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

In 2, you learned that the following are true for the Oracle database: Reads (selects) don t block writes (inserts, updates, deletes, etc.). For example, a user can be updating a row at the same time as another user is querying the same exact row. The session performing the query would see only the rows that existed at the start of the query. Writes don t block reads. Writes don t block writes unless the contending writes are writing a common set of rows. For example, if two sessions are updating the same table, but update a mutually exclusive set of rows, they don t block each other. Only when they try to update the same row(s) does one of the sessions get blocked until the other session issues a commit or a rollback to end its transaction.

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Any state information maintained on these servers must be centralized when you re using a Web Farm Since requests can be sent to any machine in the mix, a change in the memory of one machine won t show up in the memory of another Session state, a feature of both classic ASP and ASP NET, is an example of this type of user-specific, volatile information that s stored per browser instance The canonical example of what goes in the Session is the user s shopping cart This information must be maintained across different requests that the user makes It must survive for the entire session that the user maintains with the application While no solution was built into ASP for out-of-process sessions, it s accounted for in ASPNET Session information can be stored on a centralized state server, or it can be stored in SQL Server.

Now consider the following situation, which can occur in any relational database: 1 User1 queries a row 2 User2 queries the same row in a different session 3 User1 updates the row and commits 4 User2 updates the row to a different value and commits Unknown to user1, user2 in step 4 overwrote user1 s data This phenomenon is called a lost update, since all changes made by user1 in step 3 were lost because user2 overwrote those changes in step 4 Let s look at an example Imagine it has been a good sales year for our company, and all salespeople are getting a salary hike Adam, who is a clerk, is updating the database to reflect this change One salesperson in particular, Martin, has done well in clinching a mega deal for the company.

It is common to use data to drive control, and indeed in functional programming the distinction between data and control is often blurred: function values can be used as data, and data can influence control flow. One example is using a function such as List.iter to iterate over a list. Let s take a simple example: let sites = [ "http://www.live.com"; "http://www.google.com"; "http://search.yahoo.com" ] sites |> List.iter (fun site -> printfn "%s, length = %d" site (http site).Length) List.iter simply calls the given function (here an anonymous function) for each element in the input list. Here is its type:

Martin s boss, Blake, is very happy and decides to give him an additional bonus of $100 The following is how these two transactions occur, presented in the order of time (many of the following examples use hard-coded values; remember to use bind variables in a real-world application) Blake queries the database to look at Martin s current salary information: blake@ORA10G> select ename, sal from emp where ename='MARTIN'; ENAME SAL ---------- ---------MARTIN 1250 Blake gets an urgent phone call and leaves his desk to answer it Meanwhile, Adam has queried records for all employees who are salespeople and is currently looking at Martin s salary details He updates Martin s record to give Martin a 10% salary hike: adam@ORA10G> select ename, sal from emp where ename='MARTIN'; ENAME SAL ---------- ---------MARTIN 1250 adam@ORA10G> update emp set sal = sal * 110 where ename='MARTIN'; 1 row updated.

Many additional aggregate iteration techniques are defined in the F# and .NET libraries, particularly by using values of type seq<type>, discussed in Getting Started with Sequences later in this chapter.