title: Design patterns used in symfony2
body: <p>In <em>symfony2</em>, the <em>services</em> using Singleton patterns. Doctrine uses Unit of work patterns. I wonder a lot of design patterns may be used in <em>symfony2</em>. I think it will be useful to have better understanding of design patterns. My question is <em>what are the design patterns used in symfony2?</em></p>