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Multi-tenancy and cloud computing
Ricky Ho

Participants in the cloud

In my cloud computing model, a "SaaS provider" is the organization that provides a domain specific SaaS App to its users (such as SmugMug for photo sharing). In this case, the SaaS consumer is just any individual who has a SmugMug account. The SaaS provider may choose an infrastructure provider (e.g. Amazon) to host its SaaS App. In this example, SmugMug is a SaaS provider and Infrastructure consumer at the same time.

Definition of a Tenant

Now, who is the "tenant" in this picture. I think Amazon will consider SmugMug as a tenant. But I doubt SmugMug will consider its individual user a tenant.

But what if SmugMug offer a services to car manufacturers so they can store, organize and image process their photos, which will show up in the car manufacturer's website. Will SmugMug consider BMW a tenant ? I think the answer is "yes". So maybe the definition of a tenant is "my user who has her own users".

You can see there can be a value chain built up. So except the start and end point of this value chain, everyone is a "tenant" to its service provider.

Multi-tenancy

After we define what a "tenant" is, what does "multi-tenancy" mean? In my opinion, "multi-tenancy" is for the benefit of the service provider so they can manage the resource utilization more efficiently, but multi-tenancy is not to the tenant's advantage at all. In the fake example I gave above, would BMW prefers a multi-tenancy environment from SmugMug? My guess is that BMW would in fact worry if their data is sitting together with their competitors in a shared infrastructure. I bet they would prefer an environment which is isolated as much as possible.

While "multi-tenancy" indicates that some infrastructure is shared, at what layers are things being shared can make a big difference. For example, Amazon AWS is multi-tenant at the hardware level in that its users may be sharing a physical machine. On the other hand, Force.com is multi-tenant at the DB level in that its users are sharing data in the same DB tables. And Amazon is relying on the hypervisor to provide the isolation between tenants while Force.com is relying on a query rewriter to do the same.

While "multi-tenancy" at the highest layer basically advocates a shared-DB approach, does it enables better collaboration or sharing between tenants? I don't think so. I think all we need is to have an authentication model such that spontaneous workgroup can be formed and membership can be identified. Then it is just a matter of a requesting tenant to presents his membership to another tenant when making a SaaS service call. What I mean is they are using an SOA approach to access data, rather than directly access a shared-DB.

Amazon approach (Hypervisor isolation) and Salesforce approach (DB isolation) are both valid but attract a different set of audiences.

First of all, increase efficiency through sharing is a fundamental value proposition underlying all cloud computing initiatives, there is no debate that ...

  1. We should "share resource" to increase utilization and hence improve efficiency
  2. We should accommodate highly dynamic growth and shrink requirement rapidly and smoothl
  3. We should "isolate" the tenant so there is no leakage on sensitive information

But at which layer should multi-tenancy be facilitated? Hypervisor level or DB level.

Hypervisor level Isolation

Hypervisor is a very low-level layer of software that maps the physical machine to a virtualized machine on which a regular OS runs on. When the regular OS issue system calls to the VM, it is intercepted by the Hypervisor which maps to the underlying hardware. The hypervisor also provide some traditional OS functions such as process scheduling to determine which VM to run. Hypervisor can be considered to be a very lean OS that sits very close to the bare hardware.

Depends on the specific implementation, Hypervisor introduces an extra layer of indirection and hence incur a certain % of overhead. If we need a VM with capacity less than a physical machine, Hypervisor allows us to partition the hardware into finer granularity and hence improve the efficiency by having more tenants running on the same physical machine. For light-usage tenant, such increment in efficiency should offset the lost from the overhead.

Since Hypervisor focus on low-level system level primitives, it provides the cleanest separation and hence lessens security concerns. On the other hand, by intercepting at the lowest layer, Hypervisor retain the familiar machine model that existing system/network admin are familiar with. Since Application is now completely agnostic to the presence of Hyervisor, this minimizes the change required to move existing apps into the cloud and makes cloud adoption easier.

Of course, the downside is that virtualization introduces a certain % of overhead. And the tenant still need to pay for the smallest VM even none of its user is using it.

DB level Isolation

Here is another school of thought, if tenants are running the same kind of application, the only difference is the data each tenant store. Why can't we just introduce an extra attribute "tenantId" in every table and then append a "where tenantId = $thisTenantId" in every query? In other words, add some hidden column and modify each submitted query.

In additional, the cloud provider usually needs to re-architect the underlying data layer and move to a distributed and partitioned DB. Some of the more sophisticate providers also need to invest in developing intelligent data placement algorithm based on workload patterns.

In this approach, the degree of isolating is as good as the rewritten query. In my opinion, this doesn't seem to be hard, although it is less proven than the Hypervisor approach.

The advantage of DB level isolation is there is no VM overhead and there is any minimum charge to the tenant.

However, we should compare these 2 approaches not just from resource utilization / efficiency perspective, but also other perspectives as well, such as ...

Freedom of choice on technology stack

Hypervisor isolation gives it tenant maximum freedom of the underlying technology stack. Each tenant can choose the stack that fits best to its application's need and in-house IT skills. The tenant can also free to move to latest technologies as they evolve.

This freedom of choice comes with a cost though. The tenant needs to hire system administrators to configure and maintain the technology stack.

In a DB level isolation, the tenants are live within a set of predefined data schemas and application flows. So their degree of freedom is limited to whatever the set of parameters that the cloud provider exposes. Also the tenants' applications are "lock-in" to the cloud provider's framework, and a tight coupling and dependency is created between the tenant and the cloud provider.

Of course, the advantage is that there is no administration needed in the technology stack.

Reuse of Domain Specific Logic

Since it focuses on the lowest layer of resource sharing, Hypervisor isolation provides no reuse at the app logic level. Tenants need to build their own technology stack from the ground up and write their own application logic.

In the DB isolation approach, the cloud provider pre-defines a set of templates in DB schemas and Application flow logic based on their domain expertise (it is important that the cloud provider must be the recognized expert in that field). The tenant can leverage the cloud provider's domain expertise and focus in purely business operation.

Conclusion

I think each approach will attract a very different (and clearly disjoint) set of audiences.

Notice that DB-level isolation commoditizes everything and make it very hard to create product feature differentiations. If I am a technology startup company trying to develop a killer product, then my core value is my domain expertise. In this case, I won't go with the DB-level isolation which imposes too much constraint on me to distinguish my product from "anyone else". Hypervisor level isolation much better because I can outsource the infrastructure layer and focus in my core value.

On the other hand, if I am operating a business but not building a product, then I would like to outsource all supporting functions including my applications as well. In this case, I would pick the best app framework provided by the market leader and follow their best practices (also very willing to live by their constraints), the DB level isolation is more compelling in this case.

Ricky Ho is an architect specializing in SaaS and Cloud computing and runs the blog Pragmatic Programming techniques.