The Two Fundamental Types of Architecture Diagrams

Two diagrams joined by a yin-yang symbol

There are two fundamental types of architecture diagrams from which all others derive. Together they form a duality; their purposes and capabilities are opposite and complementary. Understanding both is critical for creating clear, informative, and relevant architecture diagrams for technical systems.

First, what is an architecture diagram?

Before continuing, we must first define “architecture diagram”. According to alibabcloud

“An architectural diagram is a diagram of a system that is used to abstract the overall outline of the software system and the relationships, constraints, and boundaries between components.”

Components are commonly understood to be discrete, tangible, and bounded resources that provide value to a system. Components may vary in importance, but generally a component is something that is required in order for a system to run properly.

By this definition, components include machines, artifacts, services, repositories, databases, clases, functions, teams, and people, among many others. It excludes intangible things like decisions, use cases, states, and actions, as well as indiscrete resources like money and time.

An architecture diagram of a distributed load testing system running on Amazon Web Services

An architecture diagram of a distributed load testing system running on Amazon Web Services

We’re limiting the definition of architecture diagrams to be only those that include components as defined above. This restriction excludes popular (and useful) diagrams like flow charts and state machine diagrams. We do this to limit the scope of this article, certainly, but it is also in line with the commonly (if vaguely) understood meaning of “architecture diagram”.

Relation diagrams

Relation diagrams describe the invariant relations between components in a system - that is, relations between components that are always true. Common relations include dependency, ownership, belonging, unitilization, and capability. Relation diagrams come in many forms, but are usually as simple as boxes (components) with labeled arrows going between them (relations).

“Lambda getIlograph reads from DynamoDB table Permissions” is an example of an invariant relation. The below diagram doesn’t detail when or how getIlograph reads from Permissions. It merely tells us that the relation between these components exists:

A relation diagram

Relation diagrams are static. Unlike sequence diagrams (discussed below), there is no “arrow of time”. The diagrams do not need to be read in any order, and they have no start or end.

Describing these relations visually in a diagram offers benefits over describing them in text. Viewers can quickly trace the relations in the diagram to find transitive relations. If another resource X invokes Lambda getIlograph, then X has a transitive relation on DynamoDB table Permissions. Finding such transitive relations is easier in a diagram than text:

Click to view interactive

Note that these transitive relations can extend multiple levels and cross context layers.

Additionally, when viewed as a diagram, the scope of these relations is easily read by the viewer. If the diagram includes contextual elements, this information is also conveyed in an unobtrusive way.

What does “always true” mean?

“Always true” should really be “always true from a given perspective”. For example, when diagramming dependency relations of a system, we can (and should) split up run-time dependencies and deploy-time dependencies. By doing this, we separate relations that are “always true” when deploying an architecture from relations that are “always true” when the architecture is actually running.

To take an example, at deploy time, a deployment pipeline may have dependencies on package registries (npmjs, pypi), container image libraries (Docker Hub), code repositories (in, say, GitHub), and the like. At run time, however, those dependencies do not (normally) exist. GitHub going down would not affect a system that has already been deployed, for instance, and splitting up run-time and deploy-time dependencies allows you to clearly convey this:

Click to view interactive

This example illustrates the general need for multiple relation diagrams, one for each perspective of a system. Medium-sized systems could include a half-dozen perspectives. Perspectives could show component relations from security, physical, team and other points of view. You can even have separate perspectives for the current state and a future state(s) of a system that is being actively developed.

Sequence diagrams

Sequence diagrams are distinct from and complimentary to relation diagrams. While relation diagrams show how components are statically related, sequence diagrams show how components dynamically interact.

Sequence diagrams show things that happen between components, when they happen, and how. For example, while the relation diagram above shows that Lambda getIlograph reads from DynamoDB, a sequence diagram shows examples of when this happens.

A sequence diagram

In this example sequence diagram (which is a kind of UML diagram), we see some of the same components as in the relation diagram. It shows us a very specific activity being performed: an Ilograph diagram being retrieved from S3. The diagram shows us what components are involved, how they interact with each other, and in what order those interactions happen.

While relation diagrams give static context to components, sequence diagrams give temporal context to components. They show what happens before and after key interactions between components.

Unlike (static) relation diagrams, sequence diagrams “have an arrow of time”. They are designed to be read in a certain order, and have a start and an end. They are read like a book, that is, left-to-right and top-to-bottom (when written in left-to-right languages).

Sequence diagrams are generally lower-level and more detailed than relation diagrams. Just like with relation diagrams, however, the level of detail can vary:

Click to view interactive

Critically, like with relation diagrams, there are many possible sequence perspectives for a given system. In fact, there are probably dozens of useful interactions that can be diagrammed in a medium-sized project. The example below switches between five sequence diagrams of a single system (creating a test, executing a test, writing test results, listing running tests, and canceling a test):

Click to view interactive

Working together

Relation diagrams and sequence diagrams complement one another. What one can’t show well, the other typically can. For most projects, a generous mix of relational and sequence diagrams can thoroughly document an architecture. A half-a-dozen or so relation diagrams, and at least that many sequence diagrams, is a good goal for a medium-sized system.

Questions or comments? Please reach out to me @ilographs on Twitter or by email at

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