\begin{tikzpicture}[
  -latex,
  entity/.style = {
    draw = gray!30,
    thick,
    rounded rectangle,
    fill = white,
    minimum width = 2cm,
    font = \fontsize{8}{8}\selectfont,
    text = white,
  },
  ls/.style = {
    draw=none,
    font=\fontsize{6}{6}\selectfont,
  },
]
  \node[entity,fill=darkGreen](domain) at (0,0) {Subject};
  \node[entity,fill=darkRed](log) at (0,-3) {Log};
  \node[entity,fill=darkBlue](monitor) at (3,-1.5) {Monitor};
  \path[draw,thick,-latex]
    (domain) -|
      node[ls,above]{Notify if you see my certs}
    (monitor);
  \path[draw,thick,-latex]
    (monitor) --
      node[ls,sloped,above]{Found cert!}
    (domain);
  \path[draw,thick,-latex]
    (monitor) |-
      node[ls,below]{Any new certs?}
    (log);
  \path[draw,thick,-latex]
    (log) --
      node[ls,sloped,above]{List of certs}
    (monitor);
  \path[draw,thick,-latex]
    (domain.north) |-
    ($ (domain.west) + (domain.north) + (domain) + (-0.25, 0.25) $)
      node[ls,above,pos=0.75]{
        All ok?
      } |-
    (domain.west);
\end{tikzpicture}
