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Eric Larson  committed 8e363c6

Added a readme

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+======
+ Xenv
+======
+
+Xenv stands for Execution Environment. A xenv is a specific set of
+applications, libraries and files that are used when running some
+application. 
+
+
+Why?
+====
+
+Why would anyone need to use xenv? Well, the reason for using an xenv
+is that it provides a simple way to layer your execution environment
+within a larger environment. 
+
+In order to create a consistent deployment model you need to make
+agreements on what the deployed systems look like. At the lowest level
+you have standards such as POSIX and the applications that make up the
+operating system. In the Linux world, you have a distribution that
+makes some decisions regarding things like system level package
+management and directory structure. You application may rely on
+certain applications being present along with libraries. All these
+systems effectively layer upon each other in order to provide
+different levels of support.
+
+Often times developers utilize virtualization technology to create an
+execution environment for their applications. This is a very poweful
+model, yet it is somewhat inefficient in that you reproduce every
+layer of support in order to run some application. Another approach is
+to reuse much of the lower level resources. This is where xenv comes
+in. 
+
+Xenv allows you to create an application level execution environment
+that focuses on installing application level tools while reusing the
+system level resources. By providing an application level execution
+environment it allows each level of support to function
+independently. The application level has the ability to install and
+use the underlying system, while also allowing overriding those
+features as necessary. 
+
+The result is that system administrators can safely upgrade systems
+without being limited by an application requirements. An application
+has a means of using the libraries it needs without impacting the
+underlying system or other execution environments.
+
+Virtualenv and Python
+=====================
+
+Xenv is written specifically with Python applications in mind. Hence
+it uses virtualenv in order to support Python within an execution
+environment. The difference between xenv and virtualenv is that a xenv
+is meant to wrap vritualenv in a way that is compatible with other
+runtimes. This allows a consistent pattern no matter the runtime
+environment. The xenv allows deploying and running a Java application
+in the same way you would run a Python application.
+
+With all that said, virtualenv is the only system somewhat formally
+supported. There is generic support for any executable, but no effort
+has been placed in running something more specific such as Java. I'd
+expect this to change if others see use in this idea and adopt it
+directly or in concept, then I'd imagine support for a specific
+runtime being developed.
+
+How to use it
+=============
+
+Xenv provides a command line application 'xe' that is an entry point
+to the execution environment. Its primary purpose is to make sure the
+correct environment has been configured. Here is the help output: ::
+
+  usage: xe [-h] [--root ROOT] {create,run} ...
+   
+  Utilize a specific executable environment (xenv) for running applications
+   
+  positional arguments:
+    {create,run}          sub command help
+      create              Create a new xenv
+      run                 Run a command from the xenv
+   
+  optional arguments:
+    -h, --help            show this help message and exit
+    --root ROOT, -r ROOT  The xenv root. This will override using the XE_ROOT env var.
+  
+Creating a xenv will make a directory with a set of folders. The most
+important folder is the /usr directory. This is where a virtualenv is
+installed. Here is an example creating a new xenv and calling its
+Python interpreter. ::
+
+  $ xe create xenv
+  mkdir xenv (mode 511)
+  mkdir xenv/local (mode 511)
+  New python executable in xenv/usr/bin/python
+  Installing setuptools............done.
+  Installing pip...............done.
+  $ XE_ROOT=xenv xe run python
+
+