TikZ fit nodes with non-regular shape

Question

I am seeking a better way of doing the following: fitting nodes with a non regular shape, like it is in the MWE. So that I can give a label to the fitted nodes.

\documentclass{standalone}

\usepackage{tikz}
\usetikzlibrary{positioning}

\tikzset{any/.style={draw,shape=rectangle}}
\tikzset{nxs/.style={xshift=-2mm}}
\tikzset{pxs/.style={xshift=2mm}}
\tikzset{nys/.style={yshift=-2mm}}
\tikzset{pys/.style={yshift=2mm}}

\begin{document}

\begin{tikzpicture}
\node [any] (n1) {N1};
\node [any,right=of n1] (n2) {N2};
\node [any,above=of n2] (n3) {N3};

\draw ([nxs,pys]n1.north west) -- ([nxs,pys]n2.north west) -| 
      ([nxs,pys]n3.north west) -- ([pxs,pys]n3.north east) -| 
      ([pxs,nys]n2.south east) -- ([nxs,nys]n1.south west) -- cycle;

\node [yshift=4mm] at (n1.north) {Cluster}; 

\end{tikzpicture}

\end{document}

Answer 1

I would also like to know if there is any automated way to do this. In the meantime, here is an alternative which uses the fit library. This at least makes the margins consistent with standard rectangular fits.

\documentclass{standalone}

\usepackage{tikz}
\usetikzlibrary{positioning, fit}

\tikzset{any/.style={draw,shape=rectangle}}

\begin{document}

\begin{tikzpicture}
\node [any] (n1) {N1};
\node [any,right=of n1] (n2) {N2};
\node [any,above=of n2] (n3) {N3};

\node[fit=(n1) (n2)] (n12) {};
\node[fit=(n2) (n3)] (n23) {};

\draw (n12.north west) -- (n12.north west-| n23.west) -- (n23.north west) 
 -- (n23.north east) -- (n23.south east) -- (n12.south west) -- cycle;

\node [inner xsep = 0pt, anchor=south west] at (n12.north west) {Cluster}; 

\end{tikzpicture}

\end{document}