But as Fowler points out, there are good reasons (e.g. complexity) to make a networked API as fine-grained as possible. How granular/coarse should IoT microservices be? In his blog posting, Fowler suggested that granularity was an open question and that experience with different systems with different levels of microservices granularity would provide eventual insight. I agree with Fowler's view that experience is necessary to decide on 'appropriate' granularity.   I think it's particularly true for the Internet of Things, where multiple people and organizations are attempting attempting to create consistent abstractions for the relatively limited input and output capabilities exposed by newly networked devices...aka 'things'. 

But when actually defining remote services, often the first thing done is to bind the service to a particular transport+protocol+impl framework (e.g. https+json+jersey). Once bound to a transport, the service API may become very difficult to refactor and version. This is especially true once a service has been deployed, but frequently deployment is the only way to get enough real experience to be more (or less) granular!

One way to provide flexibility...and allow future change to the granularity of a service is to remain from the beginning as transport-independent as possible. As described by this article, new standards such as OSGi Remote Services/RSA and ECF's modular implementation of these specs makes transport independence much easier for the microservices designer and developer. One consequence of using such an approach is that once sufficient experience has been gained, it will be much easier to refactor a microservice to be more or less granular. It does not guarantee that all microservices granularity changes will remain possible, but it does make it easier.