cqrs-booking

Introduction

This kata is based on the CQRS Booking kata. The aim is to write a simple booking solution using the CQRS architectural pattern. It offers plenty of optional and possible extensions, both on a feature as well as infrastructural level, such as an event and command bus.

Features

The aim is to be able to create a booking solution for one hotel.

The user stories are:

1. As a user I want to book a room. This is a command.
2. As a user I want to see all free rooms. This is a query.
3. As a user I want to remove a booking. This is one more command.

As stated in the introduction, the constraint is that we have to use the CQRS pattern

To do that we will have :

• One create-booking command.

  The booking command contains the following fields:

  client id
  room name
  arrival date
  departure date
  

• One view-bookings query: Room[] freeRooms(arrival: Date, departure: Date).

  The room value object contains one field only:

  room name
  

Suggested approach

Command bus

For the sake of simplicity, start without a command bus first. Eventually, you may opt to implement a command bus as well, e.g. as has been described in this post for Python.

Event sourcing

According to implementing event sourcing with Python, event sourcing is characterized by:

• Keeping your business objects (called aggregates) as a series of replayable events. This is often called an event stream.
• Never delete any events from a system, only appending new ones.
• Using events as the only reliable way of telling in what state a given aggregate is.
• If you need to query data or present them in a table-like format, keep a copy of them in a denormalized format. This is called projection.
• Designing your aggregates to protect certain vital business invariants, such as Order encapsulates costs summary. A good rule of thumb is to keep aggregates as small as possible.

Although you can start the kata without event sourcing at first, we strongly suggest you go with event sourcing right from the start. A way in which to test your aggregates in a TDD manner is very well explained in implementing event sourcing with Python.

Event bus

It is recommended to start without an event bus. Of course, when everything works, you are free to add an event bus as well!