" MVC is often seen in web apps: the view is the html page and the code which gathers dynamic data for the page. Controller: This responds to events, typically user actions, and invokes changes on the model (but *not* the view) . Many applications use a persistent storage mechanism (such as a database) to store data. MVC does not specifically mention this data access layer. It's common to think of an application as having three main layers: presentation (UI), domain,and data access. In MVC, the presentation layer is split into controller and view. The most important separation is between presentation and domain. Model Railroad Steam Locomotive The V/C split less so. Though MVC comes in different flavors, control flow generally works as follows: The user interacts with the user interface in some way (e. "

" Bosons possess integer spin and do not obey the Pauli exclusion principle. Informally speaking, fermions are particles of matter and bosons are particles that transmit forces. For a detailed description of the differences between fermions and bosons, see the article on identical particles. In the Standard Model, the theory of the electroweak interaction (which describes the weak and electromagnetic interactions) is combined with the theory of quantum chromodynamics. All of these theories are gauge theories, meaning that they model the forces between fermions by coupling them to bosons which mediate (or "carry") the forces. The Lagrangian of each set of mediating bosons is invariant under a transformation called a gauge transformation, so these mediating bosons are referred to as gauge bosons. The bosons in the Standard Model are: Unsolved problems in physics: Parameters in the Standard Model: What gives rise to the Standard Model of particle physics? Why do its particle masses and coupling constants possess the values we have measured? Does the Higgs boson predicted by the model really exist? Why are there three generations of particles in the Standard Model? Is the Standard Model reality, a good approximation to reality, or fatally flawed? Photons, which mediate the electromagnetic interaction. Bachmann Model Railroad W and Z bosons, which mediate the weak nuclear force Eight species of gluons, which mediate the strong nuclear force. Six of these gluons are labelled as pairs of "colors" and "anti-colors" (for example, a gluon can carry "red" and "anti-green". "