IAAS, PAAS, SAAS - Cloud based software


On Premises
Software applications developers/companies used to buy hardware, OS, virtualization software (to run more OS on same hardware).  Also they would buy storage servers and networking hardware.  Then they will deploy the application managing the security, os patching, monitoring.  Here they manage on premise data center.

Off Premises
Now part of above tasks can be availed in various off the shelf manner called iaas, paas, saas.

1. iaas will provide virtualization ready systems.  Developer will need to install the guest OS, setup networking/security (network and storage is provided), install application servers middleware like tomcat and then install their application like web app.  They need to take care of defining load balancing (networking hardware for Load Balancer LB will be provided).  EC2 provides an IP and you can choose what OS you want.

2. paas will provide more relief where in developer will not need to worry about LB and application clustering.  The middlware cluster and some framework will be provided and developer merely needs to code the application and deploy.  Google App Engine is example where we develop web application and just deploy to it.  scaling of servers to manage TPS will be handled by paas.

3. saas will provide software url to work with.  developer does not need to write any application nor deploy.  gmail / google app are example where these applications url are provided to company/users.

Here is nice url giving more details.

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