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<section id="tutorial-write-your-own-history-backend">
<span id="tutorial-history-backend"></span><h1>Tutorial: Write Your Own History Backend<a class="headerlink" href="#tutorial-write-your-own-history-backend" title="Link to this heading">¶</a></h1>
<p>One of the great things about xonsh is how easy it is to customize. In
this tutorial, let’s write our own history backend based on CouchDB.</p>
<section id="start-with-a-minimal-history-template">
<h2>Start with a Minimal History Template<a class="headerlink" href="#start-with-a-minimal-history-template" title="Link to this heading">¶</a></h2>
<p>Here is a minimal history backend to start with:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">collections</span>
<span class="kn">from</span> <span class="nn">xonsh.history.base</span> <span class="kn">import</span> <span class="n">History</span>
<span class="k">class</span> <span class="nc">CouchDBHistory</span><span class="p">(</span><span class="n">History</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">append</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">cmd</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">def</span> <span class="nf">items</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">newest_first</span><span class="o">=</span><span class="kc">False</span><span class="p">):</span>
<span class="k">yield</span> <span class="p">{</span><span class="s1">'inp'</span><span class="p">:</span> <span class="s1">'couchdb in action'</span><span class="p">,</span> <span class="s1">'ts'</span><span class="p">:</span> <span class="mi">1464652800</span><span class="p">,</span> <span class="s1">'ind'</span><span class="p">:</span> <span class="mi">0</span><span class="p">}</span>
<span class="k">def</span> <span class="nf">all_items</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">newest_first</span><span class="o">=</span><span class="kc">False</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">items</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">info</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">collections</span><span class="o">.</span><span class="n">OrderedDict</span><span class="p">()</span>
<span class="n">data</span><span class="p">[</span><span class="s1">'backend'</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'couchdb'</span>
<span class="n">data</span><span class="p">[</span><span class="s1">'sessionid'</span><span class="p">]</span> <span class="o">=</span> <span class="nb">str</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">sessionid</span><span class="p">)</span>
<span class="k">return</span> <span class="n">data</span>
</pre></div>
</div>
<p>Go ahead and create the file <code class="docutils literal notranslate"><span class="pre">~/.xonsh/history_couchdb.py</span></code> and put the
content above into it.</p>
<p>Now we need to tell xonsh to use it as the history backend. To do this
we need xonsh to be able to find our file and this <code class="docutils literal notranslate"><span class="pre">CouchDBHistory</span></code> class.
Putting the following code into <a class="reference external" href="xonshrc.rst">xonshrc</a> file can achieve this.</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>import os.path
import sys
xonsh_ext_dir = os.path.expanduser('~/.xonsh')
if os.path.isdir(xonsh_ext_dir):
sys.path.append(xonsh_ext_dir)
from history_couchdb import CouchDBHistory
$XONSH_HISTORY_BACKEND = CouchDBHistory
</pre></div>
</div>
<p>After starting a new xonsh session, try the following commands:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>@ history info
backend: couchdb
sessionid: 4198d678-1f0a-4ce3-aeb3-6d5517d7fc61
@ history -n
0: couchdb in action
</pre></div>
</div>
<p>Woohoo! We just wrote a working history backend!</p>
</section>
<section id="setup-couchdb">
<h2>Setup CouchDB<a class="headerlink" href="#setup-couchdb" title="Link to this heading">¶</a></h2>
<p>For this to work, we need CouchDB up and running. Go to
<a class="reference external" href="http://couchdb.apache.org/">CouchDB website</a> and spend some time to
install it. we will wait for you. Take your time.</p>
<p>After installing, check that it’s configured correctly with <code class="docutils literal notranslate"><span class="pre">curl</span></code>:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>@ curl -i 'http://127.0.0.1:5984/'
HTTP/1.1 200 OK
Cache-Control: must-revalidate
Content-Length: 91
Content-Type: application/json
Date: Wed, 01 Feb 2017 13:54:14 GMT
Server: CouchDB/2.0.0 (Erlang OTP/19)
X-Couch-Request-ID: 025a195bcb
X-CouchDB-Body-Time: 0
{
"couchdb": "Welcome",
"version": "2.0.0",
"vendor": {
"name": "The Apache Software Foundation"
}
}
</pre></div>
</div>
<p>Okay, CouchDB is working. Now open <a class="reference external" href="http://127.0.0.1:5984/_utils/">http://127.0.0.1:5984/_utils/</a> with
your browser, and create a new database called <code class="docutils literal notranslate"><span class="pre">xonsh-history</span></code>.</p>
</section>
<section id="initialize-history-backend">
<h2>Initialize History Backend<a class="headerlink" href="#initialize-history-backend" title="Link to this heading">¶</a></h2>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="fm">__init__</span><span class="p">(</span><span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">gc</span> <span class="o">=</span> <span class="kc">None</span>
<span class="bp">self</span><span class="o">.</span><span class="n">sessionid</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">_build_session_id</span><span class="p">()</span>
<span class="bp">self</span><span class="o">.</span><span class="n">inps</span> <span class="o">=</span> <span class="p">[]</span>
<span class="bp">self</span><span class="o">.</span><span class="n">rtns</span> <span class="o">=</span> <span class="p">[]</span>
<span class="bp">self</span><span class="o">.</span><span class="n">outs</span> <span class="o">=</span> <span class="p">[]</span>
<span class="bp">self</span><span class="o">.</span><span class="n">tss</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">def</span> <span class="nf">_build_session_id</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">ts</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="n">time</span><span class="o">.</span><span class="n">time</span><span class="p">()</span> <span class="o">*</span> <span class="mi">1000</span><span class="p">)</span>
<span class="k">return</span> <span class="s1">'</span><span class="si">{}</span><span class="s1">-</span><span class="si">{}</span><span class="s1">'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">ts</span><span class="p">,</span> <span class="nb">str</span><span class="p">(</span><span class="n">uuid</span><span class="o">.</span><span class="n">uuid4</span><span class="p">())[:</span><span class="mi">18</span><span class="p">])</span>
</pre></div>
</div>
<p>In the <code class="docutils literal notranslate"><span class="pre">__init__()</span></code> method, let’s initialize
<a class="reference external" href="api/history/base.html#xonsh.history.base.History">Some Public Attributes</a>
which xonsh uses in various places. Note that we use Unix timestamp and
some random char to make <code class="docutils literal notranslate"><span class="pre">self.sessionid</span></code> unique and to keep the entries
ordered in time. We will cover it with a bit more detail in the next section.</p>
</section>
<section id="save-history-to-couchdb">
<h2>Save History to CouchDB<a class="headerlink" href="#save-history-to-couchdb" title="Link to this heading">¶</a></h2>
<p>First, we need some helper functions to write docs to CouchDB.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">_save_to_db</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">cmd</span><span class="p">):</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">cmd</span><span class="o">.</span><span class="n">copy</span><span class="p">()</span>
<span class="n">data</span><span class="p">[</span><span class="s1">'inp'</span><span class="p">]</span> <span class="o">=</span> <span class="n">cmd</span><span class="p">[</span><span class="s1">'inp'</span><span class="p">]</span><span class="o">.</span><span class="n">rstrip</span><span class="p">()</span>
<span class="k">if</span> <span class="s1">'out'</span> <span class="ow">in</span> <span class="n">data</span><span class="p">:</span>
<span class="n">data</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="s1">'out'</span><span class="p">)</span>
<span class="n">data</span><span class="p">[</span><span class="s1">'_id'</span><span class="p">]</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">_build_doc_id</span><span class="p">()</span>
<span class="k">try</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">_request_db_data</span><span class="p">(</span><span class="s1">'/xonsh-history'</span><span class="p">,</span> <span class="n">data</span><span class="o">=</span><span class="n">data</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">Exception</span> <span class="k">as</span> <span class="n">e</span><span class="p">:</span>
<span class="n">msg</span> <span class="o">=</span> <span class="s1">'failed to save history: </span><span class="si">{}</span><span class="s1">: </span><span class="si">{}</span><span class="s1">'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">e</span><span class="o">.</span><span class="vm">__class__</span><span class="o">.</span><span class="vm">__name__</span><span class="p">,</span> <span class="n">e</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">msg</span><span class="p">,</span> <span class="n">file</span><span class="o">=</span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">_build_doc_id</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">ts</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="n">time</span><span class="o">.</span><span class="n">time</span><span class="p">()</span> <span class="o">*</span> <span class="mi">1000</span><span class="p">)</span>
<span class="k">return</span> <span class="s1">'</span><span class="si">{}</span><span class="s1">-</span><span class="si">{}</span><span class="s1">-</span><span class="si">{}</span><span class="s1">'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">sessionid</span><span class="p">,</span> <span class="n">ts</span><span class="p">,</span> <span class="nb">str</span><span class="p">(</span><span class="n">uuid</span><span class="o">.</span><span class="n">uuid4</span><span class="p">())[:</span><span class="mi">18</span><span class="p">])</span>
<span class="k">def</span> <span class="nf">_request_db_data</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">path</span><span class="p">,</span> <span class="n">data</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="n">url</span> <span class="o">=</span> <span class="s1">'http://127.0.0.1:5984'</span> <span class="o">+</span> <span class="n">path</span>
<span class="n">headers</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'Content-Type'</span><span class="p">:</span> <span class="s1">'application/json'</span><span class="p">}</span>
<span class="k">if</span> <span class="n">data</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
<span class="n">resp</span> <span class="o">=</span> <span class="n">requests</span><span class="o">.</span><span class="n">post</span><span class="p">(</span><span class="n">url</span><span class="p">,</span> <span class="n">json</span><span class="o">.</span><span class="n">dumps</span><span class="p">(</span><span class="n">data</span><span class="p">),</span> <span class="n">headers</span><span class="o">=</span><span class="n">headers</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">headers</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'Content-Type'</span><span class="p">:</span> <span class="s1">'text/plain'</span><span class="p">}</span>
<span class="n">resp</span> <span class="o">=</span> <span class="n">requests</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">url</span><span class="p">,</span> <span class="n">headers</span><span class="o">=</span><span class="n">headers</span><span class="p">)</span>
<span class="k">return</span> <span class="n">resp</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">_save_to_db()</span></code> takes a dict as the input, which contains the information
about a command that user input, and saves it into CouchDB.</p>
<p>Instead of letting CouchDB provide us a random Document ID (i.e. the
<code class="docutils literal notranslate"><span class="pre">data['_id']</span></code> in our code), we build it for ourselves. We use the Unix
timestamp and UUID string for a second time. Prefixing this with
<code class="docutils literal notranslate"><span class="pre">self.sessionid</span></code>, we make history entries in order inside a single xonsh
session too. So that we don’t need any extra CouchDB’s
<a class="reference external" href="http://docs.couchdb.org/en/2.0.0/couchapp/ddocs.html">Design Documents and Views</a>
feature. Just with a bare <code class="docutils literal notranslate"><span class="pre">_all_docs</span></code> API, we can fetch history items back
in order.</p>
<p>Now that we have helper functions, let’s update our <code class="docutils literal notranslate"><span class="pre">append()</span></code> method
to do the real job - save history into DB.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">append</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">cmd</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">inps</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">cmd</span><span class="p">[</span><span class="s1">'inp'</span><span class="p">])</span>
<span class="bp">self</span><span class="o">.</span><span class="n">rtns</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">cmd</span><span class="p">[</span><span class="s1">'rtn'</span><span class="p">])</span>
<span class="bp">self</span><span class="o">.</span><span class="n">outs</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="kc">None</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">tss</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">cmd</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s1">'ts'</span><span class="p">,</span> <span class="p">(</span><span class="kc">None</span><span class="p">,</span> <span class="kc">None</span><span class="p">)))</span>
<span class="bp">self</span><span class="o">.</span><span class="n">_save_to_db</span><span class="p">(</span><span class="n">cmd</span><span class="p">)</span>
</pre></div>
</div>
<p>This method will be called by xonsh every time it runs a new command from user.</p>
</section>
<section id="retrieve-history-items">
<h2>Retrieve History Items<a class="headerlink" href="#retrieve-history-items" title="Link to this heading">¶</a></h2>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">items</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">newest_first</span><span class="o">=</span><span class="kc">False</span><span class="p">):</span>
<span class="k">yield from</span> <span class="bp">self</span><span class="o">.</span><span class="n">_get_db_items</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">sessionid</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">all_items</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">newest_first</span><span class="o">=</span><span class="kc">False</span><span class="p">):</span>
<span class="k">yield from</span> <span class="bp">self</span><span class="o">.</span><span class="n">_get_db_items</span><span class="p">()</span>
</pre></div>
</div>
<p>These two methods are responsible for getting history items for the current
xonsh session and all historical sessions respectively.</p>
<p>And here is our helper method to get docs from DB:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">_get_db_items</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">sessionid</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="n">path</span> <span class="o">=</span> <span class="s1">'/xonsh-history/_all_docs?include_docs=true'</span>
<span class="k">if</span> <span class="n">sessionid</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
<span class="n">path</span> <span class="o">+=</span> <span class="s1">'&start_key="</span><span class="si">{0}</span><span class="s1">"&end_key="</span><span class="si">{0}</span><span class="s1">-z"'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">sessionid</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">r</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">_request_db_data</span><span class="p">(</span><span class="n">path</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">Exception</span> <span class="k">as</span> <span class="n">e</span><span class="p">:</span>
<span class="n">msg</span> <span class="o">=</span> <span class="s1">'error when query db: </span><span class="si">{}</span><span class="s1">: </span><span class="si">{}</span><span class="s1">'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">e</span><span class="o">.</span><span class="vm">__class__</span><span class="o">.</span><span class="vm">__name__</span><span class="p">,</span> <span class="n">e</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">msg</span><span class="p">,</span> <span class="n">file</span><span class="o">=</span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">)</span>
<span class="k">return</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">json</span><span class="o">.</span><span class="n">loads</span><span class="p">(</span><span class="n">r</span><span class="o">.</span><span class="n">text</span><span class="p">)</span>
<span class="k">for</span> <span class="n">item</span> <span class="ow">in</span> <span class="n">data</span><span class="p">[</span><span class="s1">'rows'</span><span class="p">]:</span>
<span class="n">cmd</span> <span class="o">=</span> <span class="n">item</span><span class="p">[</span><span class="s1">'doc'</span><span class="p">]</span><span class="o">.</span><span class="n">copy</span><span class="p">()</span>
<span class="n">cmd</span><span class="p">[</span><span class="s1">'ts'</span><span class="p">]</span> <span class="o">=</span> <span class="n">cmd</span><span class="p">[</span><span class="s1">'ts'</span><span class="p">][</span><span class="mi">0</span><span class="p">]</span>
<span class="k">yield</span> <span class="n">cmd</span>
</pre></div>
</div>
<p>The <cite>try-except</cite> is here so that we’re safe when something bad happens, like
CouchDB is not running properly, etc.</p>
</section>
<section id="try-out-our-new-history-backend">
<h2>Try Out Our New History Backend<a class="headerlink" href="#try-out-our-new-history-backend" title="Link to this heading">¶</a></h2>
<p>That’s it. We’ve finished our new history backend. The <code class="docutils literal notranslate"><span class="pre">import</span></code> part is
skipped, but I think you can figure it out though. Note that in our code
an extra Python library is used: <code class="docutils literal notranslate"><span class="pre">requests</span></code>. You could easily install it
with <code class="docutils literal notranslate"><span class="pre">pip</span></code> or other library managers. You can find the full code here:
<a class="reference external" href="https://gist.github.com/mitnk/2d08dc60aab33d8b8b758c544b37d570">https://gist.github.com/mitnk/2d08dc60aab33d8b8b758c544b37d570</a></p>
<p>Let’s start a new xonsh session:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>@ history info
backend: couchdb
sessionid: 1486035364166-3bb78606-dd59-4679
@ ls
Applications Desktop Documents Downloads
@ echo hi
hi
</pre></div>
</div>
<p>Start a second xonsh session:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>@ history info
backend: couchdb
sessionid: 1486035430658-6f81cd5d-b6d4-4f6a
@ echo new
new
@ history show all -nt
0:(2017-02-02 19:36) history info
1:(2017-02-02 19:36) ls
2:(2017-02-02 19:37) echo hi
3:(2017-02-02 19:37) history info
4:(2017-02-02 19:37) echo new
@ history -nt
0:(2017-02-02 19:37) history info
1:(2017-02-02 19:37) echo new
2:(2017-02-02 19:37) history show all -nt
</pre></div>
</div>
<p>We’re not missing any history, so it looks like we’re good to go!</p>
</section>
<section id="history-garbage-collection">
<h2>History Garbage Collection<a class="headerlink" href="#history-garbage-collection" title="Link to this heading">¶</a></h2>
<p>For the built-in history backends <code class="docutils literal notranslate"><span class="pre">json</span></code> and <code class="docutils literal notranslate"><span class="pre">sqlite</span></code>, garbage collection
is triggered when xonsh is started or when the user runs <code class="docutils literal notranslate"><span class="pre">history</span> <span class="pre">gc</span></code>.
History items outside of the range defined by
<a class="reference external" href="envvars.html#xonsh-history-size">$XONSH_HISTORY_SIZE</a> are deleted.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">History</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">run_gc</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">size</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">blocking</span><span class="o">=</span><span class="kc">True</span><span class="p">):</span>
<span class="w"> </span><span class="sd">"""Run the garbage collector.</span>
<span class="sd"> Parameters</span>
<span class="sd"> ----------</span>
<span class="sd"> size: None or tuple of a int and a string</span>
<span class="sd"> Determines the size and units of what would be allowed to remain.</span>
<span class="sd"> blocking: bool</span>
<span class="sd"> If set blocking, then wait until gc action finished.</span>
<span class="sd"> """</span>
<span class="k">pass</span>
</pre></div>
</div>
<p>The History public method <code class="docutils literal notranslate"><span class="pre">run_gc()</span></code> is for this purpose. Our
<code class="docutils literal notranslate"><span class="pre">CouchDBHistory</span></code> doesn’t define this method, thus it inherits from its
parent <code class="docutils literal notranslate"><span class="pre">History</span></code>, which does nothing. We will leave the GC implementation
as an exercise.</p>
</section>
<section id="other-history-options">
<h2>Other History Options<a class="headerlink" href="#other-history-options" title="Link to this heading">¶</a></h2>
<p>There are some environment variables that can change the behavior of the
history backend. Such as <a class="reference external" href="envvars.html#histcontrol">$HISTCONTROL</a>,
<a class="reference external" href="envvars.html#xonsh-history-size">$XONSH_HISTORY_SIZE</a>,
<a class="reference external" href="envvars.html#xonsh-store-stdout">$XONSH_STORE_STDOUT</a>, etc.</p>
<p>We should implement these ENVs in our CouchDB backend. Luckily, it’s not a
hard thing. We’ll leave the implementation of those features to you,
but you can see how it’s handled for
<a class="reference external" href="_modules/xonsh/history/sqlite.html#SqliteHistory">the sqlite backend</a>.</p>
</section>
<section id="wrap-up">
<h2>Wrap Up<a class="headerlink" href="#wrap-up" title="Link to this heading">¶</a></h2>
<p>This is a barebones implementation but hopefully it will give you a sense
of how you can customize xonsh’s history backend for your own needs!</p>
</section>
</section>
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<ul>
<li><a class="reference internal" href="#">Tutorial: Write Your Own History Backend</a><ul>
<li><a class="reference internal" href="#start-with-a-minimal-history-template">Start with a Minimal History Template</a></li>
<li><a class="reference internal" href="#setup-couchdb">Setup CouchDB</a></li>
<li><a class="reference internal" href="#initialize-history-backend">Initialize History Backend</a></li>
<li><a class="reference internal" href="#save-history-to-couchdb">Save History to CouchDB</a></li>
<li><a class="reference internal" href="#retrieve-history-items">Retrieve History Items</a></li>
<li><a class="reference internal" href="#try-out-our-new-history-backend">Try Out Our New History Backend</a></li>
<li><a class="reference internal" href="#history-garbage-collection">History Garbage Collection</a></li>
<li><a class="reference internal" href="#other-history-options">Other History Options</a></li>
<li><a class="reference internal" href="#wrap-up">Wrap Up</a></li>
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