As Enterprise Architects we instinctively understand the purpose of the Conceptual Model (Requirements, Constraints and Assumptions) and the Physical Design (Vendor XYZ, Product A with setting B). Unfortunately, the Logical Design is something most architects struggle with, because it is abstract and theoretical. This is very common with Enterprise Architects who have been working with one vendor for a long time, because they are so used to building designs where Vendor XYZ has been preselected, that they overlook the need to provide a future possibility for other vendors.
Note: The VCDX program accepts the referencing of VMware technology in the Logical Design section, as long as the Physical Design section covers the VMware technology software version selection and justification.
What is the purpose of a Logical Design? The Logical Design captures the vendor agnostic, functional specification of what you are trying to achieve. It should not mention vendor specific technologies, because you want to be able to give your Conceptual Model and Logical Design to two or more vendors and ask them to produce a Physical Design that matches your requirements.
If you skip the Logical Design (most architects do), you lock yourself into one particular vendor and miss the opportunity of capturing and distilling your exact functional requirements. This leads to vendor preselection and railroading your customer into a solution that may be too expensive or complicated than what is actually needed.
A great Logical Design also allows you to future-proof your architecture design by having the option of selecting another vendor at a later time.
When you review a Logical Design, ask yourself, could I give this to any vendor Subject Matter Expert (SME) to develop a Physical Design? Are there any vendor specific technologies mentioned that would preclude another vendor from developing a Physical Design?
As you develop your Logical Design, ask yourself exactly what functions you need to meet your business requirements. Not easy to document, but it becomes less difficult with practice and you will build a library of Logical Design decisions that you can re-use in your future work.
Examples of bad Logical Design decisions (VMware vSphere/View is pre-selected with Cisco UCS, Windows Server 2012 R2 and Oracle 12c):
- Three or more ESXi Hosts will be required per HA/DRS Cluster
- vSphere HA will be used for protection against host failures and VM failures
- vSphere DRS will be used to provide workload balancing across each ESXi cluster
- A separate vCenter Server will be used for the View infrastructure
- The ESXi cluster Cisco UCS C440 hosts must be installed across multiple racks
- vCenter will be deployed to Windows Server 2012 R2
- vCenter will use an Oracle 12c Enterprise Database
- vSphere will only use Fiber Channel storage
In fact, what you see above are Physical Design decisions masquerading as Logical Design decisions.
Rewritten as great Logical Design decisions (all vendors now have a chance):
- Three or more hypervisor hosts will be required per compute cluster
- The hypervisor “workload protection mechanism” will provide the ability to restart Virtual Machines if either the Host, Virtual Machine or Virtual Machine Application fails
- The Virtual Infrastructure Management platform will provide a workload resource balancing mechanism that will optimise the use of CPU, RAM, Network and Storage resources across the cluster of hypervisor hosts
- A separate Virtualisation Infrastructure Management platform will be used to separate Virtual Desktop Infrastructure and Virtual Server Infrastructure
- Clustered hypervisor hosts must be spanned across multiple blade server chassis and/or rack-mounted servers in separate racks
- The Virtual Infrastructure Management platform can be deployed as a virtual appliance or application installation to an existing Guest Operating System
- The Virtual Infrastructure Management platform can use an internal or external database
- The hypervisor host will support Block-based and IP-based storage