Almost every virtualization provider defines the term cloud to match their own offerings. These offerings are then promoted as Cloud Services or Cloud Servers. Public Cloud, Private Cloud, Hybrids, Software as a Service (SaaS), Platform as a Service (PaaS) – the list goes on and on. In this article, we will attempt to peek through the mist of terminology and provide readers with an orientation on the topic. We will provide an overview of the properties of real-life cloud offerings that can be found on the Internet.

Definition and Features

Administrators typically will not be interested in a theoretical definition of cloud computing. Instead, they will want to know two things: Do I already have something similar to a cloud in my server room or data center that I can use? How can cloud computing supplement my existing technology and IT platforms and help me solve current and future problems in my environment? On the one hand, clouds can help support new business models and services for consumers and startups that previously required a huge amount of effort or enormous financial risk. For example, if you have programmed a new web application in Ruby, you can simply launch it in the cloud; if it takes off, your scaling options are virtually unlimited, and you can add Content Delivery Networks (CDN). As an example, SlideShare [1] integrates document downloads and Flash file hosting with Amazon S3 and CloudFront Services. But even if your blog suddenly takes off and becomes a global event, you can migrate it to the Amazon or Rackspace CDN with just a couple of clicks – or so says the theory. The cloud means a technological revolution that requires major rethinking to put it to optimum use.

Figure 1: The continuum between simple servers and full-fledged clouds.

At the end of the day, clouds are just the ongoing development of virtualization technology. Providers see themselves somewhere between shared hosting and outsourcing (Figure 1). In an ideal world, cloud computing would free administrators from the hardware headaches (e.g., scalability, availability, maintenance contracts) in a geographical sense, giving them time to get on with running their applications. Internet platforms and server rooms currently on the LAN would be partially or fully virtualized and then run on the cloud provider's technology platform.

The Mother of All Clouds

When comparing cloud computing platforms, the reference is normally Amazon's Elastic Computing Cloud (EC2) [2] by virtue of its pioneering role (Figure 2).

Amazon's Web Services comprise virtual servers (EC2), a web-based storage service (S3), and a CDN (CloudFront), all of which are fully integrated and available as self-service features. By credit card, you can pay as you go only for the resources you actually use. For example, you can configure and launch one or more virtual servers in the cloud and pay for them only until you delete them again. The price for a couple of hours' use of a virtual server will typically be far less than a dollar.

Self-service is an important component and not restricted to the web GUI. Larger automated applications (e.g., SaaS third-party business models) can use both the GUI and documented APIs. Some cloud customers see the APIs as an easier approach into the cloud and back out again. Theoretically, it should be possible to develop software that uses the programming interface to copy content from Amazon to Rackspace and thus avoid dependence on any one service provider. Emerging standards, stacks, and API frameworks for cloud computing, such as OpenNebula [3] or Deltacloud [4], are a big help.