Level 1: System Context Diagram
The System Context Diagram is the highest level of abstraction in the C4 model, providing a bird's-eye view of your system. It shows the system as a single "black box" and illustrates how it fits into the wider IT and business landscape.
This diagram is the starting point for understanding a software system, showing what it does (and doesn't do) without revealing any internal implementation details.
What it Shows
Your System: The software system being described, placed in the center.
Actors: The people (users, roles, personas) who interact directly with the system.
External Systems: The other software systems that your system interacts with (e.g., third-party APIs, other internal systems, databases).
Interactions: The high-level relationships and data flow between actors, your system, and external systems.
Purpose and Scope
Audience: Everyone, including business stakeholders, non-technical staff, developers, and architects. It's the perfect diagram to start a conversation about the system.
Goal: To show the system's scope and its connections to the outside world. It clearly defines the system's boundaries.
Key Message: "Here is the system we are building, here are the people who use it, and here are the other systems it interacts with."
What it Doesn't Show
Technology choices.
Internal architecture or implementation details.
Protocols or specific data formats.
PlantUML Example
Below is an example of a System Context diagram for a fictional Internet Banking System.