Martin Fowler says: “Don't even consider Microservice unless you have a system that's too complex to manage as a monolith.” The reason for his caution is that Microservice is not a free lunch (nothing in life that is worthy is free): there is a great overhead in implementing Microservice.
He shows the overhead in this diagram:
So where does the overhead lie? I like this diagram from http://microservices.io/patterns/index.html, which shows that the Miroservice architecture involves much more than just application design:
Underneath the application layer, there are the application infrastructure layer and the infrastructure layer. In today’s technology, the application infrastructure layer usually involves some highly robust message broker, logging gathering and analysis tools etc; while the infrastructure layer usually involves some service registry and discovery, docker orchestrating tools etc; on the application layer, you have to consider how all the services can behave nicely together, you have to consider API compatibility, failure handling, eventual consistency etc.
So not only Microservice is not a free lunch, it is quite expensive!
Martin Fowler is right to caution you about Microservice, but I think he is wrong in saying that you should start with monolith. There are great benefits in starting with Microservice in mind – even if you are building a monolithic application.
I got this awakening when I tried to dissect one of the systems I have worked on. I first listed its features, and picked one to practice. I thought it wouldn’t be too hard because it has a very clear functionary boundary, but pretty soon I found out it has a lot hidden dependencies with other functions of the system.
In an aging system, hidden dependencies and relationships are hard to find. We thought we were building a clearly layered beautiful system:
We thought we have walls along each layer and around each component. But in reality, because everything is in the same code base and in the same DB, it is so easy to break the walls and even tunnel underneath the walls. Sometimes you do this without even a second thought especially when you saw there were already holes in the wall. You ended up with this:
This is very bad: business logic is scattered in the code and in the database models.
I wanted to break out moduleA as a Microservice, and discovered its hidden dependencies through discussing with my colleagues and digging into the database models. In the process, often times, discussions like the following sprung up:
“Why did we design this model like this?”
“Probably to provide the maximum flexibility.”
“Do we have any customers using this function in this way?”
“Probably not many, but maybe a few…”
“So we went to so much trouble just for maybe a few…”
If we had started with Microservice in mind, we would have setup walls around the bounded context. We might be building a monolith, but comparing with a monolith without having Microservice in mind, the walls will be guarded more carefully because we have the idea or hope that one day we might break things apart into Microservices.
The conversation, in this situation, might be like this:
“We need this feature.”
“This feature requires communicating across walls, it is expensive. Do you really need it?”
“I didn’t know it was so expensive. We need to think about this carefully.”
Having visible walls will force us to be economic, it will force us to weigh benefits and costs. It doesn’t mean that we can’t open holes or create tunnels underneath to work around the walls – even in a Microservice architecture, we can still make a mess, at the end of the day, it is our skills that count. But having Microservice in mind force us to do deliberate thinking and make decisions explicit.
The moduleA in my system has the following data models:
It has this structure because in many of its tables, either “flexibility” is reserved for some unknown future usage or because other functions tried to piggyback on those tables (e.g. adding some fields into the tables, the new fields belong to different domain models than the tables). Applying DDD, moduleA’s core can be structured as:
Of course, I am benefited by 100% hindsight. But I believe if there is one single bullet, it is: start with simple, evolve it into complexity after it is proven.