[Tex/LaTex] How to draw an arc between two Cartesian coordinates and fit nodes into a big one

arrowstikz-pgf

I'm trying to do this diagram using TikZ but I haven't managed to draw the curved arrow (see what I tried in the commented lines). How can I draw an arc between two Cartesian coordinates? How can I fit all the diagram into a rectangle?
What I have did doesn't work…

I want something like this:

\draw [->, radius=r] (x1,y1)  arc (x2,y2);

I have tried the solution given in the comment but I'm not satisfied.

MWE

\documentclass[border=10pt]{standalone}
%\usepackage[margin=1cm]{geometry}
\usepackage{tikz,xcolor}
\usepackage{hyperref}
\usepackage{tkz-euclide}
\usetkzobj{all} 

\usetikzlibrary{shapes.geometric,arrows,positioning,fit,calc,}
\tikzset{
b/.style={draw, rectangle, rounded corners=2ex,minimum height=0.5in, minimum width=2in,align=center},
c/.style={draw, rectangle, rounded corners=2ex, minimum height=0.5in, minimum  width=2in,align=center, rotate=-90},
ar/.style={rounded corners=2ex,->,>=latex },  
myarrow/.style args={#1 colored by #2 and #3}{
    -stealth,line width=#1,#2,postaction={draw,-stealth,#3,line width=(#1)/3,
    shorten <=(#1)/3,shorten >=2*(#1)/3}, 
  }
}

\makeatletter
\newcommand{\gettikzxy}[3]{%
  \tikz@scan@one@point\pgfutil@firstofone#1\relax
  \edef#2{\the\pgf@x}%
  \edef#3{\the\pgf@y}%
}
\makeatother


\begin{document}
\begin{tikzpicture} 
       \node (client) [b] {Client};
       \node (serveur) [b, left=4 cm of client.west] {Serveur};
       \gettikzxy{(client)}{\axc}{\ayc}
       \draw[thick] (client.south) -- (\axc, \ayc - 300);
       \gettikzxy{(serveur)}{\axs}{\ays}
       \draw[thick] (serveur.south) -- (\axs, \ays - 300);
       \draw [ar] (\axc, \ayc - 100) -- node[above]{\parbox[t]{5cm}{\centering POST \nolinkurl{authentification/}\\username=user\&password=abc123}} (\axs, \ays -100);
       \draw [ar] (\axs, \ays - 150) -- node[above]{\parbox[t]{5cm}{\centering HTTP 200 OK\\ \color{green}{\{Token : "JWT"\}}}} (\axc, \ayc -150);
       \draw [ar] (\axc, \ayc - 200) -- node[above]{\parbox[t]{5cm}{\centering GET \nolinkurl{api/user/id}\\ \color{green}{JWT : Token} }} (\axs, \ays -200);

       \draw [ar] (\axc, \ayc - 250) -- node[below]{\parbox[t]{5cm}{\centering HTTP 200 OK\\ \color{green}{\{name : foo\} }}} (\axs, \ays -250);

       %\draw[->] (\axs,\ays - 200) arc (-120:30:5) ;
       % Draw the arc which center is (2,1)
       %\draw[thick,red] ([shift=(90:1cm)]\axs,\ays-175) arc (90:360:0.9cm);
      % \draw [->] (\axs,\ays-150)  arc (\axs,\ays-200,2.1);
       %\draw [<->] (0:2.1)  arc (0:60:2.1);
       %\draw [->,line width=5pt]  (\axs:-200) arc (\axs:-300);

       %\draw [ar] ([yshift=0.5cm]serveur.south)  -- node[above]{
    %                             \parbox[t]{5cm}{POST authentification/\\username=...\&password=..}} ([yshift=0.5cm]client.south);
       %\draw [ar] ([yshift=-0.5cm]client.south) -- ([yshift=-0.5cm]serveur.north);

\node [rectangle, opacity=0.2, fill=blue, fit=(client)(serveur),]{};
\draw[->, thick] (\axs-3,\ays-150) to[bend left=250] (\axs-3,\ays-200);
  \end{tikzpicture}

\end{document}

enter image description here

Best Answer

Well, here's a suggestion. It uses the accepted answer from What's the easiest way to draw the arc defined by three points in TikZ? to draw the arc. A third coordinate is placed midway between the two points, offset a bit to the left, and used as a midpoint for the arc. Modifying this offset changes the radius of the circle, although the offset is of course not the same as the radius.

I also changed how all the lines and arrows are drawn. Whether you think this is an improvement or not I'll leave for you to decide. Note that you can define a text width and alignment for nodes, so there is no need to add a \parbox in each node. See the new toptext style.

Regarding the background fill: The fit library lets you create a node that encompasses a list of other nodes. When you only add the two top nodes to this list, it will of course not cover the entire figure. What I did was to add a new node bottom at the end of one of the vertical lines, and add this to the fit= node list. (You could also use the anchors of the current bounding box node.) Using the backgrounds library I placed the fill on the background, so it doesn't cover up the other elements.

A final note: \color{green} is a switch that makes all the following text green. Hence, the final pair of braces in \color{green}{text} is not needed, and they wont limit the effect of \color. There is however a \textcolor command that does use such a syntax, \textcolor{green}{green text}. (Compare \textcolor{red}{red text} black text \color{red}{red text} still red text.) 

\documentclass[border=10pt]{standalone}

\usepackage{tikz}
\usepackage{tkz-euclide}
\usetkzobj{all} 
\usetikzlibrary{shapes.geometric,arrows,positioning,fit,calc,backgrounds}

\usepackage{hyperref}

\tikzset{
   b/.style={
       draw,
       rectangle,
       rounded corners=2ex,
       minimum height=0.5in,
       minimum width=2in,
       align=center},
   c/.style={
      draw,
      rectangle,
      rounded corners=2ex,
      minimum height=0.5in,
      minimum width=2in,
      align=center,
      rotate=-90},
   ar/.style={
      rounded corners=2ex,
      ->,
      >=latex},  
   myarrow/.style args={#1 colored by #2 and #3}{
      -stealth,
      line width=#1,
      #2,
      postaction={draw,-stealth,#3,line width=(#1)/3,
      shorten <=(#1)/3,
      shorten >=2*(#1)/3}, 
      },
   toptext/.style={    % <-- New
      above,
      text width=5cm,
      align=center}
}

\begin{document}
\begin{tikzpicture} 
\node (client) [b] {Client};
\node (serveur) [b, left=4 cm of client.west] {Serveur};

% draw the vertical lines      
\draw [thick] (serveur.south) -- ++(0,-300pt);
\draw (client.south) -- ++(0,-300pt) coordinate (bottom);

% add a coordinate below serveur for given y-offsets from bottom of serveur
\foreach [count=\i] \y in {-100,-150,-200,-250}
  \path (serveur.south) ++(0,\y pt) coordinate (level \i);

\draw [ar] (level 1 -| client.south) -- node[toptext] {POST \nolinkurl{authentification/}\\username=user\&password=abc123} (level 1);

\draw [ar] (level 2) -- node[toptext] {HTTP 200 OK\\ \textcolor{green}{\{Token : "JWT"\}}} (level 2-| client.south);

\draw [ar] (level 3 -| client.south) -- node[toptext] {GET \nolinkurl{api/user/id}\\ \textcolor{green}{JWT : Token}} (level 3);

\draw [ar] (level 4 -| client.south) -- node[toptext,below] {HTTP 200 OK\\ \textcolor{green}{\{name : foo\}}} (level 4);


% define some coordinates
\coordinate (a) at ($(level 2) + (-3pt,0)$);
\coordinate (b) at ($(level 3) + (-3pt,0)$);
\coordinate (c) at ($(a)!0.5!(b) + (-1cm,0)$);  % modify the -1cm offset here

% Just to show the points used
\foreach \x in {a,b,c}
  \fill[red] (\x) circle[radius=2pt];

% Draw the arc through the points
\tkzCircumCenter(a,b,c)\tkzGetPoint{O}
\tikzset{compass style/.append style={-latex,thick,black}}
\tkzDrawArc(O,a)(b)  

% fill background
\begin{scope}[on background layer]      
  \node [rectangle, opacity=0.2, fill=blue, fit=(client)(serveur)(bottom)]{};
\end{scope}
\end{tikzpicture}

\end{document}

enter image description here

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