CS 4720 Model-View-Controller CS 4720 – Web & Mobile Systems

CS 4720 Abstractions and Tiers Every system we seem to build we talk about tiers How do we reason about the architecture of a system? How can we communicate how the pieces fit together? 2

CS 4720 A Two-Tier System 3

CS 4720 A Two-Tier System? 4

CS 4720 Describing the World We generally refer to client/server systems as “two-tier systems” Why? What does that communicate? 5

CS 4720 A Three-Tier System How might a three-tier system be different than a two tier? Do we add a server? Do we add a client? What does it mean to have three tiers? 6

CS 4720 Context Clues It’s all in what you’re talking about. If you’re discussion the overall system, including the client, a common view of a three-tier architecture might be this: 7

CS 4720 Context Switch What if we are talking about just inside our application? We assume that the client is there, but it is somewhat immaterial How do we define the layers now? 8

CS 4720 Three-Tier Architecture 9

CS 4720 Three-Tier Architecture We will see this architecture pattern over and over again in this class –Web development –Web services –Android (basically all mobile…) Here we are discussing it in the context of a RESTful web architecture 10

CS 4720 Principles of REST Remember this one? –3. Addressable Resources –Everything has its place and every place has a thing –“Everything is a resource” This is actually key to how we go about building a three-tier web application 11

CS 4720 RESTful Tiers If every resource is a place, and every place is a resource, how do we define our resources? We know the URL would look something like: – Where: –mysite.com is the domain –users is the “category of resources” –mss2x is a specific instance of the resource 12

CS 4720 RESTful Tiers What we want the web server / web application to do is translate this: – Into effectively a function call that –Identifies what the user is asking for –Loads a particular resource –Displays the pertinent info about that resource back to the user 13

CS 4720 Model-View-Controller This is the definition of what MVC is The MVC pattern maps: –Identifies what the user is asking for –Loads a particular resource –Displays the pertinent info about that resource back to the user To Model, Controller, View (in that order) 14

CS 4720 MVC 15

CS 4720 MVC 16

CS 4720 Controller The role of the controller is basically traffic cop It takes the request from the user and (with the assistance of the server and routing rules) turns it into a method call of sorts It finds the appropriate model to load It finds the appropriate view to load It returns the whole thing back to the user 17

CS 4720 Model The model is the representation of the data This may or may not be directly linked to a database (but often is in larger apps) A model is often translated directly into a DB table, with the columns as its attributes Think “class definition w/ DB backend” Often contains relationship rules (a Student has many Classes, for instance) 18

CS 4720 View The closest thing to what you’ve been dealing with so far is the view It’s effectively an HTML template that will be populated with the appropriate data from the loaded model It often has PHP (or whatever) embedded in it All UI components go here 19

CS 4720 Putting it all Together So, if you were building a blog, what might some of the models be? What are the resources that should have addresses to them? How do they relate to each other? 20

CS REST and MVC REST and MVC aren’t directly related, but they intersect in many, many ways The idea of “nouns” in the system – of addressable resources – is a major component of both In general, a good MVC system will be RESTful, although a RESTful system does not have to follow MVC.

CS 4720 Example Code What do some of these things look like? 22