BSc IT - Web Application Development - ASP.NET

Web services are built using a set of standardized technologies and protocols that allow different applications to communicate over the internet. The main building blocks of web services include:

### 1. **XML (eXtensible Markup Language)**
- **Purpose**: Used to format and structure data for exchange between systems.
- **Role**: Acts as the foundation for data representation in web services.
- **Example**:

  <Person>
    <Name>John</Name>
    <Age>30</Age>
  </Person>

- Since XML is platform-independent, it allows diverse systems to understand shared data.

### 2. **SOAP (Simple Object Access Protocol)**
- **Purpose**: A protocol used to **send and receive messages** in a web service.
- **Features**:
  - Uses XML for message formatting.
  - Relies on HTTP or SMTP for transmission.
  - Highly secure and supports ACID compliance (used in enterprise apps).
- **Structure of a SOAP Message**:

  <soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope">
    <soap:Header></soap:Header>
    <soap:Body>
      <GetUserResponse>
        <User>John Doe</User>
      </GetUserResponse>
    </soap:Body>
    <soap:Fault></soap:Fault>
  </soap:Envelope>

- **Pros**: Secure, reliable, supports WS-Security, WS-ReliableMessaging.
- **Cons**: Heavyweight, slower due to large XML overhead.

### 3. **WSDL (Web Services Description Language)**
- **Purpose**: An XML-based file that **describes what a web service can do**, how to call it, and where it is located.
- **Contains**:
  - **Service name**
  - **Operations (methods)** available
  - **Input/output parameters**
  - **Communication protocol** (e.g., SOAP over HTTP)
  - **Endpoint URL (address)**

- **Example**:

  <definitions>
    <message name="GetUserRequest">
      <part name="userId" type="xs:int"/>
    </message>
    <portType name="UserServicePort">
      <operation name="GetUser">
        <input message="GetUserRequest"/>
        <output message="GetUserResponse"/>
      </operation>
    </portType>
    <binding type="UserServicePort" transport="HTTP">
      <soap:binding style="rpc"/>
    </binding>
    <service name="UserService">
      <port binding="UserServiceBinding">
        <soap:address location="https://example.com/userservice.asmx"/>
      </port>
    </service>
  </definitions>

- When you add a web service reference in Visual Studio, it reads the WSDL to generate client code.

### 4. **UDDI (Universal Description, Discovery, and Integration)**
- **Purpose**: A directory or registry where web services are **published and discovered**.
- **Function**:
  - Businesses can **list their web services**.
  - Clients can **search for services** they need (like a phone book).
- Example: A company publishes its payment processing web service in UDDI so others can find and use it.
- **Note**: UDDI is less commonly used today; modern APIs rely more on documentation (like Swagger) and API gateways.

### 5. **REST (Representational State Transfer) – Modern Alternative**
While not part of traditional SOAP-based web services, **REST has become a key building block** of modern web services.

- **Architecture style** (not a protocol).
- Uses standard **HTTP methods**: GET, POST, PUT, DELETE.
- Data formats: **JSON** (most common), XML.
- Lightweight and fast — ideal for mobile and web apps.

#### Example (REST API endpoint):

GET https://api.example.com/users/1
Response (JSON):
{
  "id": 1,
  "name": "John Doe"
}

- No need for WSDL or SOAP — uses simple URLs and returns easy-to-parse JSON.