Cloud Service Models: Comparing SaaS PaaS and IaaS

Defining what comprises Cloud Computing is hard because it is so many things. Many vendors do not help clarify it because labelling products as Cloud Computing makes them appear current and more relevant. Despite all the marketing hype, Cloud Computing can be readily broken down into one of three delivery models as defined by NIST and known as the SPI model. SPI stands for Software, Platform and Infrastructure. When all the hype is stripped away, these just represent hardware and software !

Cloud computing enables hardware and software to be delivered as services, where the term service is used to reflect the fact that they are provided on demand and are paid on a usage basis – the more you use the more you pay. Draw an analogy with a restaurant. This provides a food and drinks service. If we would like to eat at a restaurant, we do not buy it, just use it as we require. The more we eat the more we pay. Cloud Computing provides computing facilities in the same way as restaurants provide food, when we need computing facilities, we use them from the cloud. The more we use the more we pay. When we stop using them we stop paying.

Although the above analogy is a great simplification, the core idea holds. Since computing is many many things, Cloud Computing has a lot of things to deliver as a service.This is where the SPI model helps organise things. Lets consider these in turn.

Software as a Service This is typically end user applications delivered on demand over a network on a pay per use basis. The software requires no client installation, just a browser and network connectivity. An example of SaaS is MicroSoft Office365. Until its launch, if a user required say Word, they would have to purchase it, install it, backup files etc. With Office365 Word can be acquired for a small monthly fee, with no client installation, the files are automatically backed up, software upgrades are automatically received and the software can be accessed from anywhere. Decide you do not require Word anymore – stop paying the monthly fee. It is that simple.

Platform as a Service Used by software development companies to run their software products. Software products need physical servers to run on, with database software, often Web servers too. These are all the platform that the application runs on. Building this yourself is a time consuming task and needs to be continually monitored and updated. PaaS provides all of the platform out of the box enabling software applications to be given to the platform which will execute them with no requirement for administration of the lower level components.

InfraStructure as a Service This covers a wide range of features, from individual servers, to private networks, disk drives, various long term storage devices as well as email servers, domain name servers as well as messaging systems. All of these can be provisioned on demand and often include software license fees for operating systems and associated software installed on the servers. Organisations can build a complete computing infrastructure using IaaS on demand.

So all the services provided by Cloud Computing fit into one of the three delivery models above. End users typically use SaaS, software development teams PaaS and IT departments whose responsibility is the infrastructure use IaaS. There is much more to Cloud Computing, including aspects such as the automatic scaling and security for example, but as a starting point, categorising the deliver models should help to understand that all aspects of computing are covered and it can be potentially useful for everybody involved in, or using IT.

Chris Czarnecki

As cloud computing continues to make information technology headlines, vendors are aggressively promoting the many benefits it can provide organizations.  Our White Paper, Cloud Computing Promises: Fact of Fiction , addresses the claims and questions that are often raised in relation to cloud computing and provides a clear view of what the cloud can—and can’t—deliver in reality.

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