user962206 user962206 - 3 months ago 39
Scala Question

What is Event Driven Concurrency?

I am starting to learn Scala and functional programming. I was reading the book !Programming scala: Tackle Multi-Core Complexity on the Java Virtual Machine". Upon the first chapter I've seen the word Event-Driven concurrency and Actor model. Before I continue reading this book I want to have an idea about Event-Driven concurrency or Actor Model.

What is Event-Driven concurrency, and how is it related to Actor Model?

Answer

An Event Driven programming model involves registering code to be run when a given event fires. An example is, instead of calling a method that returns some data from a database:

val user = db.getUser(1)
println(user.name)

You could instead register a callback to be run when the data is ready:

db.getUser(1, u => println(u.name))

In the first example, no concurrency was happening; The current thread would block until db.getUser(1) returned data from the database. In the second example db.getUser would return immediately and carry on executing the next code in the program. In parallel to this, the callback u => println(u.name) will be executed at some point in the future.

Some people prefer the second approach as it doesn't mean memory hungry Threads are needlessly sat around waiting for slow I/O to return.

The Actor Model is an example of how Event-Driven concepts can be used to help the programmer easily write concurrent programs.

From a super high level, Actors are objects that define a series of Event Driven message handlers that get fired when the Actor receives messages. In Akka, each instance of an Actor is single Threaded, however when many of these Actors are put together they create a system with concurrency.

For example, Actor A could send messages to Actor B and C in parallel. Actor B and C could fire messages back to Actor A. Actor A would have message handlers to receive these messages and behave as desired.

To learn more about the Actor model I would recommend reading the Akka documentation. It is really well written: http://doc.akka.io/docs/akka/2.1.4/

There is also lot's of good documentation around the web about Event Driven Concurrency that us much more detailed than what I've written here. http://berb.github.io/diploma-thesis/original/055_events.html