Multiple dotted background on a flowchart

Question

I am trying to add multiple backgrounds to my "flow chart" and group them into two, EC2 Instance and RDS Instance.

MWE:

\documentclass[a4paper, 11pt]{report}
\usepackage{geometry}
\geometry{a4paper,
          total={170mm,257mm},
          left=20mm,
          top=20mm,
          }
\usepackage{tikz}
\usetikzlibrary{arrows.meta,
                backgrounds,
                chains,     
                fit,        
                positioning,
                shadows.blur,
                shapes.geometric}

\begin{document}

\begin{center}
    \begin{tikzpicture}[
          node distance = 12mm and 10mm,
            start chain = A going below, 
             arr/.style = {thick,-Stealth},
           block/.style = {rectangle, rounded corners, fill=blue!20,
                           text width=7em, align=center, minimum height=2em, 
                           blur shadow, on chain=A},
          blockJ/.style = {block, join=by arr},
           cloud/.style = {ellipse,fill=red!20, minimum height=2em, drop shadow},
        decision/.style = {diamond, aspect=1.2, fill=blue!20,
                           text width=4.5em, align=center, inner sep=0pt, blur shadow},
             FIT/.style = {rectangle, rounded corners, draw, fill=yellow!30,
                           inner xsep=3em, inner ysep=1em, yshift=0.5em,
                           fit=#1},
            FIT2/.style = {rectangle, rounded corners, draw, dashed, fill=green!10,
                           inner xsep=3em, inner ysep=1em, yshift=0.5em,
                           fit=#1},
            FIT3/.style = {rectangle, rounded corners, draw, dashed, fill=red!10,
                           inner xsep=3em, inner ysep=1em, yshift=0.5em,
                           fit=#1},
                        ]

% Place nodes

        \node [block] {EC2 Server};                         % A-1
        \node [block, right=of A-1] {EC2 Collector};        % A-2
        \node [block, right=of A-2] {RDS Database};         % A-3

        \begin{pgfonlayer}{background}
            \node (f1) [FIT=(A-1) (A-3), xshift=-0em] {};
            \node[below right, font=\small] at (f1.north west) {\arial AWS Cluster Server closest to Site};
            \node (f2) [FIT2=(A-1) (A-2), xshift=-0em] {};
            \node[below right, font=\small] at (f2.north west) {\arial EC2 Instance};
            \node (f3) [FIT3=(A-3) (A-3), xshift=-0em] {};
            \node[below right, font=\small] at (f3.north west) {\arial RDS Instance};

        \end{pgfonlayer}

    \end{tikzpicture}
\end{center}

\end{document}

OUTPUT:

DESIRED OUTPUT: Excuse my poor MS Paint attempt - but I would like to achieve something like this:

Answer 1

Here is a proposal to replace some of the hard distances by distances that scale with the font size, or avoid them altogether. You can fit the fits you had in a larger fit, which requires the introduction of another layer, behind.

\documentclass[a4paper, 11pt]{report}
\usepackage{geometry}
\geometry{a4paper,
          total={170mm,257mm},
          left=20mm,
          top=20mm,
          }
\usepackage{tikz}
\pgfdeclarelayer{background}
\pgfdeclarelayer{behind}
\pgfsetlayers{behind,background,main}
\usetikzlibrary{fit,        
                positioning,
                shadows.blur,
                shapes.geometric}
\def\arial{\sffamily}%<- replace by your own
\begin{document}

\begin{center}
    \begin{tikzpicture}[
             arr/.style = {thick,-Stealth},
           block/.style = {rectangle, rounded corners, fill=blue!20,
                           text width=7em, align=center, minimum height=2em, 
                           blur shadow},
             FIT/.style = {rectangle, rounded corners, draw, fill=yellow!30,
                           inner xsep=1em, inner ysep=1em,
                           fit=#1},
            FIT2/.style = {rectangle, rounded corners, draw, dashed, fill=green!10,
                           inner xsep=1.5ex, inner ysep=2ex,yshift=-1ex, 
                           fit=#1},
            FIT3/.style = {rectangle, rounded corners, draw, dashed, fill=red!10,
                           inner xsep=1.5ex, inner ysep=2ex,yshift=-1ex, 
                           fit=#1},
                        ]

% Place nodes

        \node [block] (A-1)  {EC2 Server};                         % A-1
        \node [block, right=1em of A-1] (A-2) {EC2 Collector};        % A-2
        \node [block, right=3em of A-2] (A-3) {RDS Database};         % A-3
        \node [xshift=-1ex,anchor=south west] (EC2) at (A-1.north west)  {\arial EC2
        Instance};
        \node [xshift=-1ex,anchor=south west] (RDS) at (A-3.north west)  {\arial
        RDS Instance};
        \begin{pgfonlayer}{background}
            \node (f1) [FIT2=(A-1) (A-2) (EC2)] {};
            \node (f2) [FIT3=(A-3) (RDS)] {};
        \end{pgfonlayer}
        \node [xshift=-2em,anchor=south west] (AWS) at (f1.north west)  
            {\arial AWS Cluster closest to 1};
        \begin{pgfonlayer}{behind}
            \node (f0) [FIT=(f1) (f2) (AWS)] {};
        \end{pgfonlayer}
    \end{tikzpicture}
\end{center}
\end{document}