Any of these sound familiar?
Rubbish in rubbish out.
Never trust user input.
If you’ve ever built any sort of professional application you will have had to validate user input. Whether it is to ensure a valid email address or something more complicated. There are tried and tested ‘rules’ for doing this safely while being nice the user.
But here’s a question..
If you issue commands rather than send in models, where should the validation occur?
The most obvious answer is in the domain model.
But this poses a problem…Continue reading
Private fields are not accessible outside the class. It’s C# 101 right?
Which means this code should not work…Continue reading
Making mistakes is part of programming. Spotting them early can save you time. I’ve started to notice a common set of ‘DDD Mistakes’ which many of us seem to make. And yes, I’ve made them all at some point in my career. This is by no means the definitive list – I’m sure there are more.Continue reading
Download this infographic.
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
The nice thing about programming is that everything is black and white.
When designing new features, we all go through a process of discovering requirements. If you are a good developer, you will start to ask questions and probe deeper to find the edge cases. Why? Because you know the edge cases can often take up 80% of the development time. But is that where 80% of the value is?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