Add a recipe about using Dalli for sessions stored in memcached

Author: patriciomacadden

middleware/share_session_with_memcached.md

@@ -0,0 +1,37 @@
+# How to share a session with Memcached?
+
+Suppose that you are developing a Sinatra app that will be deployed to multiple
+server instances and must be seen as a single application (it must be balanced
+with nginx, for instance). You need a mechanism for sharing user sessions
+between all the application instances. To accomplish this task, we'll use
+memcached for sharing the session and [Dalli](https://github.com/mperham/dalli)
+as the memcached client.
+
+First, add `dalli` to your `Gemfile`:
+
+```ruby
+gem 'dalli'
+```
+
+Then, add the `Rack::Session::Dalli` middleware:
+
+```ruby
+configure do
+  use Rack::Session::Dalli, cache: Dalli::Client.new
+end
+```
+
+And that's all, from now on we'll be storing the users sessions in our local
+memcached server (`localhost:11211`) and the `rack:session` namespace. If you
+want to change these settings, use the middleware like this:
+
+```ruby
+configure do
+  use Rack::Session::Dalli,
+    memcache_server: 'example.com:11211',
+    namespace: 'other.namespace'
+    cache: Dalli::Client.new
+end
+```
+
+If you need more info, refer to the [dalli gem documentation](https://github.com/mperham/dalli).