Given my (well-known and enduring) interest in all aspects of services, I have followed Martin Fowler's writing on microservices. But I will admit I always found the original paper more confusing than insightful. And in my client work I have resisted the temptation to use a microservices pattern, for precisely the reason that it would more than likely confuse. So I was interested to see the book Building Microservices by Sam Newman published last month, particularly as Newman is part of the Thoughtworks stable, which presumably means it is authoritative.
Right off the bat, Newman advises that we should "think of microservices as a specific approach for SOA in the same way that XP or Scrum are specific approaches for Agile Software development". These analogies are very interesting because my expectation was that microservices is a pattern. So I might infer that microservices is a set of process techniques as opposed to an architectural approach. Yet in the book, Newman clearly includes some elements of concept model and architecture as well as process and organization.
Probably the biggest weakness of Newman's book is the absence of a coherent reference model. He discusses a series of heuristics such as:
- Microservices are small, autonomous services that work together.
- With high cohesion.
- Gather those things that change for the same reason, and separate those things that change for different reasons.
- Using business boundaries.
- How small is small? Could be written in 2 weeks; small enough and no smaller; can be managed by a small team
- Separate entity. All communications between services are network calls. Decoupled. Minimum dependency.
- We need to think about what our services expose, and what they should allow to be hidden. If there is too much sharing our consuming services become coupled to internal representations.
And these are mostly useful principles, but are most unlikely to guide a consistent approach. Given Martin Fowler's involvement, I would have expected at least some good patterns. But mature SOA needs a defined concept model, like CBDI-SAE, SOA-RM etc.
This lack of clarity is fundamental. Many organizations treat operations as services, and they typically end up with thousands of services. They will be confused by the very term microservice because they already have what they perceive as small services. Perhaps the term microservice has been chosen because of the ubiquitous use of the ITIL service, which undoubtedly means monolith. Most mature SOA organizations define services and operations as a primary technique to establish cohesion guided by reference models such as SOA-RM and CBDI-SAE. Just as important is the clarity and precision around specifications or contracts and what is exposed to consumers (APIs) and providers. Yet the book contains nothing on the subject of rich service specification or contract as the primary means of achieving implementation independence and business alignment. Organizations adopting microservices will need a defined reference model. Without that there will be architecture governance by personal opinion. I wonder how Thoughtworks advise their clients in this area?
The book contains some quite general advice on business capability based decomposition - which is a tried and tested method of defining services. However there is little or no advice on what a well-formed business capability is, it's characteristics and governance considerations. In my experience this is an area for considerable confusion; the business capability concept is so well known, but little understood, it's an accident waiting to happen. And as for the relationship between capability and service, there's no model provided, and more importantly, the granularity of a business capability service is likely to be non-trivial.
Similarly there's no guidance on types of service. Do all services contain a mix of channel, process, business and data behaviors? Which you might expect given the explicit advice to gather those things that change for the same reason, and separate those things that change for different reasons.
The one area that might be helpful is the concept that microservices are highly autonomous, and a capability cluster of services would be a unit of deployment. But sadly for Newman, this is an idea that I and my colleagues worked on and popularized nearly 20 years ago. We called it Component Based Development, and the concept was closely allied to Rich Service Specifications which enforced the component boundaries, a concept that seems to have escaped him.
Once upon a time I had expected there to be a microservice pattern. But having read the book, all I see is a set of established ideas being rehashed in a very superficial manner, under a new name; with many key concepts missing or misunderstood. Apparently there is a lack of consensus on how to do SOA well. That failure of SOA is caused by vendors’ products, issues with granularity or picking wrong places to split a system. These assessments of the problem illustrate the lack of understanding of the author and his colleagues. Yes, there have been SOA failures, but mostly they have arisen through a) a focus on integration technology not business, or b) lack of reference model, reference architecture and governance. As an aside, in mature SOA we don’t consider “places to split a system”; we define the scope of a service using well defined heuristics and techniques that vary by type of service. To claim therefore that microservices is the answer to problems with SOA is downright disingenuous. And given the amount of hype I see about the topic generated by supposedly responsible organizations such as Thoughtworks and Gartner, that’s downright irresponsible. Disappointing.
Public Domain information on Mature SOA architecture and delivery practices
Book reference: Building Microservices by Sam Newman