Explore core concepts of Domain Driven Design, its pros and cons, modeling techniques, and hexagonal architecture to transform complex business requirements into effective software solutions.

In this course, you are going to learn the most important concepts of Domain Driven Design or DDD for short. You will see both benefits and drawbacks, techniques to model, how DDD can be implemented using hexagonal architecture and many other amazing concepts.

By the end of this course, you will be able to take complex business requirements and processes and turn them into meaningful software.

Domain Driven Design Made Easy

A user may want to know what happens in the system when something is executed in a domain and an event is produced. Queries enable the user to receive information from a system, with regards to actions and events that occur in a domain.

# What are queries? 
Queries are third in order of the three activities that can occur in a domain, and it is part of the **Command Query Responsibility Segregation (CQRS) pattern**. They represent the requests that a user makes to receive information about a domain. Typically, a system exposes some interface, which enables us to look for information. A query should always return a `h` response with information when it exists.

It is common to implement systems oriented to **CRUD**. They execute the create, read, update, and delete operations. In the case of queries, they are oriented to Read. The system state should not change when a query is executed.

## Queries are idempotent 
Queries should be idempotent. This means that queries should not change the state of the system. In other words, no matter how many times a user requests information, the system should always return the same information. The responses must be the same, as long as there are no changes in between the queries. When a command is executed between two queries, it will cause the second query to return a different response. Let us understand this behavior with an example. 

The following image shows how two queries are exactly the same when there are no changes in the state between them. Additionally, they do not change the state either:

Learn how queries enable a user to receive information from a domain.

Introduction

Mapping Domain to Model

Domain Activities

Domain Objects

Domain Abstractions

Hexagonal Architecture

Conclusion

Queries

What are queries?