CQS_and_CQRS

CQS_and_CQRS.cs

// CQS (Command-Query Separation)
// Bertrand Meyer devised the CQS principle
// It states that every method should either be a command that performs an action, or a 
// query that returns data to the caller, but not both. In other words, asking a question 
// should not change the answer. More formally, methods should return a value only if they 
// are referentially transparent and hence possess no side effects.
//
// Example: 

public class CustomerService
{
    // Commands
    void MakeCustomerPreferred(CustomerId) 
    void ChangeCustomerLocale(CustomerId, NewLocale) 
    void CreateCustomer(Customer) 
    void EditCustomerDetails(CustomerDetails)

    // Queries
    Customer GetCustomer(CustomerId) 
    CustomerSet GetCustomersWithName(Name) 
    CustomerSet GetPreferredCustomers()
}

// CQRS (Command-Query Responsibility Segregation)
// Defined by Greg Young.
// CQRS is simply the creation of two objects where there was previously only one
// http://codebetter.com/gregyoung/2010/02/16/cqrs-task-based-uis-event-sourcing-agh/
//
// Example: 

public class CustomerWriteService
{
    // Commands
    void MakeCustomerPreferred(CustomerId) 
    void ChangeCustomerLocale(CustomerId, NewLocale) 
    void CreateCustomer(Customer) 
    void EditCustomerDetails(CustomerDetails)
}

public class CustomerReadService
{
    // Queries
    Customer GetCustomer(CustomerId) 
    CustomerSet GetCustomersWithName(Name) 
    CustomerSet GetPreferredCustomers()
}

Interfaces, surely! (Couldn't resist.)