Axoniq’s essential software design concepts

A breakdown of the Introduction course offered by AxonIQ

Patrick Rachford
3 min readJun 28, 2023

I recently took AxonIQ’s course titled Introduction to DDD, CQRS and Event Sourcing. I had minimal experience with DDD and CQRS, but some understanding of Event Sourcing before taking this course. I wanted to take AxonIQ’s course because 1) it was free, and 2) as a technical writer I’m always seeking to learn how others teach and deliver content.

Overview

This course provides a comprehensive understanding of these interconnected concepts and offers building blocks of knowledge for future course.

The course structure is designed effectively to explain complex concepts through practical examples, demonstrating how to implement these concepts in real-world applications.

The course description said it would take approximately 5 hours to complete, and while I may have sped through some sections, it took me 56 minutes to earn my certification of completion from the course. I took extensive notes on the course so that I could back later. But I would say putting aside 1–2 hours for the material to sit in, is recommended.

Introduction

In the introductory part of the course, it outlines key concepts from Eric Evan’s Domain-Driven Design (DDD), highlighting the problems with traditional monolithic applications, which are described metaphorically as “big balls of mud”. The course elaborates on the importance of the Domain Model, which is the heart of an application, and presents an in-depth explanation of terms like ‘Domain’, ‘Model’, and ‘Domain Model’.

Key concepts

The course then delves into key concepts of DDD including Entities and Value Objects, followed by an in-depth explanation of Command-Query Responsibility Separation (CQRS). CQRS is an architectural pattern that separates the system into two parts: one for processing commands and another for providing information (queries). The course explores examples for designing both command and query models and highlights the benefits of CQRS, such as simpler models, data access flexibility, independent evolution, independent performance optimizations, and independent scalability.

Addressing the challenges associated with CQRS, the course discusses concepts like consistency, aggregates, behavior versus state, and complex business processes. It highlights the need for modeling dependencies and proper message modeling to address these challenges effectively.

Event Sourcing

In the segment on Event Sourcing, the course explains the importance of storing every state change that happens to the application to maintain a truthful history of events, requiring an Event Store for persisting these changes. It highlights the technical and business reasons for using Event Sourcing, such as auditing, analytics, conflict resolution, debugging, and replaying into new read models.

Common challenges

The course also presents common challenges when implementing Event Sourcing, including Event Stream Size, Efficient Aggregate Retrieval, Data Correction, Sensitive Data Management, and Event Versioning. Various solutions are proposed to overcome these challenges, including archiving, caching, compensating streams, bounded contexts, out-of-band data storage, and lenient deserialization.

Final sections

The final sections of the course elaborate on using DDD, CQRS, and Event Sourcing together. It highlights the routing of Commands and Queries, location transparency, system evolution from monoliths to micro-services, and domain modeling application design. The course recommends an event storming approach to design applications, where all events are described first and plotted on a timeline. It guides how to identify inputs and outputs, define the commands that trigger the events, identify the screens, and assign aggregates and contexts.

Conclusion

The Introduction to DDD, CQRS and Event Sourcing course offered by Axoniq was an immensely positive and enriching learning experience.

The course excels in not only presenting deep theoretical insights into Domain-Driven Design (DDD), Command-Query Responsibility Separation (CQRS), and Event Sourcing but also in providing practical examples that bring these complex concepts to life.

Its unique strength lies in its detailed exploration of Event Sourcing and its application in modern software development. The course’s engaging approach and thorough explanations make even the most complicated concepts more approachable and understandable, which is immensely beneficial for learners at all levels.

Final thoughts

Whether you’re a seasoned professional or a newcomer to these topics, this course offers invaluable insights and knowledge.

It has been a rewarding journey through the intricacies of DDD, CQRS, and Event Sourcing, and I highly recommend it to anyone seeking a comprehensive understanding of these essential software design concepts.

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Patrick Rachford

Currently at Temporal, previously at AWS. I enjoy documenting my thoughts and sharing ideas with others.