I’ve written previously about the “cloud stack” of -as-a-service models but I recently saw Microsoft’s Steve Plank (@plankytronixx) give a great description of the differences between on-premise,  infrastructure as a service (IaaS), platform as a service (PaaS) and software as a service (SaaS).

Of course, this is a Microsoft view of the cloud computing landscape and I’ve had other discussions recently where people have argued the boundaries for IaaS or PaaS and confused things further by adding traditional web hosting services into the mix*.  Even so, I think the Microsoft description is a good starting point and it lines up well with the major cloud services offerings from competitors like Amazon and Google.

Not everyone will be familiar with this so I thought it was worth repeating Steve’s description here:

In an on-premise deployment, the owning organisation is responsible for (and has control over) the entire technology stack.

With infrastructure as a service, the cloud service provider manages the infrastructure elements: network, storage, servers and virtualisation. The consumer of the IaaS service will typically have some control over the configuration (e.g. creation of virtual networks, creating virtual machines and storage) but they are all managed by the cloud service provider.  The consumer does, however, still need to manage everything from the operating system upwards, including applying patches and other software updates.

Platform as a service includes the infrastructure elements, plus operating system, middleware and runtime elements. Consumers provide an application, configuration and data and the cloud service provider will run it, managing all of the IT operations including the creation and removal of resources. The consumer can determine when to scale the application up or out but is not concerned with how those instances are operated.

Software as a service provides a “full-stack” service, delivering application capabilities to the consumer, who only has to be concerned about their data.

Of course, each approach has its advantages and disadvantages:

• IaaS allows for rapid migrations, as long as the infrastructure being moved to the cloud doesn’t rely on other components that surround it on-premise (even then, there may be opportunities to provide virtual networks and extend the on-premise infrastructure to the cloud). The downside is that many of the management issues persist as a large part of the stack is still managed by the consumer.
• PaaS allows developers to concentrate on writing and packaging applications, creating a service model and leaving the underlying components to the cloud services provider. The main disadvantage is that the applications are written for a particular platform, so moving an application “between clouds” may require code modification.
• SaaS can be advantageous because it allows for on-demand subscription-based application use; however consumers need to be sure that their data is not “locked in” and can be migrated to another service if required later.

Some organisations go further – for example, in the White Book of Cloud Adoption, Fujitsu wrote about Data as a Service (DaaS) and Business Process as a Service (BPaaS) – but IaaS, PaaS and SaaS are the commonly used models.  There are also many other considerations around data residency and other issues but they are outside the scope of this post. Hopefully though, it does go some way towards describing clear distinctions between the various -as-a-service models.

* Incidentally, I’d argue that traditional web hosting is not really a cloud service as the application delivery model is only part of the picture. If a web app is just running on a remote server it’s not really conforming with the broadly accepted NIST definition of cloud computing characteristics. There is a fine line though – and many hosting providers only need to make a few changes to their business model to start offering cloud services. I guess that would be an interesting discussion with the likes of Rackspace…