How can I draw an arc between two Cartesian coordinates and fit nodes into a big one?

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}