Caching is a cheap and effective way to improve the performance of a web application. By storing relatively
static data in cache and serving it from cache when requested, the application saves the time required to generate the data from scratch. Caching is one of the best ways to improve the performance of your application, almost mandatory on any large-scale site.
Like source code, the structure of a database evolves as a database-driven application is developed and maintained. For example, during development, a new table may be added; Or, after the application goes live, it may be discovered that an additional index is required. It is important to keep track of these structural database changes (called **migration**), just as changes to the source code is tracked using version control. If the source code and the database become out of sync, bugs will occur, or the whole application might break. For this reason, Yii provides a database migration
tool that can keep track of database migration history, apply new migrations, or revert existing ones.
The Yii framework was designed to be easily extendable. Additional features can be added to your project and then reused, either by yourself on other projects or by sharing your work as a formal Yii extension.
As a model both represents data and defines the business rules to which that data must adhere, comprehending data validation is key to using Yii. In order to learn model validation basics, please refer to [Model, Validation subsection](model.md#Validation).
The concept of URL management in Yii is fairly simple. URL management is based on the premise that the application uses
internal routes and parameters everywhere. The framework itself will then translate routes into URLs, and vice versa, according to the URL manager's configuration. This approach allows you to change site-wide URLs merely by
Authentication is the act of verifying who a user is, and is the basis of the login process. Typically, authentication uses the combination of an identifier--a username or email address--and a password. The user submits these values through a form, and the application then compares the submitted information against that previously stored (e.g., upon registration).
Good security is vital to the health and success of any application. Unfortunately, many developers cut corners when it comes to security, either due to a lack of understanding or because implementation is too much of a hurdle. To make your Yii powered application as secure as possible, Yii has included several excellent and easy to use security features.
In keeping with the MVC approach, a model in Yii is intended for storing or temporarily representing application data, as well as defining the busines rules by which the data must abide.
Yii2 includes a handy toolbar, and built-in debugger, for faster development and debugging of your applications. The toolbar displays information
about the currently opened page, while the debugger can be used to analyze data you've previously collected (i.e., to confirm the values of variables).
Yii has full featured support for console applications, whose structure is very similar to a Yii web application. A console application
consists of one or more [[yii\console\Controller]] classes, which are often referred to as "commands" in the console environment. Each controller can also have one or more actions, just like web controllers.
