Event-driven architecture 101

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I’m still a student, so my point of view could be far from reality, be gentle ;)

tl;dr: Queue messaging are cool. Use them at the core of your architecture.


I’m currently playing a lot around Kafka and Flink at work. I also discovered Vert.x at my local JUG. All three have a common word: events. Event-driven architecture is not something that I learned at school, and I think that’s a shame. It’s really powerful and useful, especially in a world where we speak more and more about “serverless” and “micro services” stuff. So here’s my attempt to make a big sum-up.


the Unix philosophy

I’m a huge fan of GNU/Linux. I just love my terminal. It’s been difficult at the beginning, but now, I consider myself fluent with it. My favorite feature ? Pipes or |. For those who don’t know, it’s the ability to pass the result of the command to another command. For example, to count how many files you have in a folder, you’ll find yourself doing something like this:

  • list files in a folder
  • From this list, manipulate/filter it. One line must correspond to one file, things like folder are omitted
  • And then count the line!

In the UNIX world, it should give you something like “ls -l | grep ^- | wc -l”. it might feels like chinese. For me, it’s just feels logical. 3 operations mapped into 3 commands. You declare a set a commands that, in the end, give you the result. It’s simple and also very fast (in fact, you can find funny articles like this one: Command-line tools can be 235x faster than your Hadoop cluster). This is only possible thanks to the UNIX philosophy, greatly describe by Doug McIlroy, Elliot Pinson and Berk Tague in 1978:

Make each program do one thing well. To do a new job, build afresh rather than complicate old programs by adding new “features”.
Expect the output of every program to become the input to another, as yet unknown, program.

Why should I care? It’s 2016, not 1978! Well…


Back in 2016

Cloud changed everything in terms of software engineering. We can now deploy applications without thinking about the underlying server. How cool is that? Let’s take some steps back. Now that you can easily deploy a huge application, what can be accomplished? Well, if I can deploy one app with ease, Why should I deploy only one huge app ? why can’t I deploy multiples applications instead of one? Let’s call theses applications micro services because we are in 2016.

OK, so now I’m applying the first rule of the UNIX Philosophy, because I have multiples programs that are doing one job each. But about the second rule? How can they communicate? How can we simulate UNIX pipes? Before answering, let’s answer to another question first: What do we really need to send through our network? Don’t forget the Fallacies of distributed computing

Let’s take an example. We are a new startup, and we are building our plateform. We’ll certainly need to handle our customers. Let’s say that for each new customer, we need to make two actions: add it to our database, and then to our mailing-list. A simple and classical way would be to just call two functions (whether on the same applications or not), and then say to the customer: “You’re successfully registered”. Like this:

Classic approach

Is there another approach? Let’s use an event-based architecture:

Let’s talk events

Let’s ask Google, what’s an event?

a thing that happens, especially one of importance.

Well, handling a new customer is a thing that happens (hopefully). For this, we’ll be using a Queue messaging system or Broker. It’s a middleware that will receive events, and making them available for another application or groups of applications.

Queue messaging architecture with 2 producers and 4 consumers

So let’s rethink our architecture. Pay attention to the words: our Register page will produce an event that will contains all the information about our client. This event will be queued, waiting to be consumed by the associated micro services.

Simple event-driven architecture

We didn’t changed much, but we enable many things over here:

  • Simplicity. Remember, the first rule ! “Make each program do one thing well”. Like this, your code base for each app will be simple as hell, and you’ll be able to easily replace your software if needed.
  • Modularity. You need to add another action to the event, for example CreateProfile ? Easy, just plug another app on the same queue. You need to test a new version of your program? Easy, just plug it on the same queue.
  • Scalability. One of your micro services is taking too much time? Just start a new instance of it. Huge traffic? Add new instances. With this approach, you can start really small and become giant.
  • Big-data friendly. This type of architecture is often used to handle a lot of data. With plateform like Apache Flink, you can do some stream processing directly. Look how easy it is.
  • Polyglotism. Most messaging system are offering libraries for many languages.Like this, you can use whatever language you want . But be aware, With great power comes great responsibility.

What about serverless?

Serverless is so 2016

Serverless is the “new” buzz word. Ignited by Amazon with their product AWS Lambda and quickly followed by Google, Microsoft, IBM and Iron.io, the goal is to offer to developers a new way of building apps. Instead of writing apps, you’ll just write a function that will respond to an event. In fact, you’ll be paying only for the time it’s running. It’s a interesting point-of-view, because you’ll be deploying an architecture built only using events. I must admit that I didn’t try it yet, but I think it’s a great idea to force developers to split their apps and really think about events, but you could just build the same thing with any cloud provider.

Additional links and talks about this topic

Meetup LeboncoinTech — AMQP 101 by Quentin ADAM (French sorry)
vert.x 3 — be reactive on the JVM but not only in Java by Clement Escoffier/Paulo Lopes DEVOXX 2015

Please, feel free to react to this article, you can reach me on Twitter, or have a look on my website.