How to evenly space out nodes or in tikz?

Question

What I want is a block diagram, with a central node called "API", with a node on the left called "User" and a node on the right called "Workflow".

However, because workflow node has lengthier text, it squishes up against the other nodes:

Now I know that I can increase the node distance argument, but is there a way to say something like "Always have nodes separated by a given distance"? So that it does not depend on the amount of size of the node "rectangle"?

Here is my source for the diagram:

\begin{tikzpicture}[node distance=15mm]
\tikzstyle{block} = [inner sep=2mm, rectangle, draw, fill=blue!20]

\node [block] (api) {API};
\node [block, right of=api] (workflow) {Workflow};
\node [block, left of=api] (user) {User};

% Connect the blocks
\draw [->] (user) to (api);
\draw [->] (api) to (workflow);

\end{tikzpicture}

EDIT: I should be able to use arrows that can go in both directions, and add new nodes above and below.

Answer 1

The chains library of TikZ specifically addresses this problem. Here is a simple example:

\documentclass{article}
\usepackage{tikz}% http://ctan.org/pkg/pgf
\usetikzlibrary{chains}
\begin{document}
\begin{tikzpicture}[
start chain,
node distance=5mm, 
every node/.style={draw,on chain,join,fill=blue!20,inner sep=2mm}, 
every join/.style={->}
]
\node {User};
\node  {API};
\node {Workflow};
\end{tikzpicture}
\end{document}

EDIT:

The arrows can be passed to the chain nodes via join= by <-> specification. Here is another simple example with random words. The library commands are explained in much detail in the manual for branching and continuing the chains.

\documentclass{article}
\usepackage{tikz}% http://ctan.org/pkg/pgf
\usetikzlibrary{chains}
\begin{document}
\begin{tikzpicture}[
     start chain,
     node distance=5mm,
     mynode/.style={draw,fill=blue!20,inner sep=2mm,join=by #1},
     mynode/.default=->
]
\node [mynode,on chain] {User};
\node [mynode,on chain] {API};
\node [mynode=<->,on chain] {Workflow};
\node [mynode=<->,on chain=going below] {Update};
\node [mynode=<-,on chain] {Bug Tracker};
\end{tikzpicture}

Often, we need to keep the chain node styles local with respect to the rest of picture so we might use scopes instead of supplying the options to the general tikzpicture environment.

Answer 2

This may not address your question sufficiently, but provides some means to an end. You can use a non-centred anchor for the surrounding nodes:

\documentclass{article}
\usepackage{tikz}% http://ctan.org/pkg/pgf
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
\tikzstyle{block} = [inner sep=2mm, rectangle, draw, fill=blue!20]

\node [block] (api) {API};
\node [block, right=1cm of api] (workflow) {Workflow};
\node [block, left=1cm of api] (user) {User};

% Connect the blocks
\draw [->] (user) to (api);
\draw [->] (api) to (workflow);

\end{tikzpicture}
\end{document}

Drawing additional nodes above and below is made easy using

\node [block, above=1cm of api] (aboveapi) {Above api};
\node [block, below=1cm of api] (belowapi) {Below api};

while arrow direction can be modified using

\draw [<->] (..) to (..);

for double arrows.