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<div class="section" id="module-development-guide">
<h1>Module Development Guide<a class="headerlink" href="#module-development-guide" title="Permalink to this headline"></a></h1>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt><a class="reference internal" href="modules.html"><em>Ansible Modules</em></a></dt>
<dd>Learn about available modules</dd>
<dt><a class="reference external" href="https://github.com/ansible/ansible/tree/master/library">Github modules directory</a></dt>
<dd>Browse source of core modules</dd>
</dl>
</div>
<p>Ansible modules are reusable units of magic that can be used by the Ansible API,
or by the <cite>ansible</cite> or <cite>ansible-playbook</cite> programs.</p>
<p>Modules can be written in any language and are found in the path specified
by <cite>ANSIBLE_LIBRARY_PATH</cite> or the <cite>&#8211;module-path</cite> command line option.</p>
<div class="section" id="tutorial">
<h2>Tutorial<a class="headerlink" href="#tutorial" title="Permalink to this headline"></a></h2>
<p>Let&#8217;s build a module to get and set the system time. For starters, let&#8217;s build
a module that just outputs the current time.</p>
<p>We are going to use Python here but any language is possible. Only File I/O and outputing to standard
out are required. So, bash, C++, clojure, Python, Ruby, whatever you want
is fine.</p>
<p>It&#8217;s obvious that you would never really need to build a module to set the system time,
the &#8216;command&#8217; module could already be used to do this. However, it makes for a decent example.
Reading the modules that come with ansible (linked above) is a great way to learn how to write
modules. Keep in mind, though, that some modules in ansible&#8217;s source tree are internalisms,
so look at <cite>service</cite> or <cite>yum</cite>, and don&#8217;t stare too close into things like <cite>async_wrapper</cite> or
you&#8217;ll turn to stone. Nobody ever executes async_wrapper directly.</p>
<p>Ok, let&#8217;s get going with an example. We&#8217;ll use Python. For starters, save this as a file named <cite>time</cite>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c">#!/usr/bin/python</span>
<span class="kn">import</span> <span class="nn">datetime</span>
<span class="kn">import</span> <span class="nn">json</span>
<span class="n">date</span> <span class="o">=</span> <span class="nb">str</span><span class="p">(</span><span class="n">datetime</span><span class="o">.</span><span class="n">datetime</span><span class="o">.</span><span class="n">now</span><span class="p">())</span>
<span class="k">print</span> <span class="n">json</span><span class="o">.</span><span class="n">dumps</span><span class="p">({</span>
<span class="s">&quot;time&quot;</span> <span class="p">:</span> <span class="n">date</span>
<span class="p">})</span>
</pre></div>
</div>
</div>
<div class="section" id="testing-modules">
<h2>Testing Modules<a class="headerlink" href="#testing-modules" title="Permalink to this headline"></a></h2>
<p>There&#8217;s a useful test script in the source checkout for ansible:</p>
<div class="highlight-python"><pre>git clone git@github.com:ansible/ansible.git
chmod +x ansible/hacking/test-module</pre>
</div>
<p>Let&#8217;s run the script you just wrote with that:</p>
<div class="highlight-python"><pre>ansible/hacking/test-module ./time</pre>
</div>
<p>You should see output that looks something like this:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="p">{</span><span class="s">u&#39;time&#39;</span><span class="p">:</span> <span class="s">u&#39;2012-03-14 22:13:48.539183&#39;</span><span class="p">}</span>
</pre></div>
</div>
<p>If you did not, you might have a typo in your module, so recheck it and try again</p>
</div>
<div class="section" id="reading-input">
<h2>Reading Input<a class="headerlink" href="#reading-input" title="Permalink to this headline"></a></h2>
<p>Let&#8217;s modify the module to allow setting the current time. We&#8217;ll do this by seeing
if a key value pair in the form <cite>time=&lt;string&gt;</cite> is passed in to the module.</p>
<p>Ansible internally saves arguments to a arguments file. So we must read the file
and parse it. The arguments file is just a string, so any form of arguments are legal.
Here we&#8217;ll do some basic parsing to treat the input as key=value.</p>
<p>The example usage we are trying to achieve to set the time is:</p>
<div class="highlight-python"><pre>time time="March 14 22:10"</pre>
</div>
<p>If no time parameter is set, we&#8217;ll just leave the time as is and return the current time.</p>
<p>Let&#8217;s look at the code. Read the comments as we&#8217;ll explain as we go. Note that this
highly verbose because it&#8217;s intended as an educational example. You can write modules
a lot shorter than this:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c">#!/usr/bin/python</span>
<span class="c"># import some python modules that we&#39;ll use. These are all</span>
<span class="c"># available in Python&#39;s core</span>
<span class="kn">import</span> <span class="nn">datetime</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="kn">import</span> <span class="nn">json</span>
<span class="kn">import</span> <span class="nn">os</span>
<span class="kn">import</span> <span class="nn">shlex</span>
<span class="c"># read the argument string from the arguments file</span>
<span class="n">args_file</span> <span class="o">=</span> <span class="n">sys</span><span class="o">.</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span>
<span class="n">args_data</span> <span class="o">=</span> <span class="nb">file</span><span class="p">(</span><span class="n">args_file</span><span class="p">)</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="c"># for this module, we&#39;re going to do key=value style arguments</span>
<span class="c"># this is up to each module to decide what it wants, but all</span>
<span class="c"># core modules besides &#39;command&#39; and &#39;shell&#39; take key=value</span>
<span class="c"># so this is highly recommended</span>
<span class="n">arguments</span> <span class="o">=</span> <span class="n">shlex</span><span class="o">.</span><span class="n">split</span><span class="p">(</span><span class="n">args_data</span><span class="p">)</span>
<span class="k">for</span> <span class="n">arg</span> <span class="ow">in</span> <span class="n">arguments</span><span class="p">:</span>
<span class="c"># ignore any arguments without an equals in it</span>
<span class="k">if</span> <span class="n">arg</span><span class="o">.</span><span class="n">find</span><span class="p">(</span><span class="s">&quot;=&quot;</span><span class="p">)</span> <span class="o">!=</span> <span class="o">-</span><span class="mi">1</span><span class="p">:</span>
<span class="p">(</span><span class="n">key</span><span class="p">,</span> <span class="n">value</span><span class="p">)</span> <span class="o">=</span> <span class="n">arg</span><span class="o">.</span><span class="n">split</span><span class="p">(</span><span class="s">&quot;=&quot;</span><span class="p">)</span>
<span class="c"># if setting the time, the key &#39;time&#39;</span>
<span class="c"># will contain the value we want to set the time to</span>
<span class="k">if</span> <span class="n">key</span> <span class="o">==</span> <span class="s">&quot;time&quot;</span><span class="p">:</span>
<span class="c"># now we&#39;ll affect the change. Many modules</span>
<span class="c"># will strive to be &#39;idempotent&#39;, meaning they</span>
<span class="c"># will only make changes when the desired state</span>
<span class="c"># expressed to the module does not match</span>
<span class="c"># the current state. Look at &#39;service&#39;</span>
<span class="c"># or &#39;yum&#39; in the main git tree for an example</span>
<span class="c"># of how that might look.</span>
<span class="n">rc</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">system</span><span class="p">(</span><span class="s">&quot;date -s </span><span class="se">\&quot;</span><span class="si">%s</span><span class="se">\&quot;</span><span class="s">&quot;</span> <span class="o">%</span> <span class="n">value</span><span class="p">)</span>
<span class="c"># always handle all possible errors</span>
<span class="c">#</span>
<span class="c"># when returning a failure, include &#39;failed&#39;</span>
<span class="c"># in the return data, and explain the failure</span>
<span class="c"># in &#39;msg&#39;. Both of these conventions are</span>
<span class="c"># required however additional keys and values</span>
<span class="c"># can be added.</span>
<span class="k">if</span> <span class="n">rc</span> <span class="o">!=</span> <span class="mi">0</span><span class="p">:</span>
<span class="k">print</span> <span class="n">json</span><span class="o">.</span><span class="n">dumps</span><span class="p">({</span>
<span class="s">&quot;failed&quot;</span> <span class="p">:</span> <span class="bp">True</span><span class="p">,</span>
<span class="s">&quot;msg&quot;</span> <span class="p">:</span> <span class="s">&quot;failed setting the time&quot;</span>
<span class="p">})</span>
<span class="n">sys</span><span class="o">.</span><span class="n">exit</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="c"># when things do not fail, we do not</span>
<span class="c"># have any restrictions on what kinds of</span>
<span class="c"># data are returned, but it&#39;s always a</span>
<span class="c"># good idea to include whether or not</span>
<span class="c"># a change was made, as that will allow</span>
<span class="c"># notifiers to be used in playbooks.</span>
<span class="n">date</span> <span class="o">=</span> <span class="nb">str</span><span class="p">(</span><span class="n">datetime</span><span class="o">.</span><span class="n">datetime</span><span class="o">.</span><span class="n">now</span><span class="p">())</span>
<span class="k">print</span> <span class="n">json</span><span class="o">.</span><span class="n">dumps</span><span class="p">({</span>
<span class="s">&quot;time&quot;</span> <span class="p">:</span> <span class="n">date</span><span class="p">,</span>
<span class="s">&quot;changed&quot;</span> <span class="p">:</span> <span class="bp">True</span>
<span class="p">})</span>
<span class="n">sys</span><span class="o">.</span><span class="n">exit</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="c"># if no parameters are sent, the module may or</span>
<span class="c"># may not error out, this one will just</span>
<span class="c"># return the time</span>
<span class="n">date</span> <span class="o">=</span> <span class="nb">str</span><span class="p">(</span><span class="n">datetime</span><span class="o">.</span><span class="n">datetime</span><span class="o">.</span><span class="n">now</span><span class="p">())</span>
<span class="k">print</span> <span class="n">json</span><span class="o">.</span><span class="n">dumps</span><span class="p">({</span>
<span class="s">&quot;time&quot;</span> <span class="p">:</span> <span class="n">date</span>
<span class="p">})</span>
</pre></div>
</div>
<p>Let&#8217;s test that module:</p>
<div class="highlight-python"><pre>../ansible/hacking/test-module ./rst/time time=\"March 14 12:23\"</pre>
</div>
<p>This should return something like:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="p">{</span><span class="s">&quot;changed&quot;</span><span class="p">:</span> <span class="n">true</span><span class="p">,</span> <span class="s">&quot;time&quot;</span><span class="p">:</span> <span class="s">&quot;2012-03-14 12:23:00.000307&quot;</span><span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="common-pitfalls">
<h2>Common Pitfalls<a class="headerlink" href="#common-pitfalls" title="Permalink to this headline"></a></h2>
<p>If writing a module in Python and you have managed nodes running
Python 2.4 or lower, this is generally a good idea, because
json isn&#8217;t in the Python standard library until 2.5.:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">try</span><span class="p">:</span>
<span class="kn">import</span> <span class="nn">json</span>
<span class="k">except</span> <span class="ne">ImportError</span><span class="p">:</span>
<span class="kn">import</span> <span class="nn">simplejson</span> <span class="kn">as</span> <span class="nn">json</span>
</pre></div>
</div>
<p>You should also never do this in a module:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">print</span> <span class="s">&quot;some status message&quot;</span>
</pre></div>
</div>
<p>Because the output is supposed to be valid JSON. Except that&#8217;s not quite true,
but we&#8217;ll get to that later.</p>
<div class="section" id="conventions">
<h3>Conventions<a class="headerlink" href="#conventions" title="Permalink to this headline"></a></h3>
<p>As a reminder from the example code above, here are some basic conventions
and guidelines:</p>
<ul class="simple">
<li>Include a minimum of dependencies if possible. If there are dependencies, document them at the top of the module file</li>
<li>Modules must be self contained in one file to be auto-transferred by ansible</li>
<li>If packaging modules in an RPM, they only need to be installed on the control machine and should be dropped into /usr/share/ansible. This is entirely optional.</li>
<li>Modules should return JSON or key=value results all on one line. JSON is best if you can do JSON. All return types must be hashes (dictionaries) although they can be nested.</li>
<li>In the event of failure, a key of &#8216;failed&#8217; should be included, along with a string explanation in &#8216;msg&#8217;. Modules that raise tracebacks (stacktraces) are generally considered &#8216;poor&#8217; modules, though Ansible can deal with these returns and will automatically convert anything unparseable into a failed result.</li>
<li>Return codes are actually not signficant, but continue on with 0=success and non-zero=failure for reasons of future proofing.</li>
<li>As results from many hosts will be aggregrated at once, modules should return only relevant output. Returning the entire contents of a log file is generally bad form.</li>
</ul>
</div>
<div class="section" id="shorthand-vs-json">
<h3>Shorthand Vs JSON<a class="headerlink" href="#shorthand-vs-json" title="Permalink to this headline"></a></h3>
<p>To make it easier to write modules in bash and in cases where a JSON
module might not be available, it is acceptable for a module to return
key=value output all on one line, like this. The Ansible parser
will know what to do.</p>
<blockquote>
<div>somekey=1 somevalue=two favcolor=red</div></blockquote>
<p>If you&#8217;re writing a module in Python or Ruby or whatever, though, returning
JSON is probably the simplest way to go.</p>
</div>
</div>
<div class="section" id="sharing-your-module">
<h2>Sharing Your Module<a class="headerlink" href="#sharing-your-module" title="Permalink to this headline"></a></h2>
<p>If you think your module is generally useful to others, Ansible is preparing
an &#8216;ansible-contrib&#8217; repo. Stop by the mailing list and we&#8217;ll help you to
get your module included. Contrib modules can be implemented in a variety
of languages. Including a README with your module is a good idea so folks
can understand what arguments it takes and so on. We would like to build
up as many of these as possible in as many languages as possible.</p>
<p><a class="reference external" href="http://groups.google.com/group/ansible-project">Ansible Mailing List</a></p>
</div>
<div class="section" id="getting-your-module-into-core">
<h2>Getting Your Module Into Core<a class="headerlink" href="#getting-your-module-into-core" title="Permalink to this headline"></a></h2>
<p>High-quality modules with minimal dependencies
can be included in the core, but core modules (just due to the programming
preferences of the developers) will need to be implemented in Python.
Stop by the mailing list to inquire about requirements.</p>
</div>
</div>
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