\documentclass{standalone}
\usepackage{tikz}
\usepackage{tikzscale}
\pgfdeclarelayer{background,foreground}
\pgfsetlayers{background,main,foreground}
\usepackage{tikz-uml}
\usepackage[english]{babel}  % load this package so tikz-uml loads, it may be due to my japanesse localle lol.

\begin{document}
\begin{tikzpicture}
	\begin{umlseqdiag}
		\umlactor{Player A}
		\umlobject{Server}
		\umlactor{Player B}
		
		% Player spawns, no other player on the map
		\begin{umlcall}[op=new-player,return=update-players]{Player A}{Server}
		\end{umlcall}
		
		% Player spawns after player A, update both
		\begin{umlcall}[dt=7.5,op=new-player,return=update-players]{Player B}{Server}
		
		\begin{umlcall}[type=return,op=update-players]{Server}{Player A} 
		\end{umlcall} 
		\end{umlcall}
		
		% Player B shoots a bullet
		\begin{umlcall}[op=shoot-bullet,return=bullets-update]{Player B}{Server}
		\begin{umlcall}[type=return,op=bullets-update]{Server}{Player A} 
		\end{umlcall} 
		\end{umlcall}
		
		% Player A Moves
		\begin{umlcall}[op=move-player,return=update-players]{Player B}{Server}
		\begin{umlcall}[type=return,op=update-players]{Server}{Player A} 
		\end{umlcall} 
		\end{umlcall}
		\begin{umlfragment}[type=loop]
		\begin{umlcallself}[dt=5,op=ServerGameLoop]{Server}
		\begin{umlcallself}[op=spawnHealthPacks]{Server}
		\end{umlcallself}
		% Update hearts, mini-ships on both sides
		\begin{umlcall}[type=return,op=healthPack-update]{Server}{Player A} 
		\end{umlcall} 
		\begin{umlcall}[type=return,op=healthPack-update]{Server}{Player B}
		\begin{umlcall}[type=return,op=bullet-update]{Server}{Player A} 
		\end{umlcall}  
		\begin{umlcall}[type=return,op=bullet-update]{Server}{Player B} 
		\end{umlcall}  
		\end{umlcall} 
				\end{umlcallself}
		\end{umlfragment}
	\end{umlseqdiag}
\end{tikzpicture}


\end{document}