Re-Defining Multi-Tenancy
Up to this point, I’ve avoided introducing the idea of multi-tenancy. It’s a word that is used heavily in the SaaS space and will appear all throughout the remainder of this book. However, it’s a term that we have to wander into gracefully. The idea of multi-tenancy comes with lots of attached baggage and, before sorting it out, I wanted to create some foundation for the fundamentals that have driven companies toward the adoption of the SaaS delivery model. The other part of the challenge here is that the notion of multi-tenancy–as we’ll define it in this book–will move beyond some of the traditional definitions that are typically attached to this term.
For years, in many circles, the term multi-tenant was used to convey the idea that some resource was being shared by multiple tenants. This could apply in many contexts. We could say that some piece of cloud infrastructure, for example, could be deemed multi-tenant because it was allowing tenants to share some resource under the hood. In reality, many services running in the cloud may be running in a multi-tenant model to achieve their economies of scale. As a cloud consumer, this may be happening entirely outside of your view. Even outside the cloud, teams could build solutions where compute, databases, and other resources could be shared amongst customers. This created a very tight connection between multi-tenancy and the idea of a shared resource. In fact, in this context, this is a perfectly valid notion of multi-tenancy.
Now, as we start thinking about SaaS environments, it’s entirely natural for us to bring the mapping of multi-tenancy with us. Afterall, SaaS environments do share infrastructure and that sharing of infrastructure is certainly valid to label as being multi-tenant.
To better illustrate this point, let’s look at a sample SaaS model that brings together the concepts that we’ve been discussing in this chapter. The image in Figure 1-5 provides a view of a sample multi-tenant SaaS environment.
Figure 1-5. A sample multi-tenant environment
Here we have landed the shared infrastructure of our application services inside surrounding services that are used to introduce tenancy, manage, and operate our SaaS environment. Assuming that all of our tenants are sharing their infrastructure (compute, storage, and so on), then this would fit with the classic definition of multi-tenancy. And, to be fair, it would not be uncommon for SaaS providers to define and deliver their solution following this pattern.
The challenge here is that SaaS environments don’t exclusively conform to this model. Suppose, for example, I create a SaaS environment that looks like the drawing in Figure 1-6.
