Identify parameters and results
Once you have identified your web service you need to find out how it
will be used. In most cases web services have a set of functions (or in
simple cases just one function). Each function may accept a set of
parameters and will perform its function returning some value (which may
be structured in some way). ASP.NET web services provide a useful human
interface for evaluating the services and determining how to use them.
Web services in other languages may need a more mechanised approach
using the development tools to allow you to figure out how they work. If
you are lucky there will be online help associated with each web
service.
For example Amazon, PayPal and Google web services have comprehensive
online documentation available for developers who wish to use the
services. We will look at some simple ASP.NET web services.
Example 1: Global Weather
The web service interface on
http://www.webservicex.net/globalweather.asmx
display the set of functions, in this case GetCitiesByCountry and
GetWeather. In session 1 we looked at the GetWeather function which
required two parameters and returned a string which was an XML
representation of the weather in the requested city. The simple test
form can be used to get a feel for the way in which the function works,
but a more detailed formal description is found in the SOAP interface
and the WSDL schema. We don't necessarily need to know too much about
SOAP and WSDL as the development tools can take care of handling all the
required interactions with the web service. The important thing is to
know the input parameters and the structure of the returned results.
In this case the input parameters are called CityName and CountryName.
Example 2: Currency Converter
If we check out the web service interface on
http://www.webservicex.net/CurrencyConvertor.asmx
we will see that there is just one function called ConversionRate. The
function makes use of some coded three letter currency codes (e.g. USD,
GBP, EUR). By clicking on the ConversionRate link we see the test form
which tells us that the function requires two parameters (FromCurrency
and ToCurrency). The function returns a floating point number in double
format which is the current conversion rate between the two currencies.
Example 3: Validate Email Address
The web service interface on
http://www.webservicex.net/ValidateEmail.asmx
shows us that there is a single function called IsValidEmail. The
function takes a single parameter called Email and returns a boolean
(true/false) result.