.. _c4-label:



*******************************************************************************
C4 Lightweight Software Architecture Description Method
*******************************************************************************


Introduction
===============================================================================


.. c4-intro-begin-content

    *Big design up front is dumb, but doing no design up front is even
    dumber.* Dave Thomas

The `C4 Model <https://c4model.com/>`__ is a lightweight software
architecture description method. It consists of a set of 4 diagrams that
describe the **static** structure of a software system.

Overall, C4 strives for clarity and communication of the story, and
follows `Shneiderman's
mantra <http://www.ifp.illinois.edu/nabhcs/abstracts/shneiderman.html>`__:

    *Overview first, zoom and filter, then details-on-demand*

It is not formal UML e.g. the UML actor stickman is deliberately not
used as it causes confusion between a person or a system.

.. c4-intro-end-content

.. tip ::
    Multiple independent views of a system are better than just one view. 
    
    PlantUML sequence diagrams can give a good **dynamic** view of a system. Similar to C4, they can start at the top level, and zoom into the details in different diagrams.

.. figure:: ./c4.png
    
    The 4 C's


.. csv-table:: 
   :header: "The **4C's**"
   :widths: 15, 30

    "**Context**", "A high-level diagram that sets the scene; including key system dependencies and people (actors/roles/personas/etc). `Context diagrams <https://en.wikipedia.org/wiki/System_context_diagram>`__ are standard in software engineering (even if not used often)."
    "**Container**", "A container diagram shows the high-level technology choices, how responsibilities are distributed across them and how the containers communicate."
    "**Component**", "For each container, a component diagram lets you see the key logical components and their relationships. 4."
    "**Classes (or Code)**", "This is optional and is the lowest level of detail."
    

CheatSheet
===============================================================================

The
`CheatSheet <http://www.codingthearchitecture.com/2017/04/27/visualising_and_documenting_software_architecture_cheat_sheets.html>`__
gives a good summary of the C4 model and diagrams.

Video Presentation
===============================================================================

See `youtube video from NDC2017
conference <https://www.youtube.com/watch?v=Ym9nhVZs89o>`__ by Simon
Brown on C4 for visualisation.

Related Methods
===============================================================================

There are a number of related models and templates:

4+1 model
-------------------------------------------------------------------------------

C4 is inspired by `the 4+1 model for software
architecture <https://en.wikipedia.org/wiki/4%2B1_architectural_view_model>`__


`ARC42 <https://arc42.org//>`__
-------------------------------------------------------------------------------

C4 can be combined with arc42 documentation template. The diagrams map
as follows:

+---------------------------------+--------------------------+
| Arc42                           | C4                       |
+=================================+==========================+
| Context and Scope               | System Context diagram   |
+---------------------------------+--------------------------+
| Building Block View (level 1)   | Container diagram        |
+---------------------------------+--------------------------+
| Building Block View (level 2)   | Component diagram        |
+---------------------------------+--------------------------+
| Building Block View (level 3)   | Class diagram            |
+---------------------------------+--------------------------+

Data-flow diagrams
-------------------------------------------------------------------------------

`Data-flow diagrams
(DFD) <https://en.wikipedia.org/wiki/Data-flow_diagram>`__ (to describe
data **activity**) use a similar hierarchical approach starting with the
contextual DFD0. They use a multi-level numbering scheme - DFD0: 1 -
DFD1: 1.1., 1.2, - DFD2: 1.1.1, 1.1.2,

Books
-------------------------------------------------------------------------------

These books, written by [@simonbrown](https://twitter.com/simonbrown),
are available to buy from
https://leanpub.com/visualising-software-architecture.


.. |Book1| image:: ./swarchv1.png

.. |Book2| image:: ./swarchv2.png


|Book1|  |Book2|  


.. |`Software Architecture for Developers: Volume 1 - Technical leadership and the balance with Agility <https://leanpub.com/b/software-architecture>`__| image:: swarchv1.png
.. |`Software Architecture for Developers - Volume 2 - Visualise, document and explore your software architecture <https://leanpub.com/visualising-software-architecture>`__| image:: swarchv2.png