Above observations and concerns are leading us to one of the great bugaboos of software applications over the years, which is their size. The problem is that we simply tend to build too big systems! Although above statements seem to be quite obvious, we still do same mistakes all over again.
Let's think for a while and ask ourselves a question: what we do, when a given class is doing too many things? We simply go and refactor it, so that it sticks to SRP principle. Second question: what we do when our system is doing too much? Basically, we keep adding new features. It is extremely counter intuitive, but that is what we do. In fact, I am not even surprised, as the most common way of teaching TDD is to apply it together with SOLID principles on a unit level. Of course it is true, but it might be extended to all levels of abstraction. However, having a tangled application, it is our duty to apply SRP principle on a system level. Such an approach may lead us, but is not limited, to micro services architecture.
Micro services characteristics
There is no official definition of micro services, but they do have their own, specific characteristics. Micro services might be treated as a result of applying refactoring, with an SRP principle in mind, on a system level. They enable easier testing and re-usage of services. Also, you may treat them as fine-grained SOA approach. Moreover, micro services create real options, as you may adapt different (right) tools for different jobs (Unix system analogy). Exploiting micro services style we are getting independent deployment and scale, easier test and a change (side effect of small systems!) and very important option - flexibility.
Architect and his job
Let's step back for a second and describe what an architect does? An architect is a person who is the 'chief builder' of a system. His role is to satisfy clients' needs. In addition, he should do 'just enough architecture and design upfront'.
In terms of architect's work, it might be compared to city planning. All major requirements have to be envisaged in the plan and embraced accordingly. For instance, there has to be a place for industrial and housing zones, possibly some sub-zones for light and heavy industry, too. I am sure, people would also appreciate nice city centre with greenery and leisure bits. On the flip side, it would not be a good idea to build a playground for children in the middle of industrial zone, instead of housing area. What is more, such a city should have shared utilities like: electricity, waterworks and gas pipelines.
Getting maximum flexibility with upfront design
'Just enough' is in fact very fuzzy quantifier of 'upfront design', so in real life we need more concrete guidelines. We should be starting from asking a question, how we can provide just enough upfront design in order to provide flexibility in our architecture?
Having city planning example in mind, it may lead us to a conclusion that such a high level plan is actually something what business people call a capability map. In simple terms, it is a diagram, which maps business needs and requirements into a set of services talking to each other, where services might be zones and interfaces between services might be utilities, in our city planning example.
In order to get a flexible design, apparently contradictory, but in fact complementary concepts are used i.e. evolutionary architecture and emergent design.
On the other hand, there is a concept of emergent design. It is an idea of deriving a system design in the continuous process of adding incremental changes.
Here comes another, tricky question, how we should use these techniques? Luckily, the rule itself is fairly simple. We apply emergent design on a level of bounded contexts/services (zones) and evolutionary architecture on a level of gaps between bounded contexts i.e. interfaces.
Hexagonal architecture
Also, while designing the general architecture and communication between components/services, it is very important not to obstruct the natural, creative flow of the thoughts by distorted diagrams.
We should be using right tools from the very beginning - hexagonal architecture approach is one of them. It gives us extremely clear view on what should happen in the core of a given application and how that application should communicate with external systems. Apart from that hexagonal architecture introduces more generic concept of looking at a system. In contrary to common approach, we are able to freely express more than two integration points in our model (i.e. UI and DB integrations).
Incidentally, if you were imaging a system that is described in terms of hexagonal architecture and sticking to SRP principle, instantly you would get a handy tool for depicting micro services and making SRP fully visible on a system level! Apart from that such a micro service also conforms to Bovine's conjecture: "objects should be no bigger than one's head", which in our case applies to the system.
Integration
Assuming we have micro services in place, we can start introducing well-defined interfaces (contracts) between them. It will give us a power of communication with external world e.g. micro services.
There are two, very important attributes of decent software:
- high cohesion
- loose coupling
High cohesion relates to a service. Basically it means service should have a single reason to change - that is where SRP comes from. In terms of loose coupling, it might be treated as description of components dependencies to other components. In this case it should be as loose, as possible.
Over the years, in software design we went through many integration patterns and anti-patterns, as well. It would be worth going through majority of them and pointing out theirs characteristics.
Data oriented integration (dependency on DB level):
- no loose coupling
- hard to reason about
- brittle approach
- difficult to maintain and change
Procedure oriented integration (RCP, CORBA, WSDL-binding, JAXB, Java Serialization):
- method calls across boundaries
- sharing serialized object (tight dependency)
- adding methods with different input parameters doing similar things
- above leads to 'God classes' syndrome
- above leads to very difficult change
- above leads to 'objects explosion'
- above leads to SRP and interface segregation principle (ISP) violation
Document oriented integration (JMS, messaging in general):
- asynchronous (decoupled)
- allows additive changes without breaking existing clients
- adding a field in document without break
- renaming a field in document, by adding new field - no breaks
- requires middleware
Resource oriented integration (REST, not HTTP):
- language agnostic
- expose state
- noun oriented, not verb oriented like HTTP
When we talk about integration, it would be hard to forget about Postel's law: "Be liberal in what you do, conservative in what you expect". It might be translated into slightly more practical hint: "only bind to what you need, to reduce breaking service consumption".
As we see, there are many integration styles. However, RESTful approach looks like the most promising one. It is a fairly light concept, providing desired components' properties like loose coupling and it does not repeat errors from the past.
Summary
I believe I have outlined main issues in software design, prodding us to better understanding of the nature of the problem and finding out some better solutions. Searching for a right design is a continuous process and starts from the very beginning. Collecting and defining clients' requirements, the time architects spend on thinking about all use cases and satisfying client defined criteria, developers correctly modelling and implementing ideas can never be neglected. All these actions are crucial in the process of building anti-fragile and well-designed software.
