You know what a cloud is, right? Of course, right. So why is it so much trouble to determine how you want to deploy your cloud? I don’t know, but I get asked all the time, so here is my quick and dirty method for determining what kind of cloud you’ll need.

Let’s start with the basics. No matter what kind of cloud you have, the servers and virtual machines (VMs) it runs on have to live somewhere. It’s where those CPUs run that determines the three fundamental types of clouds. A public cloud runs on servers at a third-party’s data center or centers. A private one runs on your servers at your data center. And a hybrid runs its services on both public and private clouds.

Before you pick one, you need to realize that this isn’t an either/or choice. You can mix and match these models. For instance, you might run your web servers on a public cloud, your corporate databases on a private one, and the front end to a software-as-a-service (SaaS) from a public cloud while doing its authentication from your private-cloud back-end servers.

So which kind of cloud is right for you? Well, it depends on what you want it to do. If your cloud to-do list says you need to:

  • Add computing resources dynamically
  • Run standardized application (for example, e-mail for multiple users)
  • Provide developers or DevOps staff with a testbed
  • Collaborate work with other organizations
  • Stay within a limited budget

you want a public cloud.

But if the following factors are vital to you (and you can afford to run a data center), a private cloud may be your best choice:

  • Absolute control of your data and/or applications
  • Strict adherence to data control regulations and laws, such as Health Insurance Portability and Accountability Act (HIPPA)

I say “may” because you can lose a lot of the advantages of a cloud with a private cloud. As Jason Bloomberg observed in his 2013 book, The Agile Revolution, there are two major problems with private clouds.

First, one of the major wins with a cloud is elasticity. Did your company workload just explode? With a public cloud, you automatically rent more capacity and your work continues as usual. With a private cloud, unless you have a lot of money sunk into your data-center infrastructure, you simply can’t economically ramp up to meet unexpected demand bursts.

Second, while a public cloud shifts capital expenses to operational expenses, with a private cloud you’re still left paying for new servers and operations. It adds up. Can your company really match the economies of scale of an Amazon, Google, Microsoft, or Rackspace? I doubt it.

Don’t get me wrong. There are times when a private cloud makes a lot of sense. That’s why I think for many businesses that have a mixture of IT requirements, a hybrid model is probably the best bet.

With a hybrid model you keep private the crown jewels of your company, say your customer databases, while relying on the public cloud for the heavy lifting of SaaS. Indeed, many SaaS applications are already using hybrid clouds.

Before he became Microsoft’s chief Cloud & Enterprise group strategist, former Forrester’s cloud principal analyst James Staten blogged:

Hybrid isn’t a future state after you have a private cloud in place and IT Ops chooses to connect that private cloud to a public cloud. Look at it through the lens of a business process or application service which is composed of different components, some cloud-based, some on-premise. From an Infrastructure & Operations perspective, hybrid cloud means a cloud service connected to any other corporate resource (a back office app, your web site, your intranet, another SaaS app you have under contract and yes, even your private cloud).”

For some of you, public clouds are all you’ll ever need. For others, you need the calm that comes from having all your services under your control. And for many companies, hybrid clouds make a great deal of sense. They give you most of the cost savings and elasticity of the public cloud with the security of a private cloud.

At day’s end, you need to carefully consider all your IT requirements. Only then will you be able to work out which cloud approach you should use before moving from your early 21st century server room and data center to your own special cloud services blend.