Infrastructure as a Service. IaaS services would logically appear to the buyer as a virtual machine host that is essentially an extension of the buyer’s own data center virtual resource pool. Running an application on a virtual host means creating a machine image that includes the application and its operating system software and middleware, and then loading it on the selected cloud server.
Since the application image and operating software is provided by the user, the cloud provider has limited options for managing the internal behavior of the application. The virtual machine host can be managed but not the machine image software. Still, this application is what made Amazon’s EC2 famous, and operators almost universally expect to support it. A virtualized set of servers and a director function are required to assign free virtual machines to the best available server resource.
Platform as a Service. PaaS services can be viewed as an IaaS framework that has defined one or more middleware “services” that are available to the applications within the cloud. Therefore, these applications are not included in the machine image provided by the user. The most common platform service is Database as a Service, followed closely by management services.
To the user, the value in consuming a platform service versus having a completely customer-provided machine image of the application is that the platform service is cloud-aware and can be managed and optimized for a cloud-hosted execution.
Cloud databases are almost mandatory for applications that will involve data exchange among application components. Otherwise, these components would have to develop an internal mechanism for data sharing that could be relied upon to work efficiently within a cloud, where resource-to-application assignment is nearly invisible to the user.
Cloud management tools, including tools to manage application performance, allow both users and the cloud provider to merge application performance issues with virtual machine resource performance, giving a better management picture.