How to Unit Test Akka.NET Actors with Akka.TestKit

An Introduction to the Akka.TestKit

In this post we introduce the Akka.TestKit module - a library that makes it easy to unit test Akka.NET actors using all of the popular .NET unit testing frameworks.

A Brief End-to-End `Akka.TestKit` Example

Before going deep into the TestKit, here’s an end-to-end example of what a test usually looks like.

[TestFixture] //using NUnit
public class UserIdentityActorSpecs : TestKit{

    [Test]
    public void UserIdentityActor_should_confirm_user_creation_success()
    {
        var identity = Sys.ActorOf(Props.Create(() => new UserIdentityActor()));
        identity.Tell(new UserIdentityActor.CreateUserWithValidUserInfo());
        var result = ExpectMsg<UserIdentityActor.OperationResult>().Successful;
        Assert.True(result);
    }

}

80% of your tests will be this simple: create an actor, send it a message, and expect a response back. Let’s explore how to do it.

The concepts in this post were first introduced in a talk at the 2015 Cassandra Summit.

I’ve been a .NET developer for roughly 10 years now - since the summer after my freshman year in college in 2005 I’ve been developing in Visual Studio and .NET. I’ve founded three startups on .NET, worked for Microsoft, and founded multiple successful OSS projects in .NET - I say all of this in evidence to the depth of my commitment and investment in the .NET ecosystem.

But it’s time we, the .NET community, address a major elephant in the room - .NET is, and often has been, way behind in other ecosystems in terms of overall innovation, openness to new ideas, and flexibility.

The Experience of Being a .NET Developer

.NET has been, historically, an expensive ecosystem to play in.

Because to play in it, you have to do so by Microsoft’s rules.

It's Ballmer's World, Bro

You have to buy a license for Visual Studio, or more likely, an MSDN subscription.
You have to buy a license for Windows Server.
You have to buy a license for Windows Server.
You have to develop your web applications in a framework built by Microsoft, like ASP.NET or WCF.
You have to host your web applications in IIS.

And the list goes on - the point being that our entire way of developing our own products depends on a single company in Redmond choosing what tools it wants to make available to us at any given time and price.

The Impact of Marrying .NET to Microsoft

I want to share with you a little story from a point in my career with Microsoft during 2010-2011. By 2011 HTML5 had become mostly standardized and its...

How to Stop an Actor... the Right Way

Stopping an actor is a routine operation that developers often find confusing. This is because each actor is like a micro process, and all interaction with an actor is asynchronous. So shutting down an actor is more complex than stopping entities in procedural code.

In this post, I’m going to review one of the common questions that I’ve heard lately:

How do I stop an actor?!

The Ways of Stopping an Actor

In short, there are three ways to stop an actor:

Stop() the actor: stops the actor immediately after it finishes processing the current message.
Kill the actor: this throws an ActorKilledException which will be logged and handled. The actor will stop immediately after it finishes processing the current message.
Send the actor a PoisonPill: the actor will finish processing the messages currently in its mailbox, and then Stop.

Now let’s go over each in detail.

1) The Default: `Stop()` an Actor

This is the go-to method to stop an actor, and should be your default approach.

What Happens When I `Stop()` an Actor?

This is the sequence of events when you Stop() an actor:

Actor receives the Stop message and suspends the actor’s Mailbox.
Actor tells all its children to Stop. Stop messages propagate down the hierarchy below the actor.
Actor waits for all children to stop.
Actor calls PostStop lifecycle hook method for resource cleanup.
Actor shuts down.

The point of this sequence is to make sure that an actor—and any hierarchy beneath it—have a clean shut down.

How Do I Use `Stop()`?

You Stop() an actor...

The Top 7 Mistakes Newbies Make with Akka.NET

The actor model is a radically new concept for the majority of Akka.NET users, and therein lies some challenges. In this post we’re going to outline some of the common mistakes we see from beginners all the time when we take Akka.NET questions in Gitter chat, StackOverflow, and in our emails.

7. Making message classes mutable

One of the fundamental principles of designing actor-based systems is to make 100% of all message classes immutable, meaning that once you allocate an instance of that object its state can never be modified again.

The reason why immutable messages are crucial is because you can send the same message to 1000 actors concurrently and if each one of those actors makes a modification to the state of the message, all of those state changes are local to each actor.

There are no side effects to other actors when one actor modifies a message, which is why you should never need to use a lock inside an actor!

For example, this is an immutable message class:

public class Foo{ public Foo(string name, ReadOnlyList<int> points){ Name = name; Points = points; } public string Name {get; private set;} public ReadOnlyList<int> Points {get; private set;} } 

This class is immutable because:

string is an immutable class - any modifications to it produce an entirely new string. The original is never modified, so to all of the other actors processing this Read more




How to Integrate Akka.NET and ASP.NET (and Nancy!)


 NOTE: A lot has changed in both the .NET and Akka.NET ecosystems since this post was originally written in 2015 and we have since released an updated blog post: “Best Practices for Integrating Akka.NET with ASP.NET Core and SignalR”.
 
 Lots of folks have been asking about Akka.NET and ASP.NET MVC integration on StackOverflow and in our Gitter chat room, so we thought it was time we created the definitive post on how to integrate these two amazing technologies together.
  Note: Everything in this article also applies to Web API, Nancy, WCF, and ASP.NET WebForms.
 
 Use Cases
 So when would you want to use Akka.NET and a web framework like ASP.NET together at the same time?
 Well, you might be like Joel in our “Akka.NET Goes to Wall Street” case study and need to manage concurrent reads / writes to a shared object model.
 If you’re building anything resembling a chat room, web-based game, collaboration software, and more - then congratulations: managing concurrent mutations to shared state is something you’re going to have to do. Akka.NET actors are a tremendously better option than sprinkling your code with locks.
 In general, what most people use Akka.NET for in the context of ASP.NET is to communicate with others network services the ASP.NET app might depend on, such as remote Windows Services via Akka.Remote and Akka.Cluster. This is especially useful if you need to do any sort of stateful web application programming, but that’s a story for a different day.
 How to Start an ActorSystem in ASP.NET
 If you’ve gone through Akka.NET Bootcamp, and you should if you haven’t yet, you know how to start an ActorSystem:
 

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The Inevitable Rise of the Stateful Web Application


Our Akka.NET Virtual Meetup on August 12th was a huge success - we had about 100 users on the livestream with us and the recording of the meetup has been viewed several hundred times since.
One of the topics I covered during my presentation about using Akka.NET in production at MarkedUp was the concept of stateful applications built with actors - the idea that state can reside within your application rather than outside of it by default, and how it was this idea that made our marketing automation product possible to build.
This tweet from an attendee sums up the realization well:
@Aaronontheweb A key take away for me is Actors are stateful and I do not have to get/post constantly from Redis. Wow that is liberating.
— akshay123 (@akshay123) August 12, 2015

Exactly right! Actors can be stateful - and that means we no longer have to factor round-trip times to SQL Server, Redis, Cassandra, or whatever into the design of our applications. The application already has the state it needs to do its job by the time a request arrives!
The Limitations of Stateless Design
The traditional way of developing web applications is stateless - and that’s a natural consequence of HTTP, itself an inherently stateless protocol.


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Akka.NET Goes to Wall Street


Many people in the Akka.NET community have been asking for case studies over the last few weeks, since we shared the MarkedUp case study. There are a ton of production deployments, but getting actual case studies out always has some lag time.
 To that end, I wanted to share an email case study that I received from Joel Mueller, an Akka.NET community member, that just BLEW MY MIND (reprinted with permission).
 Check out Joel’s story of how Akka.NET changed the trajectory of his business, which is now a part of McGraw Hill—owners of this little thing called Standard & Poor’s—and now proud users of Akka.NET, acquiring SNL Financial for $2 billion:
    Joel Mueller, Software Architect, SNL Financial
 One of the features of a larger product I’ve been working on for years is a budgeting/forecasting module for community banks. Picture an ASP.NET project that is roughly the equivalent of 200-300 interrelated Excel workbooks, using a custom mostly-Excel-compatible formula engine, lots of business rules, lots of back-end queries against both SQL and Analysis Services, and a front-end in SlickGrid that communicates with the back-end over XHR and Web API. The object model for Forecast instances is (was) stored in ASP.NET session state until changes are saved to the application database. That, of course, means one instance per session, even if two people open the same forecast.
 Then, as an afterthought bullet point at the end of a list of other new features being requested, “oh by the way, can you make it work for multiple concurrent users in a single forecast, Google Docs style?”
 After I was done freaking out and yelling at people, I sat down to figure out how...


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Announcing: Akka.NET Virtual Meetup


This is a short update, but an important one.

The first Akka.NET Virtual Meetup will be next Wednesday, August 12 @ 18:30UTC. RSVP Here.

The first ever community-wide meetup for Akka.NET will be taking place next Wednesday, August 12. You can RSVP here.
When is the meetup?
18:30UTC on August 12, 2015. (Click here to see that in your local time zone.) We tried to find a time when people from all over the world could attend, given the geographical diversity of the Akka.NET community.
Why should I come?
I can think of a ton of reasons, but here are three:

To ask questions & swap ideas with other community members
To learn about interesting case studies or use cases for Akka that you don’t already know about
To have connect and have fun with other really smart developers like you!

What will be covered?
This will be a combination of speakers sharing their stories / case studies, and an open forum for Q&A with the core team. In particular, Aaron will be sharing a powerful Akka.NET case study and story. We will also:

providing an update on the state of the project and its trajectory
give timelines for upcoming Akka.NET major releases
have open Q&A for anyone to ask whatever they want!

Where do I join?
RSVP here. Or, if you don’t have a Google account, here is the watch page link.
See you next Wednesday!


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Large Messages and Sockets in Akka.NET


One of the questions that has been coming up a lot lately as many people are building with Akka.Remote is this:
  How big of a message can I send over the network?
 
 I’ve been asked about this four or five times this week alone, so it’s time to put out a blog post and stop re-writing the answer.  This is a great question to cover, as it starts to reveal more about what is going on under the hood with Akka.Remote and the networking layer.
 Up until I worked on Akka.NET, I honestly hadn’t thought much about the networking layer and so this was a fun question for me to dig into and research.
 When Does This Come up?
 This comes up all the time when people have large chunks of data that they need to transmit and process. I’ve been asked about this lately in the context of doing big ETL jobs, running calculations on lab data, web scraping, for video processing, and more. People asking about sending files that range anywhere from 20MB to 5GB.
 All of these contexts involve large amounts of data that need to be processed, but no clear way to link that up with the distributed processing capabilities that Akka.NET provides.
 So what’s a dev to do?
 Here’s the first answer people try: “network-shmetwork! JUST SEND IT!”
 Why This Is A Bad Idea
 This is basically what that does to the network:
 
 That is a python trying to eat an entire pig. Ewwwww. Gross.
 If it could feel, that’s how the network would feel about our large messages.
 What else do people try to do? Here are some of the common approaches I’ve seen:
  crank up...


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Meet the Top Akka.NET Design Patterns


Just the other day, I saw this tweet and knew this post was long overdue:
Is there any list of actor model design patterns ? @AkkaDotNET
— Laurent Lepinay (@foobarcode) June 24, 2015

Quite understandably, people want to know what the design patterns in Akka.NET are! There are quite a few-our team has cataloged 30+ across various areas of the framework so far-but there is also that rarefied set of patterns that show up again, and again, and again.
These patterns cover four broad categories:

Actor Composition
Messaging
Reliability
Testability

Let’s get going.
Actor Composition Patterns
“Actor Composition” patterns are used to create groups of interrelated actors in order to accomplish specific goals. Patterns of this type help you think about how many actors you need, and how to structure the relationships between them in your hierarchy.
These patterns are aimed at allowing you to use Akka.NET’s supervision hierarchy to your advantage in order to achieve maximum reliability and transparency when working with actors. They will also make your architecture more intuitive.

Composition Pattern: Child-Per-Entity
The Child-per-Entity pattern occurs whenever some /parent actor is responsible for a domain of entities and elects to represent each entity as its own actor. The parent maintains a mapping to know which child actor corresponds with which domain entity. The /parent can then lookup/create/kill child actors as domain object enter or leave its going concerns.


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Beyond HTTP

How to Unit Test Akka.NET Actors with Akka.TestKit

An Introduction to the Akka.TestKit

A Brief End-to-End `Akka.TestKit` Example

The New .NET Stack

The Experience of Being a .NET Developer

The Impact of Marrying .NET to Microsoft

How to Stop an Actor... the Right Way

The Ways of Stopping an Actor

1) The Default: `Stop()` an Actor

What Happens When I `Stop()` an Actor?

How Do I Use `Stop()`?

The Top 7 Mistakes Newbies Make with Akka.NET

7. Making message classes mutable

How to Integrate Akka.NET and ASP.NET (and Nancy!)

Use Cases

How to Start an `ActorSystem` in ASP.NET

The Inevitable Rise of the Stateful Web Application

The Limitations of Stateless Design

Akka.NET Goes to Wall Street

Announcing: Akka.NET Virtual Meetup

When is the meetup?

Why should I come?

What will be covered?

Where do I join?

Large Messages and Sockets in Akka.NET

When Does This Come up?

Why This Is A Bad Idea

Meet the Top Akka.NET Design Patterns

Actor Composition Patterns

Composition Pattern: Child-Per-Entity

Petabridge

Beyond HTTP

An Introduction to the Akka.TestKit

A Brief End-to-End Akka.TestKit Example

The Experience of Being a .NET Developer

The Impact of Marrying .NET to Microsoft

The Ways of Stopping an Actor

1) The Default: Stop() an Actor

What Happens When I Stop() an Actor?

How Do I Use Stop()?

7. Making message classes mutable

Use Cases

How to Start an ActorSystem in ASP.NET

The Limitations of Stateless Design

When is the meetup?

Why should I come?

What will be covered?

Where do I join?

When Does This Come up?

Why This Is A Bad Idea

Actor Composition Patterns

Composition Pattern: Child-Per-Entity

Petabridge

A Brief End-to-End `Akka.TestKit` Example

1) The Default: `Stop()` an Actor

What Happens When I `Stop()` an Actor?

How Do I Use `Stop()`?

How to Start an `ActorSystem` in ASP.NET