Month: July 2009

What is cloud computing? Cloud computing main idea explained

The cloud computing is the style of computing which is linearly scalable over network.

There are at least 2 variations of concept:

  1. Client-side cloud computing, which is client-based software which is at the same time client and server to many other instances. Examples are:  Skype, BitTorrent, SETI@home.
  2. Server-side cloud computing, which is automatically scalable systems like Amazon S3 or Google Apps Engine  or Salesforce which provide services on their own platform and hardware and just platforms such as Appistry for deployment on your own hardware.

We will focus on questions and concerns about server-side case.

Server-side cloud computing is when the parts of the stack where your application is running is cloned on demand if load is increasing and freed on load decrease. This is of course mostly about web and business-logic parts of the stack but also for databases. I.e. if you can automatically get more computing resources and/or more scaled database.

This is all, of course, is nothing new and could be implemented with “old” software technologies, but usually all cloud providers supply their own API optimized for cloud computing. Currently most of clouds implementations supports databases, distributed caching and transactions.

Concerns

The main concern is, of course, security, since for cloud-services providers your software is deployed on other-company’s servers. Another concern is that this architecture have issues with short high spikes loads, when it starts to scale after spike.

When to use

  1. If you don’t know what load is going to be for your application;
  2. When you don’t want to invest in your new hardware, clouds will be able to reuse your existing resources or you can rent the service, which will charge you only for used resources;

The anatomy of cloud computing is another good resource on this subject.

Berkeley’s view on the subject.

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Why do I need inheritance in OOP? Real-world examples.

The problem in general is that it is sometimes unclear from books why do we need some particular technology. In this case we are going to discuss why do you need inheritance and where it is used in real-life applications. Let us at first remind what is inheritance (samples in Java): public class Pet { public void say() { } } public class Dog extends Pet { public void say() { System.out.println("I am a dog."); } } public class Cat extends Pet { public void say() { System.out.println("I am a cat."); } } public class Test{ public static void main(String[] args) { Pet pet1 = new Dog(); Pet pet2 = new Cat(); pet1.say(); pet2.say(); } } This program will output: I am a dog. I am a cat. The idea is very simple: despite pet1 and pet2 are of type Pet, pet1 is pointing to object of class Dog and pet is pointing to object of class Cat. The common question which is usually raised is: why we don’t have just Dog pet1 = new Dog(); and Cat pet2 = new Cat(); ? Why do we need to access it via Pet? Let me give you some real-world examples where do we need it: 1. Servlets. When we create a servlet we inherit base servlet class and override method doGet() or doPost()to add our functionality to the servlet. The server (for example

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Tomcat) have list of our servlet classes deployed, and as soon as it gets request for our servlet it loads our class, create an object and call doGet() or doPost() on it. As soon as server have no idea what classes do we have it address object of our class via variable of type HttpServlet. 2. The similar idea was used in early versions of Struts library. 3. In .NET as well as in Java you override Exception class or one of it’s successors to create exception specific to your application. The system (Java or .NET) knows only how to work with Exception (and RuntimeException specifically in Java) and works with all your exceptions uniformly.

2 eggs and a K-floors building puzzle

You have absolutely identical 2 eggs and empty K-story building. You can throw eggs from any floor and see if it was broken or not. If not, you can reuse it again momentarily. You need to identify the lowest floor, starting from which eggs is broken if thrown (“breaking floor”) in minimum possible steps in worst case.

Solution is here: the-problem-of-eggs-and-a-building.pdf