System block diagram

\documentclass[border=8pt]{standalone}

% --- packages (mirror these in template.meta.json "requires") ---
\usepackage{tikz}
\usetikzlibrary{positioning, arrows.meta, shapes.geometric, fit, backgrounds}

% --- palette (canonical source: reference/color-palettes/color-palettes.md; light variant) ---
\definecolor{otblue}{HTML}{0072B2}
\definecolor{otorange}{HTML}{E69F00}
\definecolor{otteal}{HTML}{009E73}
\definecolor{otpurple}{HTML}{CC79A7}
\definecolor{otgray}{HTML}{5A5A5A}

\begin{document}
% ==== parameters (edit these) ============================================
\def\rowsep{1.1}    % vertical gap between rows (cm)
\def\colsep{1.5}    % horizontal gap between columns (cm)
\def\boxminw{1.9}   % minimum process/external box width (cm)
\def\boxminh{1.0}   % minimum box height (cm)
% labels
\def\clientlabel{Client}
\def\gatewaylabel{API\\Gateway}
\def\svconelabel{Service A}
\def\svctwolabel{Service B}
\def\dblabel{Database}
\def\cachelabel{Cache}
\def\grouplabel{Backend}
% =========================================================================

\begin{tikzpicture}[
    node distance=\rowsep cm and \colsep cm,
    >={Stealth[length=2.4mm]},
    comp/.style={
      rounded corners=3pt, line width=0.7pt, align=center,
      minimum width=\boxminw cm, minimum height=\boxminh cm, font=\sffamily\small,
    },
    external/.style={comp, draw=otgray!60, fill=otgray!10, dashed},
    process/.style={comp, draw=otblue!70!black, fill=otblue!16},
    store/.style={
      shape=cylinder, shape border rotate=90, aspect=0.25,
      draw=otteal!70!black, fill=otteal!16, line width=0.7pt,
      minimum width=1.7cm, minimum height=1.0cm, align=center,
      font=\sffamily\small, inner sep=2pt,
    },
    cachestore/.style={store, draw=otorange!80!black, fill=otorange!22},
    link/.style={draw=otgray!75, line width=0.9pt, ->},
    group/.style={
      draw=otgray!55, dashed, rounded corners=6pt, line width=0.8pt,
      inner sep=11pt, fill=otgray!4,
    },
  ]

  % --- components (left to right; services stacked) ---
  \node[external] (client) {\clientlabel};
  \node[process, right=of client] (gateway) {\gatewaylabel};
  \node[process, above right=of gateway] (svc1) {\svconelabel};
  \node[process, below right=of gateway] (svc2) {\svctwolabel};
  \node[store,      right=of svc1] (db)    {\dblabel};
  \node[cachestore, right=of svc2] (cache) {\cachelabel};

  % --- connections ---
  \draw[link] (client) -- (gateway);
  \draw[link] (gateway) -- (svc1);
  \draw[link] (gateway) -- (svc2);
  \draw[link] (svc1) -- (db);
  \draw[link] (svc2) -- (cache);

  % --- subsystem boundary drawn behind the components ---
  \begin{scope}[on background layer]
    \node[group, fit=(svc1)(svc2)(db)(cache)] (backend) {};
    \node[font=\sffamily\footnotesize, text=otgray, anchor=north west]
      at ([xshift=4pt, yshift=-3pt]backend.north west) {\grouplabel};
  \end{scope}

\end{tikzpicture}
\end{document}