PlantUML

“PlantUML is used to draw UML diagrams, using a simple and human-readable text description. Be careful, because it does not prevent you from drawing inconsistent diagrams (such as having two classes inheriting from each other, for example). So it’s more a drawing tool than a modelling tool.” - PlantUML.org

Diagrams

C4 Model

The C4 model is a lean graphical notation technique for modeling the architecture of software systems. It is based on a structural decomposition (a hierarchical tree structure) of a system into containers and components and relies on existing modelling techniques such as Unified Modeling Language (UML) or entity–relationship diagrams (ERDs) for the more detailed decomposition of the architectural building blocks.

Source code
@startuml
   skinparam dpi 300
   !include <C4/C4_Container>

   AddElementTag("v1.0", $borderColor="#d73027")
   AddElementTag("v1.1", $fontColor="#d73027")
   AddElementTag("backup", $fontColor="orange")

   AddRelTag("backup", $textColor="orange", $lineColor="orange", $lineStyle = DashedLine())

   Person(user, "Customer", "People that need products")
   Person(admin, "Administrator", "People that administrates the products via the new v1.1 components", $tags="v1.1")
   Container(spa, "SPA", "angular", "The main interface that the customer interacts with via v1.0", $tags="v1.0")
   Container(spaAdmin, "Admin SPA", "angular", "The administrator interface that the customer interacts with via new v1.1", $tags="v1.1")
   Container(api, "API", "java", "Handles all business logic (incl. new v1.1 extensions)", $tags="v1.0+v1.1")
   ContainerDb(db, "Database", "Microsoft SQL", "Holds product, order and invoice information")
   Container(archive, "Archive", "Audit logging", "Stores 5 years", $tags="backup")

   Rel(user, spa, "Uses", "https")
   Rel(spa, api, "Uses", "https")
   Rel_R(api, db, "Reads/Writes")
   Rel(admin, spaAdmin, "Uses", "https")
   Rel(spaAdmin, api, "Uses", "https")
   Rel_L(api, archive, "Writes", "messages", $tags="backup")

   SHOW_LEGEND()
@enduml
Diagram