❖Cloud computing delivers different computing facilities to consumers as utility services. NISTmodel divides these services primarily into three categories: Infrastructure-as-a-Service (IaaS),Platform-as-a-Service (PaaS) and Software-as-a-Service (SaaS).

❖The three service models (SaaS-PaaS-IaaS) together are known as SPI model of cloud. IaaSprovides infrastructure like server, storage and network facilities; PaaS provides applicationdevelopment and deployment environments and SaaS delivers software applications.

❖PaaS layer interacts/communicates with the underlying IaaS layer by using the set of APIs(application program interface) provided by respective IaaS solutions.

❖Computing vendors have launched many other specialized cloud services which ultimatelyfall under these three primary services of SPI model.

❖Offerings like Security Management-as-a-Service, Storage-as-a-Service, Database-as-a-Servicehave shown high rate of adoptability among the consumers.

❖ is a popular commercial SaaS offering for enterprises that was pioneer amongearly cloud services and was launched towards the end of the last century. It delivers customerrelationship management (CRM) solutions.

❖Apart from proprietary cloud service offerings from vendors like, Amazon,Google, many open-source communities have also come up with their own solutions.

❖Eucalyptus, Nebula, Nimbus, OpenStack, CloudStack are few examples of popular open cloudsolutions which are mainly used to build private cloud environments.

❖Open cloud solutions which are compatible with some public cloud service systems canbe used to create hybrid cloud environment. For instance, Eucalyptus or OpenNebula arecompatible with Amazon cloud.

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