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C4 diagrams provide a way to visualize software architecture at different levels of abstraction. The C4 model consists of hierarchical diagrams showing system context, containers, components, and code.
C4 diagrams are experimental in Mermaid. The syntax and properties may change in future releases. Syntax is compatible with PlantUML.

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Diagram types

Mermaid supports five types of C4 diagrams:
  1. System Context (C4Context) - Shows the system and its users
  2. Container (C4Container) - Shows high-level technical building blocks
  3. Component (C4Component) - Shows components within a container
  4. Dynamic (C4Dynamic) - Shows runtime behavior and collaborations
  5. Deployment (C4Deployment) - Shows deployment topology

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System context diagram

Shows the big picture of the system and how it fits into the world:

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Container diagram

Shows the high-level technology choices and how containers communicate:

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Component diagram

Shows how a container is made up of components:

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Dynamic diagram

Shows how elements collaborate at runtime:

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Deployment diagram

Shows how containers are deployed to infrastructure:

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Styling elements

Customize the appearance of elements:
UpdateElementStyle(elementName, $bgColor="color", $fontColor="color", $borderColor="color")
Example:
UpdateElementStyle(customerA, $fontColor="red", $bgColor="grey", $borderColor="red")
Customize the appearance of relationships:
UpdateRelStyle(from, to, $textColor="color", $lineColor="color", $offsetX="value", $offsetY="value")
Example:
UpdateRelStyle(customerA, SystemAA, $textColor="blue", $lineColor="blue", $offsetX="5")
Adjust the layout configuration:
UpdateLayoutConfig($c4ShapeInRow="number", $c4BoundaryInRow="number")
Example:
UpdateLayoutConfig($c4ShapeInRow="3", $c4BoundaryInRow="1")

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Parameter assignment

There are two ways to assign optional parameters (those with ? prefix):

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Positional parameters

UpdateRelStyle(customerA, bankA, "red", "blue", "-40", "60")

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Named parameters

Parameter names must start with $: 
UpdateRelStyle(customerA, bankA, $offsetX="-40", $offsetY="60", $lineColor="blue", $textColor="red")
UpdateRelStyle(customerA, bankA, $offsetY="60")
Use named parameters for better readability and when you only need to set specific optional parameters.

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Supported elements

  • Person(alias, label, ?descr, ?sprite, ?tags, $link)
  • Person_Ext - External person
  • System(alias, label, ?descr, ?sprite, ?tags, $link)
  • SystemDb - System database
  • SystemQueue - System queue
  • System_Ext - External system
  • SystemDb_Ext - External database
  • SystemQueue_Ext - External queue
  • Boundary(alias, label, ?type, ?tags, $link)
  • Enterprise_Boundary(alias, label, ?tags, $link)
  • System_Boundary
  • Container(alias, label, ?techn, ?descr, ?sprite, ?tags, $link)
  • ContainerDb - Container database
  • ContainerQueue - Container queue
  • Container_Ext - External container
  • ContainerDb_Ext - External database
  • ContainerQueue_Ext - External queue
  • Container_Boundary(alias, label, ?tags, $link)
  • Component(alias, label, ?techn, ?descr, ?sprite, ?tags, $link)
  • ComponentDb - Component database
  • ComponentQueue - Component queue
  • Component_Ext - External component
  • ComponentDb_Ext - External database
  • ComponentQueue_Ext - External queue
  • Rel(from, to, label, ?techn, ?descr, ?sprite, ?tags, $link)
  • BiRel - Bidirectional relationship
  • Rel_U, Rel_Up - Upward relationship
  • Rel_D, Rel_Down - Downward relationship
  • Rel_L, Rel_Left - Left relationship
  • Rel_R, Rel_Right - Right relationship
  • Rel_Back - Backward relationship

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Limitations

The following features are not currently supported:
  • Sprites (custom icons)
  • Tags
  • Links
  • Legend
  • Layout statements (Lay_U, Lay_D, Lay_L, Lay_R)
  • Custom element and relationship tags (AddElementTag, AddRelTag)
C4 diagrams have a fixed style. Different CSS themes do not affect C4 diagram appearance.

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