Simplicity: One Concept
For a system to survive long-term incremental development, it helps to have one concept that stays true.
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I've been thinking and writing about simplicity in architecture and design as a result of reading Niklaus Wirth's article on programming languages. The first article was about how arguments over design can result from having multiple kinds of simplicity, while the second was about the need for architects to communicate using one diagram.
This time, I want to discuss a kind of simplicity related to architecture and communication. In any complex system, there are a lot of interrelated pieces with associated interfaces. It is impossible to have a complete mental model of all of the pieces down to the lowest level of detail. So we need an abstraction.
But the kind of abstraction we pick can make a large difference in the ability to understand the system and make useful predictions about its behavior. This is hard to talk about in general, so let's take a specific example. The core idea of a Service Oriented Architecture (SOA) seems like a very simple and useful abstraction. And there is some value to it. It suggests that the various pieces of the system will be encapsulated in services that will be as independent from each other as possible. These services will use each other via defined, typically language agnostic interfaces.
So by saying that a system uses SOA we have said something useful about it that enables us to think about the system without having to go into every detail about every piece. But I think there's an insufficiency here that is the reason that so many attempts to do SOA (or similarly to do a microservice architecture) go badly wrong. We've established rules for how one piece of our system talks to another, but we haven't established any rules as to which pieces generally should communicate with each other,when, or what they should communicate.
I'll show my age a little by pointing out that SOA (and even some microservice architectures) remind me of CORBA. CORBA also specified a model for interaction between components in a system. We would ignore process boundaries, locality, and programming languages, and define objects that accepted and responded synchronously to messages as defined in an interface description language.
Unfortunately, with only the how of communication specified, many CORBA systems became a hodge-podge of objects communicating with and dependent on each other, resulting in brittle systems that struggled with performance. And also, unfortunately, there's been a share of that with SOA and more recently with microservice architectures as well.
So what is an example of a single unifying concept that is abstract enough to unite the pieces of a single system, while having enough meat to specify not just interactions but also rules of behavior? I don't think I can identify a rule for coming up with such an abstraction, but I've seen a couple that worked for real-world systems.
A while back I was the architect for a system that provides near-real-time data updates from various specialized pieces of hardware, while also allowing the system users to control that hardware. This system uses an n-tier architecture, but since there are multiple hardware devices and events are fed into the system from both the users and the hardware, it had the potential to become uncontrolled and therefore difficult to manage state. To avoid this, the concept we chose was "synchronous down, asynchronous up". User input is fed into the application via synchronous remote calls to a stateless application layer, where the user interface is notified that the input was received successfully. This constrains the user interface to assemble complete requests to the system, since there is no mechanism to fire off partial requests. Meanwhile, data from the hardware devices is fed asynchronously to the application when received, even if the hardware device is queried via polling. This has the effect of decoupling the interfaces used by the user interface from those used by the hardware (since one is remote invocation while the other is messaging), effectively separating the two types of inputs to the system. Similarly, data updates to the user interface go asynchronously, which simplifies the behavior of the application tier and permits it to be agnostic as to how many users there are and how they use the data.
All of which are a great many words to say things that are inherent in the concept of "synchronous down, asynchronous up". I think this is a good post hoc indicator that the single system concept is truly one that simplifies thinking about the system while also specifying its behavior; it should be possible to use the concept to illuminate a large number of system behaviors that otherwise do not appear related.
On another recent system, we were presented with the challenge of building an architecture for a set of unknown applications, with the desire of integrating arbitrary future applications. For this system, we chose a concept of "distributed integration" based on the Enterprise Integration Patterns. The preferred backbone communication is publish-subscribe messaging using a canonical data model, but if individual applications show up with custom data models or with custom interfaces, the system accommodates this by adding integration capabilities that are as local to the application as is practical. Similar to the above, this concept serves as a guide for the "right way" to perform a variety of behaviors in the system. For example, if an application produces data that must be persisted to a database, rather than having a single persistence application that knows all about every application, a new small piece is added to perform persistence for that specific application's data. Or, if Application A relies on a capability provided by Application B, but expects a REST interface rather than directly following the Request-Reply pattern, that REST interface should be created as a separate piece rather than requiring Application B to add that REST interface. This is another valuable thing to look for in a simplifying system concept; it should make it easier to make decisions about how to implement the system in cases where the decision could go either way.
So while it might not be easy to identify a process for devising the "one concept" for a system, I think there are a few lessons we can draw from these examples. First, it's necessary but not sufficient to identify a category of architecture like "SOA" or "microservice". Those categories are valuable to get people thinking in the right direction, but they allow too much flexibility in implementation to have a consistent system. Second, the system concept has to be specific to the end goals of the system. There's an interesting parallel to the "As a... I want... so that..." template used for user stories; the system concept should tie back directly to key system goals. Finally, the system concept should work both retrospectively (to explain to maintainers why the system works the way it does) and prospectively (to help maintainers understand how to add capability without breaking the system).
As I said above, I don't think I have a good process or approach for coming up with this concept; it probably falls into a "magic happens here" step. But as architects, time spent thinking about the system and what it needs to accomplish is well spent anyway, and ending up with a good diagram and a good single concept might enable our system to last longer and work better.
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