Document other prerequisites.

doc/installation.rst

@@ -28,6 +28,12 @@ used in other environments with some modification.
 Prerequisites
 -------------
 
+* Python. The backend of the framework, which takes care of running jobs, is implemented in Python. It requires Python 2.6 or
+  2.7 (it is not tested with Python 3).
+
+* Perl. The frontend, which handles user submission of jobs and the display of results, is implemented in Perl, and uses a
+  number of Perl modules, most notably CGI and DBI.
+
 * SGE. The framework expects to be run on a machine that is a submit host to a Sun GridEngine compute cluster
   (or a compatible product, such as `OGE <http://www.oracle.com/us/products/tools/oracle-grid-engine-075549.html>`_
   or `OGS <http://gridscheduler.sourceforge.net/>`_). The framework talks to SGE via DRMAA, so you will also need
@@ -44,8 +50,24 @@ Prerequisites
   to use the path of the Sali lab's own installation; to use outside of the Sali lab you will need to edit
   :file:`perl/saliweb/SConscript` and change the path to :file:`config.py`.
 
-* Web server (e.g. Apache)
+* Web server. The frontend consists of Perl CGI scripts which need to be hosted by a web server. Apache is used in the
+  Sali lab, but other web servers would probably work too (the only assumption made in the code is that files uploaded
+  to the web server end up owned by the `httpd` user, but this would be easy to change). The web server does not have
+  to run on the same machine as the framework, but it does need access to the same filesystems (see below).
 
-* Filesystems (local, NFS)
+* MySQL database. Both the frontend and backend need access to a MySQL database in which the jobs are stored. This requires
+  the Perl DBI and DBD-MySQL packages, plus the Python MySQLdb package. The framework itself needs no special access to the
+  database, but each web service that uses the framework needs its own database. (The MySQL server can reside on a different
+  machine to the framework if desired.)
+
+* Unix user. Each web service that uses the framework runs under its own user ID, and the jobs are run on the SGE cluster
+  using the same ID. Thus, generally admin access to the cluster is required to add these users.
 
-* MySQL database
+* Filesystems. Each web service that uses the framework needs access to a local filesystem on the SGE submit host. This will
+  be used by the web server to deposit newly-submitted jobs (the 'incoming' directory). The filesystem needs to support
+  `POSIX ACLs <http://www.vanemery.com/Linux/ACL/POSIX_ACL_on_Linux.html>`_ (this generally rules out NFS) since the directory
+  will be owned by a Unix user unique to that web service, but will have a POSIX ACL applied to allow the Apache httpd user
+  to write files into it. Each web service will also need a directory on a shared volume, visible to the cluster nodes,
+  where the files for jobs that run on the cluster are placed (the 'running' directory). (If space is limited on the network
+  storage, web services can also be configured to move the files back to the cheaper local filesystem - the 'completed'
+  directory - when the job is done.)