Pinned: List of CQRS / Event Sourcing Resources

CQRS Pattern

CQRS stands for Command Query Responsibility Segregation. As the name suggests, we split the application in two parts: Command-Side and Query-Side. Now one will ask what this command and query means? Okay, so –

• Commands are the ones that change the state of the object or entity, also called modifiers or mutators.
• Queries are those that returns the state of the entity, and do not change anything. The another term for that will be accessors.

Why is it required?

In traditional data management systems, both commands and queries are executed against the same set of entities, having a single reprsentation or view. The CRUD operations are applied to a single datastore, the same entity or object is accessed to handle both read and write operations.

Issues with having single view for both read and write side –

• Introduces the risks of data contention.
• Managing permissions and security become complex as same objects are exposed to both read and write operations.

How CQRS solves this problem?

The CQRS pattern holds the idea that the method should either change the state of the entity, or returns the result, but not both. Segregating models for read and write side reduces the complexity that comes with having single view for both of them.

Benefits of CQRS –

• Separate command and query models, results in simplified design and implementation of the system and overall reduction of complexity.
• One can easily optimise the read side of the system separately from the write side, allows scaling each differently as per the load on the side. For example – Read datastores often encounters greater load, and hence can be scaled without affecting the write datastores.
• You can provide multiple views of your data for querying purpose depending on the use cases.

How CQRS works?

CQRS is mainly used in conjuction with Event Sourcing. The write side model of the CQRS-based system handles the events persistence, acting as a source of information for the read side. The read model of the system provides materialized views of the data, typically as highly denormalized views.

Another Representation

Example Implementation in C#

https://github.com/Odonno/cqrs-dotnet-core-example

https://www.programmingwithwolfgang.com/cqrs-in-asp-net-core-3-1/

https://www.codeproject.com/Articles/555855/Introduction-to-CQRS

Click to access cqrs_documents.pdf

References & additional resources:

http://microservices.io/patterns/data/cqrs.html

https://microservices.io/patterns/data/cqrs.html

https://www.codeproject.com/Articles/555855/Introduction-to-CQRS

https://docs.microsoft.com/en-us/azure/architecture/patterns/cqrs

The Most Easy to Use ViewModeBase

This is something I wrote for an application I’m developing. This is an implementation of INotifyPropertyChanged that requires no backing fields, just call Set(value) or Get(). Internally it uses a dictionary to store the state and even reuses ChangedEventArgs

The source code can be found at:

https://github.com/ebalynn/StatefulViewModel/

Here is the extract from the ViewModel class that does all the heavy lifting:

Worker Services in .NET Core 3

Note to myself – Excellent article on the worker services in .NET Core 3:

Introduction to Worker Services in .NET Core 3.0

Creating your own primitive type

It’s a very rare requirement, but sometimes in .NET you have to create your own primitive and make it behave as close as possible to a native CTS (common type system) type. “That shouldn’t be hard” would be your first thought, until you start considering all the scenarios in which it could be used.

Does the usage of Thread.Sleep(…) warrant making the method Async in an API

Say you are developing an API where you have to use Thread.Sleep(…) since you are working with some device via a COM and you need to wait for the predefined amount of time before you can read from the device and there is no way around it. For example:

Rule in designing APIs in my head, is that you shouldn’t make something Async unless the underlying API(s) you are working with is also Async or uses BeginX EndX, continuations, i.e. something that is already asynchronous. By prematurely making something Async you are making an implementation decision for the consumer of the code. The code provided above doesn’t expose any such API.

C# Regrets: Top Worst C# Features

Stumbled upon this article written by no other than Eric Lippert listing the top 10 design faults of C# language. Here is the summary,  the source to the full article is at the bottom. 

#10: The empty statement does nothing for me

Reflects on the fact that lone “;” is a legal statement

#9: Too much equality

There are too many ways check for equality: ==, Equals, ReferenceEquals, CompareTo(…).

From personal experience double.NaN == double.NaN is false but double.NaN.Equals(double.NaN) is true

#8: That operator is shifty

Weirdness around << and >> operators

#7: I’m a proud member of lambda lambda lambda

The way C# 2.0 implements anonymous delegates

#6: Bit twiddling entails parentheses

Flags Enums

#5: Type first, ask questions later

C# borrows the “type first” pattern from C and many of its other successor languages – something I got used to and the “correct” way now seems illogical to me

#4: Flag me down

The fact that you can create invalid enum values and have to manually check for this in the code

#3: I rate plus-plus a minus-minus

++i, i++, i +=1 etc. how much confusion and the pain it caused.

#2 I want to destruct finalizers

Agree with the author that finilisers in C# are symptoms of a bug. Seen it way too many times myself.

#1 You can’t put a tiger in the goldfish tank, but you can try

“array covariance” and how this could lead to run-time exceptions.

Source: http://www.informit.com/articles/article.aspx?p=2425867

AForge.Net

Definitely something to play with when my brain has more oxygen (suffering from anaemia right now)

Deneme 1 2 3!

AForge.NET  is a C# framework designed for developers and researchers in the fields of Computer Vision and Artificial Intelligence – image processing, neural networks, genetic algorithms, machine learning, robotics, etc.

• AForge.Imaging – library with image processing routines and filters;
• AForge.Vision – computer vision library;
• AForge.Neuro – neural networks computation library;
• AForge.Genetic – evolution programming library;
• AForge.Fuzzy – fuzzy computations library;
• AForge.MachineLearning – machine learning library;
• AForge.Robotics – library providing support of some robotics kits;
• AForge.Video – set of libraries for video processing
• etc.

are mainly topic in this framework. İf you want more information and help  visit: https://code.google.com/p/aforge/

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This sort of code makes me very angy!

Tell me what’s wrong with this simple piece of code:

public class MyService : IMyService, IDisposable 
{
    private readonly IOtherService _otherService;
    public MyService(IOtherService otherService)
    {
        _otherService = otherService;
    }

    public void Dispose()
    {
       _otherService.Dispose();
    }
}

The answer is very simple: “I created you and I will be your end”, yet MyService didn’t create IOtherService, thus it has not right to call Dispose() on the service.”

Yet, at Mizuho, I saw many developers lacking this simple, yet critical design concept!

</rant>

Object Oriented, Test Driven Design in C# and Java: A Practical Example Part #1

Download the code in C#

For a brief overview, please refer to this post.

At this point, many tutorials start by launching into a “Hello, World” style tutorial, with very little practical basis.

This isn’t the most exciting concept, so let’s try a more practical example. Instead of churning out boring pleasantries, our application is going to do something a bit more interesting…build robots.

Specifically, our application is going to build awesome robots with big guns.

Ok, let’s get started with a narrative description of what we’re going to do.

“Mechs with Big Guns” is a factory that produces large, robotic vehicles designed to shoot other large, robotic vehicles. Robots are composed of several robotic parts, delivered by suppliers. Parts are loaded into a delivery bay, and are transported by worker drones to various rooms; functional parts such as arms, legs, etc., are dispatched to an assembly room. Guns…

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SOLID principle

Chanmingman's Blog

There is a lot of debate on SOLID principle.

The video Applying SOLID Principles in .NET   (http://channel9.msdn.com/events/TechEd/Europe/2014/DEV-B210) is worth to watch.

I like the last slide Consideration

As he mentioned.

1. Someone just over engineering using the term SOLID

2. How readable for your Dependency Injection if you have large set of object.

3. When talking about maintainable, before SOLID is 50 lines of code, after SOLID is 230 lines of code previewing the same functionality. So you think 50 lines of code is easier to maintain or 230?

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