What is Private versus Public Cloud? (Triple-A Cloud Part 2)

In “What Really Is Cloud Computing?” I tried to break down the essence of cloud computing as abstraction, automation and agility.  Part of the idea there was to break down where the value of cloud computing comes from, and step back a bit from the private versus public versus hybrid discussions.  Some of the points I tried to make include:

  • Cloud is not just outsourcing or a product that you build.  It’s a complete transformation of mindsets, culture, processes and org structure within IT.
  • Don’t wait to sort out the private/public/hybrid debates to get started.  If you’re not ready for public cloud, start by chasing 100% virtualization and automation (OPEX benefits) today as this will better position you for public cloud in the future.

Having said all that I wanted to further confuse everyone by explaining why I think that private cloud is a good choice in the short term, but public cloud (in whatever form it evolves into) will be the longer term trend.  Before I get into that post, I thought it might be beneficial to have a brief discussion on defining just what is private versus public cloud.

Private Cloud

The formal definition of private cloud is fairly straight forward, yet there’s several different variations of it in the market place (more on that in a bit).

The current Wikipedia entry describes private cloud as “… infrastructure operated solely for a single organization, whether managed internally or by a third-party and hosted internally or externally.”

In other words you’re adopting or pursuing the three A’s of cloud within a traditional infrastructure intended for use by a single organization.  You control the routers, firewalls, switches, SAN, workloads, etc. for your organization and you don’t share any of them with anyone else.

Now to make this a bit more confusing there are hosting providers offering “private cloud hosting” solutions which might not really conform with the definition of private cloud.  Sometimes one or more components of so called “private cloud hosting” can be shared with another tennant — and  technically this is not private cloud.  True private cloud means that none of your resources — hosts, SAN, firewalls, etc — are shared with another tenant.  There may be other tenants in the same datacenter but none of them should share or have access to any elements of  your private cloud infrastructure.

Public Cloud

Public cloud as we know it today, is a multi-tennant design.  The benefit of this is fairly straight forward — by knocking down walls and barriers it is possible to leverage hardware more efficiently in a larger pool.  Efficency goes up and costs go down.  You gain a more elastic capacity, utlity pricing and managed operations.

Thus in a true public cloud solution, you may have several tenents sharing the same physical server, the same SAN, the same firewall, etc., but a variety of technologies (including VMware vShield and vCloud Director in several cases) are used to provide a logical boundary/barrier for the tenants.

While security is natually a big concern in public cloud model, this does not necessarilly mean that private clouds are inherently more secure either.  Standards, audits and transparency are needed in the public cloud and over time, both process and technology should continue to evolve in this area.

Hybrid Cloud

Hybrid is when an organization has adopted both private and public cloud elements.  Often times a private cloud will be used for “planned” or budgeted capacity while public cloud will be sometimes leveraged for initiatives for which the capacity was not anticipated or budgeted.

What’s important here is not just the portability of workloads between private and public clouds, but maintaining the security and the operational model as you move from one to another (more on this in a future post).


That’s my quick take on the private/public/hybrid models.  In a future post I’ll be going into much more detail on why I think the longer term trend is the public cloud model, but that private cloud is very valid and will likely grow considerably over the next several years.  Also I’ll be looking into the vCloud ecosystem a bit.

But another quick point that I want to restate is that organizations should not let uncertainty deter them from pursuing the three A’s of cloud computing.  While there are some great products and solutions being offered, there is no magic bullet to escape the fact that cloud computing is a fundamental trnasformation of how IT is approached and executed.  Start with 100% virtualization and automation and start building a cloud (and transforming your IT org).

Do you or agree or disagree?  Have a comment?  Join the discussion below….

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