Queries with Mediator and Command Patterns

Queries with Mediator and Command PatternsI recently got a really great comment from my post on using Query Objects instead of Repositories.  Although that blog post is now 2 years old, I still use the same concepts today in my applications.

What I thought may be relevant is to elaborate more on why and when I use the mediator and command pattern for the query side.

It may seem obvious on the command side, but not really needed on the query side.

Here a portion of the comment from Chris:

I’m struggling a bit to see the killer reason for using Query objects over repository. I can see the benefits of CQS but it’s this command pattern type implementation of the Q part I struggle with. I can see for Commands (mutators) the benefits of having separate handlers as you can use decorator pattern to wrap them with additional functionality e.g. Transaction, Audit etc.

However for queries I’m struggling to see why you wouldn’t just use a normal repository. Essentially your IQuery object defined what would be a method signature in the repository and the handler defines the implementation of the method. However at some point you have to compose the IQuery class to it’s handler either using a dependency injection framework or Mediator pattern as in your following blog.

Query Objects

The primary place I use query objects (mediator + command pattern) is when I want to be decoupled from an integration boundary.  Often times this is between my application and the web framework I’m using.

I view my query objects as my public API.

Which means I usually want to create a pipeline for those queries.  I will use a request pipeline for handling things such as authorization, validation and logging in the query pipeline.

Since this is the same implementation I use on the command side, it’s easy to use with library like MediatR which handles both commands and queries as a “Request”.

Repositories

I still use repositories, but just very differently than I did before.  Since my query objects are my public API, my repositories are generally only used internally within a the core of a given application (or bounded context).

My repositories are usually pretty narrow and also handle things like caching.

CQRS

There are many ways to implement the separation between reads and rights.  I’ve failed at making this point clear in many of my prior posts.

I’ve had a few encounters recently that make me feel like there’s still a lot of misconceptions about what people think CQRS is.  I’ll keep posting quote from Greg Young.

CQRS is simply the creation of two objects where there was previously only one.

The separation occurs based upon whether the methods are a command or a query (the same definition that is used by Meyer in Command and Query Separation, a command is any method that mutates state and a query is any method that returns a value).

That’s all it is folks.  Not really that interesting.  What’s interesting are all the possibilities because of this simple idea.

How you implement that is up to you.

One of the ways I’ve been implementing this is with the Mediator + Command Patterns.  It’s not the only way!

Comments

How are you implementing the query side?  Always enjoy hear your feedback, commands and questions.  Please let me know on twitter or in the comments.


 

Developing Features not Layers

Developing and thinking about features not layers is something I’ve moved towards over the last several years.

I’ve mentioned it in several blog posts and I don’t think I ever explicitly created a post about it.

CQRS

The real enabler has been CQRS.  For those unfamiliar with CQRS or if you think it’s complicated, let me share a quote from Greg Young:

CQRS is simply the creation of two objects where there was previously only one.

The separation occurs based upon whether the methods are a command or a query (the same definition that is used by Meyer in Command and Query Separation, a command is any method that mutates state and a query is any method that returns a value).

That’s it.  It’s nothing more complicated than that.

I think where the confusion lies is a lot of articles, blog posts, etc where the content describes much more than CQRS.

CQRS is not event sourcing.

CQRS is not domain driven design.

CQRS does not mean multiple data stores.

CQRS does not mean using a service bus.

CQRS does mean having eventual consistency.

CQRS is simply the creation of two objects where there was previously only one.

Features vs Layers

For me a feature in the technical sense is a command or a query.  It could also be a combination of a few put together.

I often have commands and queries dictate on their own how they are layered inside.

This means that an individual command decides how it may do data access.  It may not be a shared concern between another command.

For comparison, here’s is how I visualize the difference.

Layered

Layered

In the layered approach, we separate and organize our code by technical concerns.  Authentication, Business Logic Layer (BLL) and Data Access Layer (DAL) are all contained.  Once layer invokes the next.

CQRS

Features not Layers

The difference in our CQRS approach is that our Commands and Queries contain the layers of technical concerns within them.  We don’t have layers spanning over the entire application.

This has some pretty profound implications in terms of being able to make different decisions in smaller units.

As well the biggest distinction is we can start thinking, developing and organize our code by business concerns rather than technical concern.

Organizing by Feature

If you want to see how this works in practice, I’ve blogged about writing your code this way.  Most of my sample code uses the MediatR library for handling things like Commands and Queries.

I recommend checking out my Fat Controller CQRS Diet: Vertical Slices post which demonstrates how to do this in ASP.NET MVC Core.

Testing

One aspect that took longer to come around was testing in the same manner.

Meaning, I start writing tests and thinking about them around my features rather than layers within a feature.

My new few series of posts will be around testing features / vertical slices.

I love hearing your comments, questions and recommendations as it usually leads me down to other blog posts.  Please post a comments or on Twitter.


MicroBus: In-Process Mediator

I’ve blogged about the mediator pattern a lot.  Primarily because it’s been a good fit in several of the applications I’ve developed over the last few years.

Mediator Pattern

For those completely unfamiliar with the mediator pattern, here’s a brief summary:

With the mediator pattern, communication between objects is encapsulated with a mediator object. Objects no longer communicate directly with each other, but instead communicate through the mediator. This reduces the dependencies between communicating objects, thereby lowering the coupling.

Libraries

I’ve written my own implementations of the mediator library from project to project.  Once I finally found the MediatR library, I’ve pretty much been using it instead of my own.

In a recent post about MediatR v3’s new Behaviors, Daniel Little commented about his implementation called MicroBus.

I love checking out new libraries to see what’s out there.  I’ve gotten a lot of feedback about other OSS libraries and tools I’ve blogged about.

I figured this was a good opportunity to mention library that look pretty interesting and may be worth taking a deeper look at.

MicroBus

This library actually looks more like what I’ve built myself in the past.

The first thing to notice is there are two separate interfaces for defining a Command or a Query.

There isn’t anything surprising when you look at implement your handlers.  It’s pretty straight forward and what you would expect.  Familiar things are nice.

Cross Cutting Handlers

What’s really interesting is how it handles cross cutting concerns.  You can create global handlers via the IDelegatedHandler interface or create a pipeline with IPipeline interface.

BusBuilder

Another interesting aspect is how you build up registering your handlers.  There is built in support for Autofac

Comments

From the looks of it MicroBus looks like a pretty nice implementation of the mediator pattern with a lot of features people are looking for such as pipelines.

Check it out on GitHub.  It is a very active project (as of this posting).

If you want to see a real world demo, possibly of my MusicStore sample app but using MicroBus, please let me know!

If you have another library you would like me to blog about feel please let me know in the comments or on Twitter.