The starting page for User Guide is done

Partially implements blueprint docs-for-v02

Change-Id: Ic6a632337b426794fdab3b97d10ffb97dd3e7d7c

doc/source/index.rst

@@ -20,9 +20,10 @@ User guide
 
    userdoc/installation.guide
    horizon/installation.guide
-   userdoc/rest_api_v1.0
+   userdoc/overview
    userdoc/features
    userdoc/hadoop-swift.rst
+   userdoc/rest_api_v1.0
 
 
 Developer Guide

doc/source/userdoc/overview.rst

@@ -0,0 +1,64 @@
+General Overview
+================
+
+Clusters
+--------
+
+A cluster deployed by Savanna consists of node groups. Node groups vary by
+their role, parameters and number of machines. The picture below
+illustrates example of Hadoop cluster consisting of 3 node groups each having
+different role (set of processes).
+
+.. image:: ../images/hadoop-cluster-example.jpg
+
+Node group parameters include Hadoop parameters like `io.sort.mb` or
+`mapred.child.java.opts`, and several infrastructure parameters like flavor
+for VMs or storage location (ephemeral drive or Cinder volume).
+
+A cluster is characterized by its node groups and its parameters. Like a node
+group, cluster has Hadoop and infrastructure parameters. An
+example of cluster-wide Hadoop parameter is `dfs.replication`. For
+infrastructure an example could be image which will be used to launch cluster
+VMs.
+
+Templates
+---------
+
+In order to simplify cluster provisioning Savanna employs concept of templates.
+There are two kind of templates: node group template and cluster template. The
+former is used to create node groups, the later - clusters. Essentially
+templates have the very same parameters as corresponding entities. Their aim
+is to remove burden of specifying all the required parameters each time user
+wants to launch a cluster.
+
+Provisioning Plugins
+--------------------
+
+A provisioning plugin is a component responsible for provisioning Hadoop
+cluster. Generally each plugin is capable of provisioning a specific Hadoop
+distribution. Also plugin can install management and/or monitoring tools for
+a cluster.
+
+Since Hadoop parameters vary depending on distribution and Hadoop version,
+templates are always plugin and Hadoop version specific. A template could not
+be used with plugun/Hadoop version different than ones it was created for.
+
+Image Registry
+--------------
+
+OpenStack starts VMs based on pre-built image with installed OS. The image
+requirements for Savanna depend on plugin and Hadoop version. Some plugins
+require just basic cloud image and install Hadoop on VMs from scratch. Some
+plugins might require images with pre-installed Hadoop.
+
+The Savanna Image Registry is a feature which helps filter out images during
+cluster creation. Before creating a cluster, user should register an image in
+Image Registry and tag it with specific tags. Each plugin has its own set of
+tags which help it to determine if an image is suitable for a specific
+Hadoop version.
+
+Features
+--------
+
+Savanna has several interesting features. The full list could be found there:
+:doc:`features`