* Fault Tolerance. Unified communications is a business critical application making built-in fault tolerance a must have. Multi-tenant based platforms have built-in fault-tolerance that multi-instance platforms do not have. In a multi-tenant architecture, customers are served from clusters of nodes performing the same function. If a particular node goes down, the work will be handled by other nodes that are still up and operating normally. In the multi-instance architecture, the service provider must setup backup/failover VMs that have the correct tenant configuration on them. Further, the provider has to make sure that the configurations for these VMs stay up to date with the production VMs.
* Ease of Development and Management. Enterprises need to be able to not only add new features, they need to be able to easily test them and provision them. A multi-tenant platform is easier to develop and manage because there is only one code base and all of the tenants are on the same version at all times. Contrast this with the multi-instance architecture where potentially different tenants could be on different code versions. Keeping a single code base is essential to making sure service provider engineering effort and feature development stays productive and testing, provisioning, and troubleshooting can be handled efficiently.
* Ability to Integrate B/OSS. The importance of integrated B/OSS to create a scalable cloud UC platform cannot be stressed enough. It's much easier to implement B/OSS tools around a UC feature set if there is one instance of the software and a common version to integrate to. This is not to say a service provider can't create a B/OSS toolset for a multi-instance architecture, it's just much harder to do since all those VMs are potentially running different software code versions that will need to be managed. Software configuration data often resides in the VM itself, so data from the B/OSS to the VM instances and vice versa must be constantly synchronized for user initiated changes.
* Track record. Look around at a range of Software as a Service (SaaS) / cloud providers that have achieved some degree of business scale and ask yourself if they have a multi-tenant or multi-instance architecture. Most successful cloud services companies have multi-tenant platforms. An example is Salesforce.com. Do Salesforce.com customers receive a dedicated instance of Salesforce.com in a VM when they sign up? Not at all. Instead, Salesforce.com and many other operators understand that a multi-tenant architecture is the way to create a scalable cloud platform from which to deploy services.
Don't be surprised if the issue of multi-tenancy vs. multi-instance hasn't come up in your discussion with UCaaS vendors. But given the impact of the choice and the interest of analysts, it will likely become an important evaluation tool as UCaaS continues to grow in usage.
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