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layout: default
navsection: userguide
title: "Concurrent Crunch tasks"
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Copyright (C) The Arvados Authors. All rights reserved.

SPDX-License-Identifier: CC-BY-SA-3.0
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{% include 'pipeline_deprecation_notice' %}

In the previous tutorials, we used @arvados.job_setup.one_task_per_input_file()@ to automatically create concurrent jobs by creating a separate task per file.  For some types of jobs, you may need to split the work up differently, for example creating tasks to process different segments of a single large file.  This tutorial will demonstrate how to create Crunch tasks directly.

Start by entering the @crunch_scripts@ directory of your Git repository:

<notextile>
<pre><code>~$ <span class="userinput">cd $USER/crunch_scripts</span>
</code></pre>
</notextile>

Next, using @nano@ or your favorite Unix text editor, create a new file called @concurrent-hash.py@ in the @crunch_scripts@ directory.

notextile. <pre>~/$USER/crunch_scripts$ <code class="userinput">nano concurrent-hash.py</code></pre>

Add the following code to compute the MD5 hash of each file in a collection:

<notextile> {% code 'concurrent_hash_script_py' as python %} </notextile>

Make the file executable:

notextile. <pre><code>~/$USER/crunch_scripts$ <span class="userinput">chmod +x concurrent-hash.py</span></code></pre>

Add the file to the Git staging area, commit, and push:

<notextile>
<pre><code>~/$USER/crunch_scripts$ <span class="userinput">git add concurrent-hash.py</span>
~/$USER/crunch_scripts$ <span class="userinput">git commit -m"concurrent hash"</span>
~/$USER/crunch_scripts$ <span class="userinput">git push origin master</span>
</code></pre>
</notextile>

You should now be able to run your new script using Crunch, with "script" referring to our new "concurrent-hash.py" script.  We will use a different input from our previous examples.  We will use @887cd41e9c613463eab2f0d885c6dd96+83@ which consists of three files, "alice.txt", "bob.txt" and "carol.txt" (the example collection used previously in "fetching data from Arvados using Keep":{{site.baseurl}}/user/tutorials/tutorial-keep.html#dir).

<notextile>
<pre><code>~/$USER/crunch_scripts$ <span class="userinput">cat &gt;~/the_job &lt;&lt;EOF
{
 "script": "concurrent-hash.py",
 "repository": "$USER/$USER",
 "script_version": "master",
 "script_parameters":
 {
  "input": "887cd41e9c613463eab2f0d885c6dd96+83"
 }
}
EOF</span>
~/$USER/crunch_scripts$ <span class="userinput">arv job create --job "$(cat ~/the_job)"</span>
{
 ...
 "uuid":"qr1hi-xxxxx-xxxxxxxxxxxxxxx"
 ...
}
~/$USER/crunch_scripts$ <span class="userinput">arv job get --uuid qr1hi-xxxxx-xxxxxxxxxxxxxxx</span>
{
 ...
 "output":"e2ccd204bca37c77c0ba59fc470cd0f7+162",
 ...
}
</code></pre>
</notextile>

(Your shell should automatically fill in @$USER@ with your login name.  The job JSON that gets saved should have @"repository"@ pointed at your personal Git repository.)

Because the job ran in concurrent, each instance of concurrent-hash creates a separate @md5sum.txt@ as output.  Arvados automatically collates theses files into a single collection, which is the output of the job:

<notextile>
<pre><code>~/$USER/crunch_scripts$ <span class="userinput">arv keep ls e2ccd204bca37c77c0ba59fc470cd0f7+162</span>
./md5sum.txt
~/$USER/crunch_scripts$ <span class="userinput">arv keep get e2ccd204bca37c77c0ba59fc470cd0f7+162/md5sum.txt</span>
0f1d6bcf55c34bed7f92a805d2d89bbf alice.txt
504938460ef369cd275e4ef58994cffe bob.txt
8f3b36aff310e06f3c5b9e95678ff77a carol.txt
</code></pre>
</notextile>