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There are a couple of options for a component to vote on the outcome of a transaction from code. One is declarative and the other imperative. The declarative method is very easy. A method can be flagged with the AutoComplete attribute, and as long as the method body does not throw an exception, the method s vote will be to commit the transaction. The method must be coded such that if something goes wrong, an exception is thrown. If an exception is thrown by the database, it must be left to ascend the call stack, or be trapped, wrapped, and rethrown. Any violation of business rules that results in the method being unable to complete its work must also be handled by throwing an exception. (You can find this code in XActional.cs in the Serviced project.) [Transaction(TransactionOption.Required)] public class CustomerService { [AutoComplete(true)] public void IncreaseCreditLimit( int customerNum, double increaseAmount) { try { Customer cust = new Customer(customerNum); double max = cust.MaxAllowableCredit; double current = cust.CreditLimit; if (max < current + increaseAmount) { throw new Exception("Max Credit Limit Exceeded"); } cust.CreditLimit += increaseAmount; cust.Save(); } catch (Exception ex) { throw new Exception( "Attempt to increase limit failed", ex); } } } Notice the explicitly thrown exception captures the violation of a business rule. The try/ catch block traps and wraps the explicitly thrown exception, or any other exception that bubbles up the call stack from your calls into the Customer object. The other option is to explicitly vote on the transaction outcome from within the body of your method. In this case, you would omit the AutoComplete attribute, and use the ContextUtil properties to indicate the success or failure of the work that s been done.

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The procedure do_nt_insert inserts a given number of parent records and a given number of children for each parent record into the table components_nt: 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 procedure do_nt_insert( p_num_of_parents in int := 50, p_num_of_children in int := 500 ) as l_part_type_tab part_type_tab; l_part_type part_type; l_part_or_id number := 1; begin l_part_type_tab := part_type_tab(); l_part_type_tab.extend( p_num_of_children ); for i in 1..p_num_of_parents loop for j in 1..p_num_of_children loop l_part_type_tab(j) := part_type( i, l_part_or_id, 'part'||i||j, 'part desc '||i||j ); l_part_or_id := l_part_or_id + 1; end loop; insert into components_nt values ( i, 'component'||i, l_part_type_tab ); end loop; commit; end do_nt_insert;

Capturing common control patterns such as cancellation and progress reporting is an absolutely essential part of mastering concurrent programming However, one of the problems with NET classes such as BackgroundWorker is that they are often more imperative than an F# programmer may want, and they force other common patterns to be captured by using mutable data structures shared between threads This leads to the more difficult topic of shared-memory concurrency, which we discuss later in the chapter Furthermore, the way BackgroundWorker handles cancellation means that flag-checks and early-exit paths have to be inserted in the executing background process Finally, BackgroundWorker is not useful for background threads that perform asynchronous operations, since the background thread will exit too early, before the callbacks for the asynchronous operations have executed.

The procedure do_or_update updates one column of all rows in the object view components_or_view: 102 103 104 105 106 107 109 110 procedure do_or_update as begin update components_or_view set component_name = component_name || ' or update'; end; procedure do_or_child_update as

set { txtTimestamp.Text = BitConverter.ToString((byte[])value); } } This code uses a BitConverter to transform the byte array received from the database into a string, and then uses the Convert.ToByte method to transform it back into a byte array for populating a parameter value to send to the database. By typing it as an Object, consumers of the control need not worry about their parameter values. They can set it with code like this (user control instance is named tsAuthor): tsAuthor.TimestampValue = cm.Parameters["@ts"].Value; And they can retrieve the value with this line of code: pm = cm.Parameters.Add("@ts", SqlDbType.Timestamp); pm.Value = tsAuthor.TimestampValue; This is the type of programming ease you re designing for by encapsulating this logic in the user control. So when the page first renders, you populate the author list. You ve done this several times, so this code is omitted here for brevity. You can see from the markup that when the user chooses an entry from the list, a postback occurs (AutoPostBack=true), and the BindToAuthor method is executed (from Concurrency3.aspx). protected void BindToAuthor(object sender, EventArgs e) { SqlConnection cn = new SqlConnection( ConfigurationManager.ConnectionStrings ["localPubs"].ConnectionString); SqlCommand cm = new SqlCommand( "select @fname = au_fname, @lname = au_lname, @ts = ts " + "from authors_ts where au_id = @id", cn); cm.Parameters.Add("@id", SqlDbType.Char, 11) .Value = ddlAuthors.SelectedValue; cm.Parameters.Add("@fname", SqlDbType.VarChar,20) .Direction = ParameterDirection.Output; cm.Parameters.Add("@lname", SqlDbType.VarChar,40) .Direction = ParameterDirection.Output; cm.Parameters.Add("@ts", SqlDbType.Timestamp) .Direction = ParameterDirection.Output; cn.Open(); cm.ExecuteNonQuery(); cn.Close();

For this reason, it can often be useful to build abstractions that are similar to BackgroundWorker but that capture richer or different control patterns, preferably in a way that does not rely on the use of mutable state and that interferes less with the structure of the overall computation Much of the rest of this chapter will look at various techniques to build these control structures We start with a case study where we build a type IterativeBackgroundWorker that represents a variation on the BackgroundWorker design pattern Listing 13-2 shows the code Listing 13-2 A Variation on the BackgroundWorker Design Pattern for Iterative Computations open SystemComponentModel open SystemWindowsForms /// An IterativeBackgroudWorker follows the BackgroundWorker design pattern /// but instead of running an arbitrary computation it iterates a function /// a fixed number of times and reports intermediate and final results.

In this chapter, you ll learn about connection pooling and caching, and how they can improve the performance of your application. You ll look at the 9i and 10g implementations separately. Finally, you ll examine Oracle s OCI driver connection pooling feature.