diagram.tex

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows.meta, positioning, decorations.pathreplacing}

\pgfdeclarelayer{bg}    % declare background layer
\pgfsetlayers{bg,main}  % set the order of the layers (main is the standard % layer)

\begin{document}
\begin{tikzpicture}[>=stealth, shorten >= 5pt, node distance=4em, scale=1]
  \tikzstyle{vertex} = [circle, scale=0.5]
  \tikzstyle{O_0} = [vertex, fill=black!30!green]
  \tikzstyle{O_1} = [vertex, fill=black!30!blue]
  \tikzstyle{O_2} = [vertex, fill=black!30!red]

  \tikzstyle{to} = [-{Stealth[scale=1.2]}]
  \tikzstyle{toO_0} = [to, color=black!30!green]
  \tikzstyle{toO_1} = [to, color=black!30!blue]
  \tikzstyle{toO_2} = [to, color=black!30!red]

  \tikzstyle{op} = [midway, above=-3pt, sloped, text=black, font=\small]

  %% Server receives operations in this order
  \node (s) at (4, 6) {$S$};
  \coordinate (se) at (4, 0) {};
  \node[O_0, below = 2em of s,] (s1) {};
  \node[O_1, below = 2em of s1,] (s2) {};
  \node[O_2, below = 2em of s2,] (s3) {};
  \node[O_1, below = 3em of s3,] (s4) {};

  %% User 0 generates/receives in this order
  \node (u0) at (10, 6) {$u_0$};
  \node (u0e) at (10, 0) {};
  \node[O_2, below = 2em of u0,] (u01) {};
  \node[O_0, below = 2em of u01,] (u02) {};
  \node[O_1, below = 3em of u02,] (u03) {};
  \node[O_2, below = 2em of u03,] (u04) {};

  %% User 1 generates/receives in this order
  \node (u1) at (7, 6) {$u_1$};
  \node (u1e) at (7, 0) {};
  \node[O_0, below = 1em of u1,] (u10) {};

  %% User 2 generates/receives in this order
  \node (u2) at (0, 6) {$u_{new}$};
  \node (u2e) at (0, 0) {};
  \node[O_1, below = 2em of u2] (u20) {};
  \node[O_1, below = 10em of u20] (u21) {};

  \begin{pgfonlayer}{bg} % select the background layer
    \draw[to, color=black!30] (s) -- (se);
    \draw[to, color=black!30] (u0) -- (u0e);
    \draw[to, color=black!30] (u1) -- (u1e);
    \draw[to, color=black!30] (u2)  -- (u2e);

    % Life of O_0
    \draw[toO_0] (u10) -- (s1) node [op, near end] {$O_0$};
    \draw[toO_0] (s1) -- (u02) node [op, pos=0.6] {$O_0$};

    % Life of O_1
    \draw[toO_1] (u20) -- (s2) node [op] {\texttt{connect-request}};
    \draw[toO_1] (s2) -- (u03) node [op, pos=0.72] {\texttt{buffer-request}};
    \draw[toO_1] (u03) -- (s4) node [op, near end] {$O_a$};
    \draw[toO_1] (u03) -- (s4) node [op, draw=none, near end, below=-2pt] {\texttt{buffer-response}};
    \draw[toO_1] (s4) -- (u21) node [op] {$O_1 \circ O_0 \circ O_1^{-1} \circ O_a$};
    \draw[toO_1] (s4) -- (u21) node [op, draw=none, below=-2pt] {\texttt{operations}};
    \draw [decorate,shorten >=0pt,decoration={brace, amplitude=10pt}] (u21.west) -- (u20.west) node [midway,xshift=-0.8cm] {\footnotesize wait};

    % Life of O_2
    \draw[toO_2] (u01) -- (s3) node [op, pos=0.6] {$O_1$};
    \draw[toO_2] (s3) -- (u04) node [op, pos=0.35] {$O_1 \circ O_0 \circ O_1^{-1}$};
  \end{pgfonlayer}
\end{tikzpicture}
\end{document}