CSCI E-143 CertPrep: Web Services (Kalani ch. 7)

CSCI E-143 CertPrep: Web Services (Kalani, chapter 7)

Saturday, Dec. 18, 2004

This Saturday we cover Kalani's chapter 7, "Web Services". This is a very short chapter. In fact, the part you are actually responsible for is shorter than the chapter itself, because Kalani briefly shows how to set up a web service, and for this test, you only have to know how to use a web service, not how to write one. Nevertheless, some tricky questions might come up.

Terms

A Web Service is a program that exposes is methods for use over the internet. It differs from a "web app" because it does not return a web page. It returns its output in a coded form for use by your Windows client. You must have a Windows client to pass data to the web service and to display the output you receive from it.

A Web Method is a routine in a web service that you can invoke across the internet. (A web service class may of course have its own private methods not exposed to web access.)

SOAP. Simple Object Access Protocol. Encapsulates object calls as XML. Used in passing data both ways between a Windows client and the web service it uses.

Discovery is finding out about web services: where they are, the names of the classes, and the names and natures of the web methods they expose.

Disco. A command-line web services discovery tool used by Microsoft.

It involves the use of registries, which compile information about available web services on the 'net.
It can be used to form proxies for an individual web service, by reading that service's .asmx or .wsdl files. (Disco can't use the .asmx file directly; it uses it to try to find the .wsdl file.)
In addition, .disco is the file extension for a file containing information retrieved by this program, including URLs, such as that for the .wsdl file. (Example: Kalani, p. 583)

UDDI. Universal Description, Discovery, and Integration. Another web services tool, also involving a registry. But there is no .uddi file extension.

WSDL. Web Services Description Language. Tells the SOAP message types possible for this service. In addition, .wsdl is a file extension for a file containing such information, which might be produced by the various tools for discovering web services.

Creating a Web Service

You will not be asked about creating a web service on this exam. (It is covered on exam 70-320.) But it may clarify things if we do a simple example. (Kalani, pp. 575-576)

Start a new Visual C# / ASP.NET Web Service project. This creates a file named project.asmx.
The class for this project must have the attribute
```
      [WebService(Namespace="http://Some/Place")]
```
This attribute is not supplied by the IDE. You have to add it yourself. The namespace is arbitrary, but it must exist.
Make sure all the methods you want to expose over the web are public and have the following attribute:
```
      [WebMethod()]
```
Otherwise, the methods are like ordinary C# methods. Private methods you do not want to expose over the 'net do not have this attribute.
Build the solution.
To test, run the project under the IDE. You will get a browser displaying information about the available service. Try entering data and invoking the service. You will get results in XML format. (This use of the browser is not to be confused with a web service discovery tool, which will set up a proxy on your local machine.)

Instantiating and Invoking Web Services from a Windows Application

Some of this might be on the exam.

There are two ways to do this: the hard way and the easy way.

The Hard Way: Using disco.exe and wsdl.exe

Run disco.exe, for example with a command like (Kalani, p. 583)

      disco  http://live.capescience.com/wsdl/AirportWeather.wsdl

This gives you two files:
- AirportWather.wsdl
- results.discomap
Now that you have the wsdl file, you use the wsdl.exe routine, for instance
```
      wsdl  /language:CS  /out:AirportWeatherProxy.cs   AirportWeather.wsdl
```
This gives you a proxy that you add to your .NET project.
You can now act as if the web service was just another routine in your app.

Some command-line options for wsdl.exe

/language. It's CS for C#.
/namespace. The output namespace. You must run this wsdl.exe routine if you want to set a namespace other than the default that would be set by using a web reference.
/out:[filename]. The output file name.

The Easy Way: Using Web References (Kalani, pp. 572-573)

In the Solution Explorer panel, right-click "References" and select "Add Web Reference".
You get a dialog box. Give the URL in the Address bar and press Enter. Information should be downloaded from the site.
Click the "Add Reference" button. This creates the same files created by the command-line options described just above.
Finish your project, creating objects from the web service as if the service was on your local machine.
- You must use a fully qualified name for a class you instantiate from the web service—for example, localhost.Class.
- You do not have make any changes to the names of methods in such a class.
Right-click on the client project in the Solution Explorer, and set it as the start-up project.
Run the (client) project.

There is another advantage to doing it this easy way: if the web service changes, you can get updated information by right-clicking the Web reference node in Solution Explorer and selecting "Update Web Reference".

Asynchronous Web Service Calls (Kalani, pp. 591-592)

Kalani gives this useful example at the end of the chapter. It uses a callback method which is invoked when the called web service returns with the information for the local program to display.

The important thing about this code is that it makes no reference to the web service that is not already made for a synchronous method. That is, such asynchronous calls require no coding on the server end.

Here is the code: (Kalani, p. 593)

private void btnGetWeather_Click(object sender, System.EventArgs e)
{
  // Declare the Web service main object

  com.capescience.live.AirportWeather aw =
    new com.capescience.live.AirportWeather();

  // Invoke the Web service. This may take some time, so call it asynchronously.
  // First, create a callback method.

  AsyncCallback wcb = new AsyncCallback(WebServiceCallback);

  // And then initiate the asynchronous call.
  // "BegingetSummary" is in the proxy class.

  aw.BegingetSummary(txtCode.Text, wcb, aw);
}

// This method will get called when the Web service call is done.

public void WebServiceCallback(IAsyncResult ar)
{
  // Retrieve the state of the proxy object.

  com.capescience.live.AirportWeather aw = 
    (com.capescience.live.AirportWeather)ar.AsyncState;

  // Call the End method to finish processing.
  // "EndgetSummary" is in the proxy class.

  com.capescience.live.WeatherSummary ws = aw.EndgetSummary(ar);

  // Display the results.

  lbResults.Items.Clear();
  lbResults.Items.Add(ws.location);
  lbResults.Items.Add(“Wind “ + ws.wind);     //etc., etc., etc.
}

Here are the relevant questions from the two tests at the end of the "short Kalani" book:

Test 1: questions 36 (page 373; answer: page 395), 37.

Test 2: questions 23 (page 415; answer: page 443), 24, 25.

(Some of these questions may duplicate those in long Kalani. Both texts are listed here.)

Last revised Dec. 17, 2004