We'll review the model-view-controller (MVC) paradigm and how Rails implements it.

1. List advantages created by compartmentalizing an application's functionality
2. Identify model-view-controller roles within a simplified example
3. Identify model-view-controller roles within Rails' implementation of the paradigm

Within the curriculum, you have been working hard learning defined, small-scale technical concepts, problems, and solutions. You've been "zoomed in" to the details of learning programming. In this README we will "zoom out" to think about software architecture. For this lesson, we are assuming the mindset of a software architect. Software architects, most likely your senior collaborator as you enter the workforce, don't focus on making a piece of code work, i.e. "...should I use .each or .map?". Rather, they are focused on making design decisions which allow for ease of change, extensibility, and avoidance of bugs.

MVC was created as a general purpose solution designed to bridge the gap between a user's mental model of a program and a computer's actual implementation. As we have been unknowing 'users' of the MVC pattern most of our lives, we need to change our perspective to understand it as programmers. When considering interfaces using the MVC pattern, the most critical shift in perspective to recognize is that the MVC pattern helps separate concerns.

At its core, MVC is designed to modularize distinct functionality within an application. While the MVC pattern can be used with many different types of applications, let's identify its distinct parts in the context of a web application:

• View: what an end user on a website experiences when interacting with a program in their browser: what they read, what they click, what flashes at them, and what they hear (despite the name "view"). In technology-speak, the vocabulary word would be interface
• Model: where the actual data, (be it information about restaurants, train times, or rare marsupials), resides and is altered
• Controller: what manages communication between the two: it takes model information and prepares it for the view and vice versa

Now that we have identified the core components, let's examine what they actually do when a user engages with our web application:

1. A user, through interaction with the view, (in this case, the browser's GUI), requests data (clicks on a link, submits a form, enters a URL in the browser's bar
2. The request is sent across the internet to the server
3. The controller (which does not change data itself!) asks the model to either provide it data (which it will send) or to change model-held data depending on the user's request
4. The model accesses/manipulates the actual 1's and 0's held on the server's database and returns the desired result to the controller
5. The controller packs the response and sends it back to the client
6. The view (on the originating client's machine) presents the data for the user

In considering the above steps, try to answer the following question:

• Do the model and view have any actual direct interaction?
• Is the data represented in the view, that the user sees and interacts with, anything but a virtual copy of the actual data a user perceives they are interacting with?

Earlier in this README, we said that we would practice thinking like a "software architect". Let's pursue that further. Imagine that the system we just described was applied to a banking system. You open a browser to your bank's site, you make requests to view your account balances, you make requests to transfer money between accounts.

Lo and behold a new device comes out, it's called a Smartphone! People can now use their PHONES to access the INTERNET! Thanks to the MVC architecture, the bank only has to build a new view, that is, an iPhone application.

Oh my goodness! Amazon invents a new home device that talks to you! Again, the bank only needs to write a new view (or "interface") that can leverage the pre-existing architecture of the bank's controller and model, so long as it can send information across the iternet.

Remember, we said that software architects focus on making design decisions which allow for ease of change, extensibility and avoidance of bugs. For every new device, we write new-device-only code (i.e. view code) which cannot introduce bugs into the pre-existing Model or Controller code. It's because of design patterns like this that 1970's banking software is still holding your account data while you view and manipulate it with 21st century technology.

• Be language- and application- agnostic
• Be modular: every part is distinct, encapsulated, and can be replaced without breaking the rest of the application
• Use a controller to facilitate communication between the view, (which the user interfaces with), and the model
• Stores 'truth', the actual data, in the model, which is far abstracted and independent from its representation in the view

Rails takes architectural guidance on how to separate concerns from the MVC pattern. Like most implementations of academic or philosophical ideals, Rails's implementation is not to-the-letter perfect MVC. That's OK. Consider the textbook definition of apple pie crust: the textbook definition describes exact ratios of salt, sugar, fats, etc. But my implementation might be different than Avi's implementation. Is mine less correct? Is his less correct? There's room for intelligent discourse and discovery of what's appropriate to the constraints.

The controller and model implementations in Rails are straightforward to align with our understanding of MVC.

Not only are they named helpfully and live in helpfully-named directories like models/ and controllers/, but they encapsulate their responsibilities as prescribed in the MVC architecture. The controller actions mediate requests and delegate lookups by calling methods on models. As expected, models work with the low-level persistent storage subsystems (e.g. databases) to UPDATE, SELECT, and DELETE data in the database. The Rails views are templates for displaying data. They are automatically turned into HTML that is sent over the internet by Rails. However, the data they need to "complete" themselves is provided by a Rails controller. Rails "automagically" shares any variable prefixed with @ in a controller action with the view.

ASIDE: Some feel that this "automagic" is "bad." DHH feels that this "automagic" is "good." Reasonable programmers can disagree over the advantages and disadvantages of this choice. However, since DHH created Rails and runs the project, his opinion is the one implemented.

• The MVC paradigm is a language- and application-agnostic pattern for separating concerns
• MVC describes an architecture of three modularized parts: model, controller, view
• While Rails emulates the majority of an MVC structure, it is not a perfect "textbook" match — by design!

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