C4 model

A way to draw software architecture at four zoom levels: system context, containers, components and code.

Simon Brown’s C4 model starts with one box for your system and the people and systems around it, then zooms into the deployable parts (apps, databases, queues), then into the components inside one part. Most teams need only the first two levels, kept current. It replaces the one-off whiteboard picture with diagrams anyone can read, which helps onboarding, audits and technical due diligence.

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Related terms

ADR (architecture decision record): A short document, kept with the code, that records one architecture decision: the context, the options, the choice and its consequences.
Runbook: A short document that tells someone else how to run a system: where it lives, how to deploy, how to recover.
Technical due diligence: A review of a company’s software, team and processes before an investment or acquisition, to price the technical risk.

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