IT Architecture languages
UML, C4 and ArchiMate all three can be used for visualizing IT Architecture. This does however not mean that they are equal. Each of the languages was designed for a specific need.
UML originated in 1994 because there was a need to describe object-oriented software. In 2006 the need for more structure than the UML arrows and boxes resulted in the C4 language.
In 2002 ArchiMate development started because of the need for a language to describe an enterprise.
From an ArchiMate perspective C4 defines a number of viewpoints.
UML
UML was designed to be a general-purpose modeling language for visualizing system design. It provides a language for many types of diagrams including behavior diagrams, interaction diagrams, and structure diagrams.
UML is said to be decreasing in popularity (source: Google Trends) but the language being around for more than 30 years and it's general-purpose nature make that it's use is still widespread.
Commonly used
As mentioned, the use of UML is widespread. Activity Diagrams are well known for visualizing system flows. Within software development these diagrams are often used:
- use case diagrams, for interactions between actors;
- class diagrams, for class relationships, attributes and methods;
- sequence diagrams, for a sequence of messages between participants.
C4
The C4 language was designed to be easy to learn and developer friendly. It reaches that goal by focussing on clarity and simplicity.
From the C4 documentation:
A software system is made up of one or more containers (applications and data stores), each of which contains one or more components, which in turn are implemented by one or more code elements (classes, interfaces, objects, functions, etc).
The four emphasized words in this one sentence correspond to the four static structure diagrams that define the core of C4.
- (system) context diagram
- container diagram
- component diagram
- code diagram
Additionally there's three more diagrams:
- system landscape diagram
- dynamic diagram
- deployment diagram
But that's it. Simple to use and easy to learn.
A few examples of C4 elements
Commonly used
C4 being specifically designed for software architecture it is mainly used by solution and software architects. Security and enterprise architects tend to choose for ArchiMate instead because of the limited C4 set of elements.
ArchiMate
From the three languages UML defines more than 100 elements. C4 is a lot simpler with no more than 10 elements. The 3.1 Specification of ArchiMate counts around 60 elements.
One of the design goals was to make ArchiMate 'as small as possible'. As in, the goal was not to cover every possible edge case.
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C4 compared to ArchiMate
C4 can be seen as a subset of ArchiMate, the C4 diagrams can be expressed with the Archimage viewpoint concept. Depending on how you look at it, the C4 elements can be mapped in a couple of ways to ArchiMate elements.
System
The C4 'System' element is described as something that delivers value to users. ArchiMate defines a 'Value Stream' element which describes behavior on the strategy layer. But since C4 also defines a 'System' as something a single software development team is building, the 'Application Function' or 'Application component' element would be a better fit.
Container
The C4 'Container' element is describes as something that needs to be running for the system to work. The ArchiMate 'Application Component' element could be used to express this.
Component
The C4 documentation states that a 'Component' is a "hugely overloaded term". It's true, the word component can be used in many different ways. C4 defines it as 'grouping of related functionality encapsulated behind a well-defined interface'. This could be an ArchiMate 'Application Component' or 'Application Service' element. The 'well-defined interface' can be modelled separately in ArchiMate by using the 'Application Interface' element.
Code
The C4 element 'Code' is hard to model in ArchiMate. The 'Application Component' element can be used to express a software library and the 'Data Object' element can be used, but concepts like 'classes', 'attributes' and 'methods' are not available. The 'Application Component' element can be used for this if you really have to, but UML and C4 are a much better suited for this.
Application Component
Simply put, all C4 elements can be expressed by ArchiMate 'Application Component' elements. It makes clear how C4 and ArchiMate differ. C4 defines four sizes of 'Application Components' whereas ArchiMate has nine different 'Application' elements like 'Interface', 'Function' and 'Service' that can be used to add more detail.
Besides the 'Application' layer, ArchiMate defines five other layers and uses Motivation elements for modelling motivations.
Structure as code
All three languages have code equivalents. PlantUml being the most popular. It can be extended to support the ArchiMate and C4 languages by using macro libraries.
