served to the client. (See 9 for details on F# quotations and workflows.) The client side can also use some .NET and F# libraries. The calls are mapped to a corresponding JavaScript functionality through fairly straightforward techniques. The client side may make asynchronous calls to the server using variations on the techniques described in 13. Some type safety is guaranteed by making modal distinctions using F# workflows. These tools were under development at the time the book was written, and some details of their use are likely to change. For this reason we don t give detailed code using these tools in this book, though this chapter gives a good grounding in the ASP.NET techniques on which the tools are based. Figure 14-4 shows part of one sample web application written using F# Web Tools. In this sample, which is authored entirely in F#, all symbolic computation is executed as JavaScript on the client side. The image displaying the curve is generated on the server side, since JavaScript has relatively weak abilities for image generation in a portable fashion.