When should one use the following: Amazon EC2, Google App Engine, Microsoft Azure and Salesforce.com?

Question

I am asking this in very general sense. Both from cloud provider and cloud consumer's perspective. Also the question is not for any specific kind of application (in fact the intention is to know which type of applications/domains can fit into which of the cloud slab -SaaS PaaS IaaS).

My understanding so far is:

IaaS: Raw Hardware (Processors, Networks, Storage).

PaaS: OS, System Softwares, Development Framework, Virtual Machines.

SaaS: Software Applications.

It would be great if Stackoverflower's can share their understanding and experiences of cloud computing concept.

EDIT: Ok, I will put it in more specific way -

Amazon EC2: You don't have control over hardware layer. But you can take your choice of OS image, Dev Framework (.NET, J2EE, LAMP) and Application and put it on EC2 hardware. Can you deploy an applications built with Google App Engine or Azure on EC2?

Google App Engine: You don't have control over hardware and OS and you get a specific Dev Framework to build your application. Can you take any existing Java or Python application and port it to GAE? Or vice versa, can applications that were built on GAE be taken out of GAE and ported to any Application Server like Websphere or Weblogic?

Azure: You don't have control over hardware and OS and you get a specific Dev Framework to build your application. Can you take any existing .NET application and port it to Azure? Or vice versa, can applications that were built on Azure be taken out of Azure and ported to any Application Server like Biztalk

Answer 1

Well, all these offerings fit into different categories:

App Engine, Azure, and Salesforce are all Platform as a Service. They offer varying levels of integration. Although Azure lets you run arbitrary background services, App engine, on the other hand, is oriented around a short-lived request handler tasks. Also, Salesforce provides CRM which is Software as a Service. So, basically, for all your CRM related and platform related such as building application things can be done over Salesforce.

Talking about EC2, it is Infrastructure as a Service. You basically get VM instances and do with them as you wish. Rackspace Cloud Servers are more or less the same.

Cloud offerings which fall under the SaaS or Software as a Service include all the infrastructural part along with platform-related things. So, basically, you need to run the application a sit is. Such as Gmail, Facebook.

You can learn about this from this video: