#590 add explanation for DTO

data-transfer-object/README.md

@@ -9,8 +9,89 @@ tags:
 ---
 
 ## Intent
-Pass data with multiple attributes in one shot from client to server,
-to avoid multiple calls to remote server. 
+Pass data with multiple attributes in one shot from client to server, to avoid multiple calls to remote server. 
+
+## Explanation
+
+Real world example
+
+> We need to fetch information about customers from remote database. Instead of querying the attributes one at a time, we use DTOs to transfer all the relevant attributes in a single shot.     
+
+In plain words
+
+> Using DTO relevant information can be fetched with a single backend query. 
+
+Wikipedia says
+
+> In the field of programming a data transfer object (DTO) is an object that carries data between processes. The 
+motivation for its use is that communication between processes is usually done resorting to remote interfaces 
+(e.g., web services), where each call is an expensive operation. Because the majority of the cost of each call is 
+related to the round-trip time between the client and the server, one way of reducing the number of calls is to use an 
+object (the DTO) that aggregates the data that would have been transferred by the several calls, but that is served by 
+one call only.
+
+**Programmatic Example**
+
+Let's first introduce our simple customer DTO class.
+
+```java
+public class CustomerDto {
+  private final String id;
+  private final String firstName;
+  private final String lastName;
+
+  public CustomerDto(String id, String firstName, String lastName) {
+    this.id = id;
+    this.firstName = firstName;
+    this.lastName = lastName;
+  }
+
+  public String getId() {
+    return id;
+  }
+
+  public String getFirstName() {
+    return firstName;
+  }
+
+  public String getLastName() {
+    return lastName;
+  }
+}
+```
+
+Customer resource class acts as the server for customer information.
+
+```java
+public class CustomerResource {
+  private List<CustomerDto> customers;
+
+  public CustomerResource(List<CustomerDto> customers) {
+    this.customers = customers;
+  }
+
+  public List<CustomerDto> getAllCustomers() {
+    return customers;
+  }
+
+  public void save(CustomerDto customer) {
+    customers.add(customer);
+  }
+
+  public void delete(String customerId) {
+    customers.removeIf(customer -> customer.getId().equals(customerId));
+  }
+}
+```
+
+Now fetching customer information is easy since we have the DTOs.
+
+```java
+    var allCustomers = customerResource.getAllCustomers();
+    allCustomers.forEach(customer -> LOGGER.info(customer.getFirstName()));
+    // Kelly
+    // Alfonso
+```
 
 ## Class diagram
 ![alt text](./etc/data-transfer-object.urm.png "data-transfer-object")