3.5 Microservices with a UI?
This book recommends that you equip microservices with a UI. The UI should offer the functionality of the microservice to the user. In this way, all changes in regards to one area of functionality can be implemented in one microservice—regardless of whether they concern the UI, the logic, or the database. However, microservice experts so far have different opinions in regards to the question of whether the integration of UI into microservices is really required. Ultimately, microservices should not be too large. And when logic is supposed to be used by multiple frontends, a microservice consisting of pure logic without a UI might be sensible. In addition, it is possible to implement the logic and the UI in two different microservices but to have them implemented by one team. This enables implementation of features without coordination across teams.
Focusing on microservices with a UI puts the main emphasis on the distribution of the domain logic instead of a distribution by technical aspects. Many architects are not familiar with the domain architecture, which is especially important for microservices-based architectures. Therefore, a design where the microservices contain the UI is helpful as a first approach in order to focus the architecture on the domains.
Instead of calling this approach “Microservice with UI” you can also call it “Self-Contained System” (SCS).6 SCS define microservices as having about 100 lines of code, of which there might be more than one hundred in a complete project.
An SCS consists of many of those microservices and contains a UI. It should communicate with other SCSs asynchronously, if at all. Ideally each functionality should be implemented in just one SCS, and there should be no need for SCSs to communicate with each other. An alternative approach might be to integrate the SCSs at the UI-level.
In an entire system, there are then only five to 25 of these SCS. An SCS is something one team can easily deal with. Internally the SCS can be divided into multiple microservices.
The following definitions result from this reasoning:
SCS is something a team works on and which represents a unit in the domain architecture. This can be an order process or a registration. It implements a sensible functionality, and the team can supplement the SCS with new features. An alternative name for a SCS is a vertical. The SCS distributes the architecture by domain. This is a vertical design in contrast to a horizontal design. A horizontal design would divide the system into layers, which are technically motivated—for instance UI, logic, or persistence.
A microservice is a part of a SCS. It is a technical unit and can be independently deployed. This conforms with the microservice definition put forward in this book. However, the size given in the SCS world corresponds to what this book denotes as nanoservices (see Chapter 14).
This book refers to nanoservices as units that are still individually deployable but make technical trade-offs in some areas to further reduce the size of the deployment units. For that reason, nanoservices do not share all technical characteristics of microservices.
SCS inspired the definition of microservices as put forward in this book. Still there is no reason not to separate the UI into a different artifact in case the microservice gets otherwise too large. Of course, it is more important that the microservice is small and thus maintainable than to integrate the UI. But the UI and logic should at least be implemented by the same team.