Maintainability
While maintainability is somewhat related to availability, there are specific issues that need to be considered when deploying a topology that is maintainable. In fact, some maintainability factors are at cross purposes to availability. For instance, ease of maintainability would dictate that one should minimize the number of appserver instances in order to facilitate online software upgrades. Taken to the extreme, this would result in a single application server instance, which of course would not provide a high availability solution. In many cases, it is also possible that a single application server instance would not provide the required throughput or performance.
Some of the maintainability aspects that we consider are: