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By Ben, on April 20th, 2009
This year, 2009, we celebrate the 50th anniversary of a landmark moment for (Dutch) computing science: it is 50 years ago this year that Edsger Wybe Dijkstra, The Netherlands’ most famous computing scientist, published his Shortest Path Algorithm in Numerische Mathematik, the German journal of numerical mathematics.
The shortest path algorithm is a solution to a problem from graph mathematics. A graph is a mathematical construct consisting of vertices (or nodes) connected by edges (or sides). Each path has a value (a length, a cost, etc.). The object of the problem is to find the shortes path through the graph from one point to a certain other point. And just in case that sounds like abstract mathematical nonsense that nobody will ever have use for: this problem and its solutions have been applied to network routing, traffic pattern regulation and worldwide distribution of goods. For those of you who drive cars: next time you get in you car, take a look at your satnav — then say “thank you Edsger”.
Dijkstra’s algorithm is not the first solution to this problem. However, his solution differed from the solutions presented before in that it is simple and straightforward (most textbooks and professors covering the subject even remark that it is strange that nobody came up with this easy solution sooner). It is also one of the first solutions that was designed with implementation on a computer in mind, so it is optimal with respect to running time and storage need. This in turn undoubtedly explains its pervasiveness throughout computing even (or only?) 50 years after its invention. It’s also why Dijkstra is most famous internationlly for this algorithm; even to the point of being insulting, because it is really one of the least impressive results of his career, which literally lifted computing science from trial-and-error tinkering to a science.
Dijkstra’s career focussed mostly on developing the science of computing science, by developing techniques for constructing provably correct solutions to problems by constructing the solutions hand in hand with the proof of correctness. Dijkstra was a great believer in computing science’s mathematical roots because exactly those roots yield the properties that computing science so desperately needs: correctness, accuracy and elegance. One of Dijkstra’s axioms in fact is that Beauty is our Business. So in that spirit (and, I hope, in Dijkstra’s spirit as well) I would like to present this birthday salute to Dijkstra’s algorithm – and to the man himself as well: a construction of Dijkstra’s Algorithm.
Continue reading Dutch Direct: Celebrating the 50th anniversary of Dijkstra’s Shortest Path algorithm
By jettro, on April 13th, 2009
 The complete story of combining a lot of different technologies and a lot of different blog post and articles found on the web. There are multiple good posts around this topic, but none of them gave the exact solution I needed. Therefore I write this blog post, maybe someone else can use this as well.
 To get an idea if this post is about the approach you need, let me describe the problem or challenge I am facing.
I have an application that is showing the current stock supply of an online shop. Because obtaining all stock items takes a few seconds, I want to notify users when stock changes. To be able to do this I want to communicate changes from a java server using spring technology to a flex client through BlazeDS. The amount of users is not very big, and the changes to the stock are not important when someone obtains all stock items. Therefore the messages do not need to be persisted and we can use an in memory message server implementation provided by ActiveMq. So the flow becomes:
- Spring sends a message to a topic.
- BlazeDS pics up the message and transforms the stock payload
- All flex clients gets notified of the message
Go on to read about the solution.
Continue reading Doing flex with JMS: combining BlazeDS, spring-jms and ActiveMQ
By jettro, on April 8th, 2009
 This is a short post about the new java support for google appengine. The Basic steps are described very well at google, check the getting started manual. Here I just describe my experiences with my first java based application for google appengine. I concentrate on the usage of intellij. In coming posts I will create the same application as the one I used for my previous post: using google appengine.
Continue reading Java on Google app engine with intelliJ
By Allard, on April 6th, 2009
As many other developers, I’ve been used to the fat service layer and the anemic domain model of the transaction script pattern. In that programming model, immutability is pretty much as rare as a Dodo. However, I have been investigating the rich domain model pattern lately (as you can read in my previous post) and more importantly, a good migration path for “transaction script” developers to get acquainted with the rich Domain Model, a design pattern that has been heavily underrated (and misunderstood) by many.
In this post, I will explain some of the advantages that immutable domain objects bring us, while showing that some of the seemingly problematic side effects aren’t really that problematic.
Continue reading The power of immutability in a Rich Domain Model
By Ben, on April 5th, 2009
This year, 2009, is the 200th anniversary of Charles Darwin‘s birth and the 150th anniversary of the first publication of On the Origin of Species. In this famous publication Darwin launched his notion of how species’ survival is dictated by their ability to make the most efficient use of the available resources in their environment, how they adapt to increase that efficiency and how the combination of increased procreation and targeted adaptation mean that the most adapted and adaptable species survive longest.
Much like species of plants and animals, the software we build for our customers (and the software built by our predecessors) lives in an ever-changing environment. Business needs change and expand, fashions change, economic booms, lulls and the occasional crisis follow one another in an endless tide. Software systems face their own survival challenges, dealing with all these changes. And much like their biological counterparts it is their adaptability that rules their survival.
Continue reading On the Origin of Technologies
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