One frequent topic of discussion is at what level does one make microservices in a microservice architecture. I have made some reference to this already, but I will attempt to summarize it now.
First of all, even if one puts on the cool label of microservice architecture, one would be well advised to remember that it's still just software. For example, although the O'Reilly book "Microservice Architecture" says that "microservices are split by business domains," it's best to consider that as just one method of applying the microservice architecture and as opposed to taking that as dogma. Because software is typically complicated, making a microservice architecture means doing the work of splitting software up into understandable independent parts. Microservices with these independent parts—software components—as units exist. Usually, when breaking up software components, one is trying to use some abstraction. This is because if there wasn't an abstraction and only a concrete implementation that can be understood simply and directly, it would not reduce the complexity to break it up as a software component, and thus there would be no meaning in doing such a thing.
Therefore, the initial criterion for deciding whether or not to break something into a microservice or not becomes "does the thing work as a software component." And thus to conclude one way or another requires one to consider if the thing works as an abstraction. One yardstick as to whether or not the thing works as an abstraction is the ratio of interface to implementation. In other words, things with narrow interfaces that do a lot of work will likely be useful as microservices. (For example, consider the abstraction of a clock which has a minute hand and an hour hand. Even if one doesn't understand the inner workings that let it accurately record time, as long as one understands the interface it can be widely used. That is what one can call a good abstraction.) The book "A Philosophy of Software Design" provides practical advice on what kind of software components are good. As an example, Wantedly has a microservice called "connections" that deals with social graphs. Even if one expects that dealing with social graphs is a complicated task, by abstracting the graph and providing an API, one can see a way to use it as a simple interface in this situation. I think it works well as a software component. This is introduced as the software engineering principle that "modules should be deep."
Even so, if one just wants to make a thing into a software component, one could split it up as a library. There are many differences when comparing microservices to libraries. I will bring up examples as they come to mind. For example, because Web API or gRPC are abstracted as language agnostic protocols, the users and the implementers can use different technologies. That is one difference that is sometimes called being heterogeneous. For another example, for microservices, server process count and CPU computational resources are to some extent independent. However, a library can only utilize the resources of its consumer. This is a major difference between the two. Fault isolation is made possible by having independent computational resources. In connection with this, when deploying an update to a software component, a library requires its users to explicitly update the library as well as push out its own update. However, a microservice only requires a batched update from the component provider. In other words, multiple versions of a service existing at the same time is something that does not happen. Putting all this together, I understand microservice architecture as being a system that uses a language agnostic communication protocol to distribute things. If it is only a distributed system, then there are technologies like Erlang, and for microservices that face problems stemming from distributed systems, Erlang exists and it can solve those problems. That is why I think it's a good choice to make it clear from the designs to not just use a single language.
With all that, the second criterion for deciding whether or not to break something up into a microservice is whether or not it should be a component of a distributed system. To put it more plainly, if one wants to provide uniform behavior for many users, using a microservice is a good idea. There is also the reason of fault tolerance. Next, I will introduce a microservice called "connections." Because there is a need for a service that provides social graphs in a unified way to each place in the Wantedly platform that might want to use it, a microservice rather than a library was the desired choice. Even from the perspective of fault tolerance, when the social graph is temporarily unusable, a default fallback (such as using the normal behavior logic when the logic for dealing with relationships is unusable) is possible.
As a final point, let's talk about being heterogeneous. I have a rather unique opinion on how to decide when to break something up into a microservice. That is to say, for example, if something is written in Rails but you want to use Python and that is the only thing you want, then setting up a monolithic Python service is fine. Despite this, the problem of being heterogeneous is something that frequently comes up and there are definitely times when it is needed. I won't go into the details here but even though using heterogeneous and distributed systems have major advantages, sometimes they won't be complete at the software component level and what to do about that is a problem. I just wanted to mention that as a problem.
Any errata or comments can be made by sending me a pull request.