Added beginnings of a deployment guide w/ info on sessions.

Implements blueprint cookie-based-sessions.

Change-Id: I2cccf623055813bdddcfe53c9abb164d095e232d

docs/source/index.rst

@@ -40,6 +40,7 @@ How to use Horizon in your own projects.
 
    intro
    quickstart
+   topics/deployment
    topics/branding
 
 Developer Docs

docs/source/topics/deployment.rst

@@ -0,0 +1,119 @@
+=================
+Deploying Horizon
+=================
+
+This guide aims to cover some common questions, concerns and pitfalls you
+may encounter when deploying Horizon in a production environment.
+
+Session Storage
+===============
+
+Horizon uses `Django's sessions framework`_ for handling user session data;
+however that's not the end of the story. There are numerous session backends
+available, which are controlled through the ``SESSION_ENGINE`` setting in
+your ``local_settings.py`` file. What follows is a quick discussion of the
+pros and cons of each of the common options as they pertain to deploying
+Horizon specifically.
+
+.. _Django's sessions framework: https://docs.djangoproject.com/en/dev/topics/http/sessions/
+
+Local Memory Cache
+------------------
+
+Enabled by::
+
+    SESSION_ENGINE = 'django.contrib.sessions.backends.cache'
+    CACHES = {
+        'BACKEND': 'django.core.cache.backends.locmem.LocMemCache'
+    }
+
+Local memory storage is the quickest and easiest session backend to set up,
+as it has no external dependencies whatsoever. However, it has two significant
+drawbacks:
+
+  * No shared storage across processes or workers.
+  * No persistence after a process terminates.
+
+The local memory backend is enabled as the default for Horizon solely because
+it has no dependencies. It is not recommended for production use, or even for
+serious development work. For better options, read on.
+
+Memcached
+---------
+
+Enabled by::
+
+    SESSION_ENGINE = 'django.contrib.sessions.backends.cache'
+    CACHES = {
+        'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache'
+        'LOCATION': 'my_memcached_host:11211',
+    }
+
+External caching using an application such as memcached offers persistence
+and shared storage, and can be very useful for small-scale deployment and/or
+development. However, for distributed and high-availability scenarios
+memcached has inherent problems which are beyond the scope of this
+documentation.
+
+Memcached is an extremely fast and efficient cache backend for cases where it
+fits the depooyment need. But it's not appropriate for all scenarios.
+
+Requirements:
+
+  * Memcached service running and accessible.
+  * Python memcached module installed.
+
+Database
+--------
+
+Enabled by::
+
+    SESSION_ENGINE = 'django.core.cache.backends.db.DatabaseCache'
+    DATABASES = {
+        'default': {
+            # Databe configuration here
+        }
+    }
+
+Database-backed sessions are scalable (using an appropriate database strategy),
+persistent, and can be made high-concurrency and highly-available.
+
+The downside to this approach is that database-backed sessions are one of the
+slower session storages, and incur a high overhead under heavy usage. Proper
+configuration of your database deployment can also be a substantial
+undertaking and is far beyond the scope of this documentation.
+
+Cached Database
+---------------
+
+To mitigate the performance issues of database queries, you can also consider
+using Django's ``cached_db`` session backend which utilizes both your database
+and caching infrastructure to perform write-through caching and efficient
+retrieval. You can enable this hybrid setting by configuring both your database
+and cache as discussed above and then using::
+
+    SESSION_ENGINE = "django.contrib.sessions.backends.cached_db"
+
+Cookies
+-------
+
+If you're using Django 1.4 or later, a new session backend is available to you
+which avoids server load and scaling problems: the ``signed_cookies`` backend!
+
+This backend stores session data in a cookie which is stored by the
+user's browser. The backend uses a cryptographic signing technique to ensure
+session data is not tampered with during transport (**this is not the same
+as encryption, session data is still readable by an attacker**).
+
+The pros of this session engine are that it doesn't require any additional
+dependencies or infrastructure overhead, and it scales indefinitely as long
+as the quantity of session data being stored fits into a normal cookie.
+
+The biggest downside is that it places session data into storage on the user's
+machine and transports it over the wire. It also limits the quantity of
+session data which can be stored.
+
+For a thorough discussion of the security implications of this session backend,
+please read the `Django documentation on cookie-based sessions`_.
+
+.. _Django documentation on cookie-based sessions: https://docs.djangoproject.com/en/dev/topics/http/sessions/#using-cookie-based-sessions