When C4 becomes C5

I've been working with a number of teams recently, helping them to diagram their software systems using the C4 approach that is described in my Software Architecture for Developers book. To summarise, it's a way to visualise the static structure of a software system using a small number of simple diagrams as follows:

1. Context: a high-level diagram that sets the scene; including key system dependencies and actors.
2. Containers: a containers diagram shows the high-level technology choices (web servers, application servers, databases, mobile devices, etc), how responsibilities are distributed across them and how they communicate.
3. Components: for each container, a components diagram lets you see the key logical components/services and their relationships.
4. Classes: if there’s important information that I want to portray, I’ll include some class diagrams. This is an optional level of detail and its inclusion depends on a number of factors.

In the real-world, software systems never live in isolation and it's often useful to understand how all of the various software systems fit together within the bounds of an enterprise. To do this, I'll simply add another diagram that sits on top of the C4 diagrams, to show the enterprise context from an IT perspective. This usually includes:

• The organisational boundary.
• Internal and external users.
• Internal and external systems (including a high-level summary of their responsibilities and data owned).

Essentially this becomes a high-level map of the software systems at the enterprise level, with a C4 drill-down for each software system of interest. Caveat time. I do appreciate that enterprise architecture isn't simply about technology but, in my experience, many organisations don't have an enterprise architecture view of their IT landscape. In fact, it shocks me how often I come across organisations of all sizes that lack such a holistic view, especially considering IT is usually a key part of the way they implement business processes and service customers. Sketching out the enterprise context from a technology perspective at least provides a way to think outside of the typical silos that form around IT systems.