The Unreasonable Effectiveness of Sequence Diagrams in MermaidJS
I can highly recommend PlantUML as a simple language for creating sequence diagrams. It also supports other UML diagram types, which, despite what the article says, are also useful sometimes (so long as you don't worry about every last silly detail that's specified in the standard).<p><pre><code>   @startuml
   Alice -> Bob: Authentication Request
   Bob --> Alice: Authentication Response

   Alice -> Bob: Another authentication Request
   Alice <-- Bob: Another authentication Response
   @enduml
</code></pre>
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