Oh the irony.
So, just to be clear …
… a CQRS command is not the same as the Gang Of Four Command Pattern.
But what are the differences and how best to use them? This is what I’ll cover in this post.
Where do you put code for sending emails? Sounds simple right? The funny thing is that if you’re adding it to a CQRS system it can be a little tricky.
It all depends on when you send them. Too early and other processes may fail and you end up sending your email half cocked. And don’t forget event replay. Could be a bit embarrassing re-sending all the emails since your app launched. I was a hairs whisker away from doing just that once!
So, in a CQRS ES system, where do you put the code to send the emails?Continue reading
Events are at the heart of a CQRS Event Sourced system. Which is why changing or upgrading them can be problematic. In this post I’m going to cover a few principles to bear in mind, which should help you avoid hitting the rocks. Before I dive into ‘how to upgrade CQRS events’ I’m going to recap the role Events play in the system.Continue reading
Ok, so you have two event sourced aggregate roots. You need to call a method in one from the other. How do you do it?
If you find your self asking this question, don’t worry, your not alone. It comes up a lot.
The short answer – it suggests there could be something wrong with your aggregate root (AR) boundaries.Continue reading
An aggregate root is at the heart of your domain. I am going to dissect a simple implementation of an Aggregate Root and reveal how it works in a CQRS and Event Sourced system. Before we dive in, we need to fly through some terms often used in DDD.Continue reading
Ever tried to hold a writhing slippery eel in your hands? In case you haven’t, its hard. Building a UI for a CQRS system needn’t be that tricky but you do need to contend with some interesting challenges:
But what is ‘eventual consistency’? ‘Eventual consistency’ is when a read of the data store may return stale results. That is a command has completed but the data store is not yet reflecting the change. This state in a typical line of business application can last for ten’s to the mid hundreds of milliseconds. The actual time is dependant on the specifics of the application. This presents a challenge for designing the user experience. Here are 4 possible approaches to solving this issue.Continue reading
When starting out with CQRS, designing the ‘events’ is not always obvious or easy. The more experienced you are, the more likely your habits will lead you astray. This post will help you spot the problems early and stay on the right track.Continue reading
One of the most liberating things about the CQRS ES way, is the simplicity of the read side. However, if you are new to CQRS you may have wondered how it works. How for example, would it work for a classic master details view? You are not alone, in fact, I am writing this post in answer to exactly this question I found on stackoverflow:
“So my question is how are people handling a screen where by for example it displays a summary of customer details and then a list of their orders with a [more detail] link etc….”Continue reading
A common issue I see is understanding the flow of commands, events and queries within a typical CQRS ES based system. The following post is designed to clear up what happens at each step. Hopefully this will help you to reason about your code and what each part does.Continue reading