I often hear different ways to define the Cloud, depending on what the various stakeholders want to highlight and what they expect to achieve or position a particular project. These different definitions lead to some confusion. Some even say that the Cloud is a scam. In this post, I review what the cloud is, its characteristics, and service and deployment models.

According to the National Institute of Standards and Technology (NIST), Cloud Computing is a model for enabling ubiquitous, convenient, on-demand network access to a shared pool of configurable computing resources (e.g., networks, servers, storage, applications, and services) that can be rapidly provisioned and released with minimal management effort or service provider interaction.

The NIST defines five essential characteristics of Cloud Computing:

• On-demand self-service
  • Provisioning of computing capabilities (compute, network, storage) without human interaction.
• Broad network access
  • Services are available over the network and accessed through standard mechanisms.
• Resource pooling
  • Computing resources are pooled to serve multiple consumers using a multi-tenant model.
  • The customer generally doesn’t know the exact location of resources.
• Rapid elasticity
  • Capabilities can be elastically provisioned and released to scale rapidly.
• Measured service
  • Resource usage can be monitored, controlled, and reported.

There are three basic types of Service Models for Cloud Computing:

• Infrastructure as a Service (IaaS)
  • It refers to vendor-provided hardware and software.
• Platform as a Service (PaaS)
  • Set of tools and services designed to make coding and deploying those applications quick and efficient.
• Software as a Service (SaaS)
  • Includes standalone Web applications or thin client access that support various computing functions.

Considering the traditional IT stack (the leftmost one in the diagram below), we can easily compare the cloud computing services. The difference between them is what the Cloud Service Provider delivers (grey areas) and what the customer manages (orange squares).

The SPI model (Software, Platform, Infrastructure) is a term that encompasses the three popular types of cloud computing services.

Finally, Cloud Computing Deployment Models are:

• Private Cloud: Cloud services are only available to members of a single organization. The infrastructure can be hosted by the same organization or a third party.
• Community cloud: Cloud services are offered to community or organisation members. The infrastructure is hosted by members of the same organization.
• Public Cloud: Cloud services are offered to the general public. The infrastructure is hosted by anyone.
• Hybrid Cloud: Composition of two or more distinct cloud infrastructures, such as a Private + Public cloud combination.

Amendment November 23rd, 2019: Since the NIST defined the Cloud and the possible deployment models, the landscape for many companies has evolved and become very sophisticated to align with their business and technical needs. The terms hybrid cloud and multi-cloud have become very popular in describing specific advanced scenarios. A Hybrid Cloud is a mix of on-premises and cloud deployments within the Enterprise Architecture, while Multi-Cloud happens when more than one cloud provider is involved.
This interactive video shows that Hybrid Cloud and Multi-Cloud use a city as an example. It also highlights some aspects that need to be addressed to manage them.