While it was not the genesis of the architectural style known as microservices, the March 2014 post by James Lewis and Martin Fowler certainly put it on the software development community’s radar. Although the level of interest generated has been considerable, the article was far from an unqualified endorsement:
Despite these positive experiences, however, we aren’t arguing that we are certain that microservices are the future direction for software architectures. While our experiences so far are positive compared to monolithic applications, we’re conscious of the fact that not enough time has passed for us to make a full judgement.
One reasonable argument we’ve heard is that you shouldn’t start with a microservices architecture. Instead begin with a monolith, keep it modular, and split it into microservices once the monolith becomes a problem. (Although this advice isn’t ideal, since a good in-process interface is usually not a good service interface.)
So we write this with cautious optimism. So far, we’ve seen enough about the microservice style to feel that it can be a worthwhile road to tread. We can’t say for sure where we’ll end up, but one of the challenges of software development is that you can only make decisions based on the imperfect information that you currently have to hand.
In the course of roughly fourteen months, Fowler’s opinion has gelled around the “reasonable argument”:
So my primary guideline would be don’t even consider microservices unless you have a system that’s too complex to manage as a monolith. The majority of software systems should be built as a single monolithic application. Do pay attention to good modularity within that monolith, but don’t try to separate it into separate services.
This mirrors what Sam Newman stated in “Microservices For Greenfield?”:
I remain convinced that it is much easier to partition an existing, “brownfield” system than to do so up front with a new, greenfield system. You have more to work with. You have code you can examine, you can speak to people who use and maintain the system. You also know what ‘good’ looks like – you have a working system to change, making it easier for you to know when you may have got something wrong or been too aggressive in your decision making process.
You also have a system that is actually running. You understand how it operates, how it behaves in production. Decomposition into microservices can cause some nasty performance issues for example, but with a brownfield system you have a chance to establish a healthy baseline before making potentially performance-impacting changes.
I’m certainly not saying ‘never do microservices for greenfield’, but I am saying that the factors above lead me to conclude that you should be cautious. Only split around those boundaries that are very clear at the beginning, and keep the rest on the more monolithic side. This will also give you time to assess how how mature you are from an operational point of view – if you struggle to manage two services, managing 10 is going to be difficult.
In short, the application architectural style known as microservice architecture (MSA), is unlikely to be an appropriate choice for the early stages of an application. Rather it is a style that is most likely migrated to from a more monolithic beginning. Some subset of applications may benefit from that form of distributed componentization at some point, but distribution, at any degree of granularity, should be based on need. Separation of concerns and modularity does not imply a need for distribution. In fact, poorly planned distribution may actually increase complexity and coupling while destroying encapsulation. Dependencies must be managed whether local or remote.
This is probably a good point to note that there is a great deal of room between a purely monolithic approach and a full-blown MSA. Rather than a binary choice, there is a wide range of options between the two. The fractal nature of the environment we inhabit means that responsibilities can be described as singular and separate without their being required to share the same granularity. Monoliths can be carved up and the resulting component parts still be considered monolithic compared to an extremely fine-grained sub-application microservice and that’s okay. The granularity of the partitioning (and the associated complexity) can be tailored to the desired outcome (such as making components reusable across multiple applications or more easily replaceable).
The moral of the story, at least in my opinion, is that intentional design concentrating on separation of concerns, loose coupling, and high cohesion is beneficial from the very start. Vertical (functional) slices, perhaps combined with layers (what I call “dicing”), can be used to achieve these ends. Regardless of whether the components are to be distributed at first, designing them with that in mind from the start will ease any transition that comes in the future without ill effects for the present. Neglecting these issues, risks hampering, if not outright preventing, breaking them out at a later date without resorting to a re-write.
These same concerns apply higher levels of abstraction as well. Rather than blindly growing a monolith that is all things to all people, adding new features should be treated as an opportunity to evaluate whether that functionality coheres with the existing application or is better suited to being a service from an external provider. Just as the application architecture should aim for modularity, so too should the solution architecture.
A modular design is a flexible design. While we cannot know up front the extent of change an application will undergo over its lifetime, we can be sure that there will be change. Designing with flexibility in mind means that change, when it comes, is less likely to be an existential crisis. As Hayim Makabee noted in his write-up of Rotem Hermon’s talk, “Change Driven Design”: “Change should entail extending the system rather than refactoring.”
A full-blown MSA architecture is one possible outcome for an application. It is, however, not the most likely outcome for most applications. What is important is to avoid unnecessary constraints and retain sufficient flexibility to deal with the needs that arise.