++it18 – Channels are Useful, not only for Water

High performance computing made quite some progress lately. Maybe you heard about the reactive manifesto (you should if you are a reader of this blog ;-)), if not, it is an interesting readying … if nothing else as a conversational icebreaker.

Basically the manifesto preaches for systems more responsive and reliable by reacting to requests and dusting it into smaller requests processed by smaller and autonomous systems. This is nothing new, but quite far from traditional processing where single monolitic application took care of requests from reception to response.

There are several programming patterns useful for implementing a reactive system, channels (and streams which are quite the same thing) are one of them. A channel allows you to define a data flow with inputs, computation units, merge, split and collector of information coming in sequence into your system. Once the dataflow is defined, the data flows into it and results come out from the other end.

In this talk Felix Petriconi describes his implementation of channels for C++.

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Protecting you from danger

One of the interesting features of Scala is the trait construct. In Java you have interfaces, a stripped down class that lets you express the requirements for classes that implement it to have a given … interface. Just to keep this from an OO perspective – in a certain system all MovableObjects, must expose an interface to retrieve, say, position, speed and acceleration:

interface MovableObject {
    Point getPosition();
    Vector getSpeed();
    Vector getAcceleration();
}

class Rocket implements MovableObject {
    public Point getPosition() {
        return m_position;
    }

    public Vector getSpeed() {
        return m_speed;
    }

    public Vector getAcceleration() {
        return m_acceleration;
    }
    private Point m_position;
    private Vector m_speed;
    private Vector m_acceleration;
}

In Java the interface has no implementation part, just a list of methods that heir(s) must implement. This is a way to mitigate the lack of multiple inheritance in the Java language – a class can inherit at most from a single base class, but can implement as many interfaces as needed.

Scala traits are different in the sense they can provide an implementation part and fields. Now that I read the wikipedia definition, it looks like that traits are a bit more mathematically sound than interfaces (that’s what you can expect in functional language). Anyway, in plain English, traits are very close to C++ base classes (with some additions that is stuff for another blog post) in the sense that you can both require interface and provide common functionality.

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