Inside the `/config` folder, every folder will be parsed and injected into the global object `strapi.config`. Let's say, you added a folder named `credentials` with two files `stripe.json` and `paypal.json` into it. The content of these files will be accessible through `strapi.config.credentials.stripe` and `strapi.config.credentials.paypal`.
:::
---
## Controllers
Controllers are JavaScript files which contain a set of methods called **actions** reached by the client according to the requested route. It means that every time a client requests the route, the action performs the business logic coded and sends back the response. They represent the _C_ in the _MVC_ pattern. In most cases, the controllers will contain the bulk of a project's business logic.
```js
module.exports = {
// GET /hello
index: async ctx => {
ctx.send('Hello World!');
},
};
```
In this example, any time a web browser is pointed to the `/hello` URL on your app, the page will display the text: `Hello World!`.
### Where are the controllers defined?
The controllers are defined in each `./api/**/controllers/` folders. Every JavaScript file put in these folders will be loaded as a controller. They are also available through the `strapi.controllers` and `strapi.api.**.controllers` global variables. By convention, controllers' names should be Pascal-cased, so that every word in the file (include the first one) is capitalized `User.js`, `LegalEntity.js`.
Filters are a handy way to request data according to generic parameters. It makes filtering, sorting and paginating easy and reusable (eg. `GET /users?_limit=30&name=John`).
Models are a representation of the database's structure and lifecycle. They are split into two separate files. A JavaScript file that contains the lifecycle callbacks, and a JSON one that represents the data stored in the database and their format. The models also allow you to define the relationships between them.
The models are defined in each `./api/**/models/` folder. Every JavaScript or JSON file in these folders will be loaded as a model. They are also available through the `strapi.models` and `strapi.api.**.models` global variables. Usable everywhere in the project, they contain the ORM model object that they are refer to. By convention, models' names should be written in lowercase.
Please refer to the [many-to-many guide](../guides/models.md#many-to-many)
:::
#### One-to-many
One-way relationships are useful to link an entry to another.
::: note
Please refer to the [one-to-many guide](../guides/models.md#one-to-many)
:::
#### One-to-one
One-way relationships are useful to link an entry to another.
::: note
Please refer to the [one-to-one guide](../guides/models.md#one-to-one).
:::
#### One-way
One-way relationships are useful to link an entry to another. However, only one of the models can be queried with its populated items.
::: note
Please refer to the [one-way guide](../guides/models.md#one-way).
:::
### Lifecycle callbacks
Lifecycle callbacks are functions triggered at specific moments of the queries.
::: note
Please refer to the [lifecycle callbacks guide](../guides/models.md#lifecycle-callbacks).
:::
---
## Internationalization and localization
Internationalization and localization (i18n) allows to adapt the project to different languages and serve the right content to the users. This feature is deeply integrated into the Strapi's core. It will detect the user language preference (locale) and translate the requested content using the translation files.
::: note
Please refer to the [internationalization's guide](../guides/i18n.md).
A plugin is like a small independent sub-application. It has its own business logic with dedicated models, controllers, services, middlewares or hooks. It can also have it's own UI integrated in the admin panel.
In strapi you can install plugins in your `node_modules`. This allows for easy updates and respect best practices. To customize those installed plugins you can work in the `/extensions` directory. It contains all the plugins' customizable files.
Certain plugins will create files in these folders so you can then modify them. You can also create certain files manually to add some custom configuration for example.
Depending on the plugins you will find extension documentation directly in the plugin's documentation.
Extensions folder structure:
-`extensions/`
-`**`: Plugin Id
-`config`: You can extend a plugin's configuration by add a settings.json file with your custom configuration
Policies are functions which have the ability to execute specific logic on each request before it reaches the controller's action. They are mostly used for securing business logic easily.
Each route of the project can be associated to an array of policies. For example, you can create a policy named `isAdmin`, which obviously checks that the request is sent by an admin user, and use it for critical routes.
Policies can be:
-`global`: so they can be used within the entire project.
-`scoped`: used by single API or plugin.
### Where are the policies defined?
The API and plugins policies (scoped) are defined in each `./api/**/config/policies/` folders and plugins. They are respectively exposed through `strapi.api.**.config.policies` and `strapi.plugins.**.config.policies`. The global policies are defined at `./config/policies/` and accessible via `strapi.config.policies`.
::: note
Please refer to the [policy guide](../guides/policies.md)
:::
### Global policies
Global policies are reusable through the entire app.
::: note
Please refer to the [global policy guide](../guides/policies.md#global-policies)
:::
### Scoped policies
A policy defined in an API or plugin is usable only from this API or plugin. You don't need any prefix to use it.
::: note
Please refer to the [scoped policy guide](../guides/policies.md#scoped-policies).
:::
### Plugin policies
Plugin policies are usable from any app API.
::: note
Please refer to the [plugin policy guide](../guides/policies.md#plugins).
:::
## Public Assets
Public assets are static files such as images, video, css, etc that you want to make accessible to the outside world. Every new project includes by default, a folder named `./public`.
Services are a set of reusable functions. They are particularly useful to respect the DRY (don’t repeat yourself) programming concept and to simplify [controllers](#controllers) logic.
