digraph daas {
  rankdir="LR"

  A [ color="red" label="User" ]
  B [ color="green" label="Web Service" ]
  C [ color="blue" label="Graphviz Dot CLI" ]

  A -> B [ label="http request" ]
  B -> C [ label="system call" ]
  C -> B [ label="results" ]
  B -> A [ label="http response" ]
}