Documentation on GALAXY_SLOTS.

docs/_writing_clusters.rst

@@ -0,0 +1,31 @@
+Cluster Usage
+==============================
+
+------------------------------
+``GALAXY_SLOTS``
+------------------------------
+
+``GALAXY_SLOTS`` is a special environment variable that is set in a Galaxy
+tool's runtime environment. If the tool you are working on allows configuring
+the number of processes or threads that should be spawned, this variable
+should be used.
+
+For example, the StringTie (tool available `here
+<https://github.com/galaxyproject/tools-iuc/blob/master/tools/stringtie/stringtie.xml>`__)
+binary ``stringtie`` can take an argument ``-p`` that allows specification
+of the number of threads to be used. The Galaxy tool sets this up as follows::
+
+    stringtie "$input_bam" -o "$output_gtf" -p "\${GALAXY_SLOTS:-1}" ...
+
+Here we use ``\${GALAXY_SLOTS:-Z}`` instead of a fixed value (Z being an
+integer representing a default value in non-Galaxy contexts). The
+backslash here is because this value is interpreted at runtime as
+environment variable - not during command building time as a templated
+value. Now server administrators can configure how many processes the 
+tool should be allowed to use.
+
+For information on how server administrators can configure this value for
+a particular tool, check out `the Galaxy wiki
+<https://wiki.galaxyproject.org/Admin/Config/GALAXY_SLOTS>`__.
+
+.. _stringtie: https://ccb.jhu.edu/software/stringtie/

docs/writing_advanced.rst

@@ -9,8 +9,9 @@ installed - check out <writing> if you have never developed a Galaxy tool.
 
 .. include:: _writing_testing.rst
 .. include:: _writing_collections.rst
+.. include:: _writing_clusters.rst
 
-.. _Galaxy: (http://galaxyproject.org/)
+.. _Galaxy: http://galaxyproject.org/
 .. _Docker: https://www.docker.com/
 .. _Vagrant: https://www.vagrantup.com/
 .. _BWA: http://bio-bwa.sourceforge.net/