Most SaaS systems have some dependency on a billing system. This could be a home grown billing system or it could be any one of the commercial SaaS billing systems that are available from different billing providers. Regardless of the approach, you can see how billing is a core concept that has a natural home within the control plane.
Billing has a couple touch points within the control plane. It’s typically connected to the onboarding experience where each new tenant must be created as a “customer” within your billing system. This might include configuring the tenant’s billing plan and setting up other attributes of the tenant’s billing profile.
Many SaaS solutions have billing strategies that meter and measure tenant activity as part of generating a bill. This could be bandwidth consumption, number of requests, storage consumption, or any other activity-related events that are associated with a given tenant. In these models, the control plane and your billing system must provide a way for this activity data to be ingested, processed, and submitted to your billing system. This could be a direct integration with the billing system or you could introduce your own services that process this data and send it to the billing system.
We’ll get more into the details of billing integration in Chapter 16. The key here is to realize that billing will likely be part of your control plane services and that you’ll likely be introducing dedicated services to orchestrate this integration.
Tenant Management
Every tenant in our SaaS system needs to be centrally managed and configured. In our control plane, this is represented by our Tenant Management service. Typically, this is a pretty basic service that provides all the operations needed to create and manage the state of tenants. This includes tracking key attributes that associate tenants with a unique identifier, billing plans, security policies, identity configuration, and an active/inactive status.
In some cases, teams may overlook this service or combine it with other concepts (identity, for example). It’s important for multi-tenant environments to have a centralized service that manages all of this tenant state. This provides a single point of tenant configuration and allows tenants to easily manage them through a single experience.
We’ll explore the elements and permutations of implementing tenant management more in Chapter 5.
Now that we have a better sense of the core concepts with the control plane, let’s start looking at the common areas where multi-tenancy shows up in the application plane. While the control plane typically has a consistent set of common services, the application plane is a bit more abstract. How and where multi-tenancy is applied within the application plane can vary significantly based on a wide range of factors. That being said, there are still a range of themes that will surface, albeit in different forms, within your application plane. So, even though there is variation here, every SaaS architect will need to consider how/where they will introduce these themes into the application plane of their solution.
As you dig into the application pane, you’ll find that your technology stack and deployment footprint will have a significant influence on how these concepts are applied. In some cases, there may be ready-made solutions that fit your use case precisely. In other cases, you may find yourself inventing solutions to fill gaps in your technology stack. While building out something to fill these gaps may add complexity and overhead to the build of your solution, in most cases you’ll want to take on this added work to ensure that your SaaS solution is not compromising on important elements of your multi-tenant architecture.
In subsequent chapters we’ll look at real-world working examples that provide a more concrete view of how these constructs are realized within your application plane. For now, though, let’s come up a level and establish a core set of application plane principles that should span every SaaS architecture.
Tenant Context
One of the most fundamental concepts in our application plane is the notion of tenant context. Tenant context does not map to any one specific strategy or mechanism. Instead, it’s a broader concept that is meant to convey the idea that our application plane is always functioning in the context of specific tenants. This context is often represented as a token or some other construct that packages all the attributes of your tenant. A common example that you’ll use here is a JSON Web Token (JWT) which combines your user and tenant information in one construct that is shared across all the moving parts of your multi-tenant architecture. This JWT becomes our passport for sharing tenant information (context) with any service or code that relies on this context. It’s this token that is referred to as your tenant context.
Now, you’ll see that this tenant context has a direct influence on how your application architecture processes tenant requests. This may affect routing, logging, metrics, data access, and a host of other constructs live within the application plane. Figure 2-6 provides a conceptual view of tenant context in action.
Figure 2-6. Applying tenant context
The flow in Figure 2-6 shows tenant context being applied across the different services and resources that are part of a multi-tenant environment. This starts on the left-hand side of the diagram where my tenants authenticate against the identity that was created during onboarding and acquire their tenant context. This context is then injected into a service of my application. This same context flows into each downstream interaction of my system, enabling you to acquire and apply that context across a range of different use cases.
This represents one of the most fundamental differences of a SaaS environment. Our services don’t just work with users–they must incorporate tenant context as part of the implementation of all the moving parts of our SaaS application. Every microservice you write will use this tenant context. It will become your job to figure out how to apply this context effectively without adding too much complexity to the implementation of your system. This, in fact, is a key theme that we’ll address when we dig into SaaS microservice in Chapter 8.
As a SaaS architect, this means that you must be always thinking about how tenant context will be conveyed across your system. You’ll also have to be thinking about the specific technology strategies that will be used to package and apply this tenant context in ways that limit complexity and promote agility. This is a continual balancing act for SaaS architects and builders.
Multi-tenancy, by its very nature, focuses squarely on placing our customers and their resources into environments where resources may be shared or at least reside side-by-side in common infrastructure environments. This reality means that multi-tenant solutions are often required to apply and implement creative measures to ensure that tenant resources are protected against any potential cross-tenant access.
To better understand the fundamentals of this concept, let’s look at a simple conceptual view of a solution running in our application plane (shown in Figure 2-7).
Figure 2-7. Implementing tenant isolation
Here you’ll see we have the simplest of application planes running a single microservice. For this example, our SaaS solution has chosen to create separate databases for each tenant. At the same time, our microservice is sharing its compute with all tenants. This means that our microservice can be processing requests from tenants 1 and 2 simultaneously.
While the data for our tenants are stored in separate databases, there is nothing in our solution that ensures that tenant 1 can’t access the database of tenant 2. In fact, this would be the case even if our tenants weren’t running in separate, dedicated microservices.
To prevent any access to another tenant’s resources, our application plane must introduce a construct to prevent this cross-tenant access. The mechanisms to implement this will vary wildly based on a number of different considerations. However, the basic concept–which is labeled Tenant Isolation–spans all possible solutions. The idea here is that every application plane must introduce targeted constructs that strictly enforce the isolation of individual tenant resources–even when they may be running in a shared construct.
We’ll dig into this concept in great detail in Chapter 10. It goes without saying that tenant isolation represents one of the most fundamental building blocks of SaaS architecture. As you build out your application plane you’ll need to find the flavor and approach that allows you to enforce isolation at the various levels of your SaaS architecture.
The services and capabilities within our application plane often need to store data for tenants. Of course, how and where you choose to store that data can vary significantly based on the multi-tenant profile of your SaaS application. Any number of factors might influence your approach to storing data. The type of data, your compliance requirements, your usage patterns, the size of the data, the technology you’re using–these are all pieces of the multi-tenant storage puzzle.
In the world of multi-tenant storage, we refer to the design of these different storage models as data partitioning. The key idea here is that you are picking a storage strategy that partitions tenant data based on the multi-tenant profile of that data. This could mean the data is stored in some dedicated construct or it could mean it lands in some shared construct. These partitioning strategies are influenced by a wide range of variables. The storage technology you’re using (object, relational, nosql, etc.) obviously has a significant impact on the options you’ll have for representing and storing tenant data. The business and use cases of your application can also influence the strategy you select. The list of variables and options here are extensive.
As a SaaS architect, it will be your job to look at the range of different data that’s stored by your system and figure out which partitioning strategy best aligns with your needs. You’ll also want to consider how/if these strategies might impact the agility of your solution. How data impacts the deployment of new features, the up-time of your solution, and the complexity of your operational footprint are all factors that require careful consideration when selecting a data partitioning strategy. It’s also important to note that, when picking a strategy, this is often a fine-grained decision. How you partition data can vary across the different services within your application plane.
This is a much deeper topic that we’ll cover more extensively in Chapter 9. By the end of that chapter, you’ll have a much better sense of what it means to bring a range of different strategies to life using a variety of different storage technologies.
In this simplest of SaaS architecture models, you may find that all tenants are sharing their resources. However, in most cases, your architecture is going to have variations where some or all of your tenant’s infrastructure may be dedicated. In fact, it would not be uncommon to have microservices that are deployed on a per tenant basis.
The main point here is that SaaS application architectures are often required to support a distributed footprint that has any number of resources running in a combination of shared and dedicated models. The image in Figure 2-8 provides a simplified sample of a SaaS architecture that supports a mix of shared and dedicated tenant resources.
Figure 2-8. Routing on tenant context
In this example, we have three tenants that will be making requests to invoke operations on our application services. In this particular example, we have some resources that are shared and some that are dedicated. On the left, tenant 1 has an entirely dedicated set of services. Meanwhile, on the right-hand side, you’ll see that we have the services that are being used by tenants 2 and 3. Here, note that we have the product and rating services that are being shared by both of these tenants. However, these tenants each have dedicated instances of the order service.
Now, as you step back and look at the overall configuration of these services, you can see where our multi-tenant architecture would need to include strategies and constructs that would correctly route tenant requests to the appropriate services. This happens at two levels within this example. If you start at the top of our application plane where our application plane is receiving requests from three separate tenants, you’ll notice that there’s a conceptual placeholder for a router here. This router must accept requests from all tenants and use the injected tenant context (that we discussed earlier) to determine how and where to route each request. Also, within the tenant 2/3 box on the right, you’ll see that there is another placeholder for routing that will determine which instance of the order service will receive requests (based on tenant context).
Let’s look at a couple concrete examples to sort this out. Suppose we get a request from tenant 1 to look up a product. When the router receives this request, it will examine the tenant context and route the traffic to the product service on the left (for tenant 1). Now, let’s say we get a request from tenant 3 to update a product that must also update an order. In this scenario, the top-level router would send the request to the shared product service on the right (based on the tenant 2 context). Then, the product service would send a request to the order service via the service-to-service router. This router would look at the tenant context, resolve it to tenant 2, and send a request to the order service that’s dedicated to tenant 2.
This example is meant to highlight the need for multi-tenant aware routing constructs that can handle the various deployments footprint we might have in a SaaS environment. Naturally, the technology and strategy that you apply here will vary based on a number of parameters. There are also a rich collection of routing tools and technologies, each of which might approach this differently. Often, this comes down to finding a tool that provides flexible and efficient ways to acquire and dynamically route traffic based on tenant context.
We’ll see these routing constructs applied in specific solutions later in this book. At this stage, it’s just important to understand that routing in multi-tenant environments often adds a new wrinkle to our infrastructure routing model.
Multi-Tenant Application Deployment
Deployment is a pretty well understood topic. Every application you build will require some DevOps technology and tooling that can deploy the initial version of your application and any subsequent updates. While these same concepts apply to the application plane of our multi-tenant environment, you’ll also discover that different flavors of tenant application models will add new considerations to your application deployment model.
We’ve already noted here that tenants may have a mix of dedicated and shared resources. Some may have fully dedicated resources, some may have fully shared, and others may have some mix of dedicated and shared. Knowing this, we have to now consider how this will influence the DevOps implementation of our application deployment.
Imagine deploying an application that had two dedicated microservices and three shared microservices. In this model, our deployment automation code will have to have some visibility into the multi-tenant configuration of our SaaS application. It won’t just deploy updated services like you would in a classic environment. It will need to consult the tenant deployment profile and determine which tenants might need a separate deployment of a microservice for each dedicated microservice. So, microservices within our application plane might be deployed multiple times. That, and our infrastructure automation code may need to apply tenant context to the configuration and security profile of each of these microservices.
Technically, this is not directly part of the application plane. However, it has a tight connection to the design and strategies we apply within the application plane. In general, you’ll find that the application plane and the provisioning of tenant environments will end up being very inter-connected.
