C4 diagram online
The editor below opens with a system context diagram for a fictional outreach product. Rename the people and systems, add the external services yours depends on and label each relationship. Your work is saved in this browser.
What goes in a system context diagram
One box for the system you are describing, one for each person who uses it and one for each external system it talks to. Label every arrow with what crosses it, such as "sends campaign email through". Leave the internals out; that is what container and component diagrams are for.
Keep it next to the rest of your docs
In a free ArchDocs account, each box can be a reference to an object in your catalog. The context diagram, the container diagram and the runbook then point at the same service, and renaming it updates all three.
Questions
Do I need an account?
No. The editor runs in your browser and keeps your drawings there. A free account lets you open them from another computer and share them with your team.
Is this strict C4 notation?
It follows the C4 system context level. You can still draw anything else on the same canvas, such as a note or an arrow the model has no name for.
Can I write it as text?
Yes. Open the YAML panel to edit the elements and relationships directly. The canvas redraws as you type.