Implementing RESTful Web Service APIs
=====================================

Yii provides a whole set of tools to greatly simplify the task of implementing RESTful Web Service APIs.
In particular, Yii provides support for the following aspects regarding RESTful APIs:

* Quick prototyping with support for common APIs for ActiveRecord;
* Response format (supporting JSON and XML by default) and API version negotiation;
* Customizable object serialization with support for selectable output fields;
* Proper formatting of collection data and validation errors;
* Efficient routing with proper HTTP verb check;
* Support `OPTIONS` and `HEAD` verbs;
* Authentication via HTTP basic;
* Authorization;
* Caching via `yii\web\HttpCache`;
* Support for HATEOAS: TBD
* Rate limiting: TBD
* Searching and filtering: TBD
* Testing: TBD
* Automatic generation of API documentation: TBD


A Quick Example
---------------

Let's use a quick example to show how to build a set of RESTful APIs using Yii.
Assume you want to expose the user data via RESTful APIs. The user data are stored in the user DB table,
and you have already created the ActiveRecord class `app\models\User` to access the user data.

First, check your `User` class for its implementation of the `findIdentityByToken()` method.
It may look like the following:

```php
class User extends ActiveRecord
{
	...
	public static function findIdentityByToken($token)
	{
		return static::find(['api_key' => $token]);
	}
}
```

This means your user table has a column named `api_key` which stores API access keys for the users.
Pick up a key from the table as you will need it to access your APIs next.

Second, create a controller class `app\controllers\UserController` as follows,

```php
namespace app\controllers;

use yii\rest\ActiveController;

class UserController extends ActiveController
{
	public $modelClass = 'app\models\City';
}
```

Third, modify the configuration about the `urlManager` component in your application configuration:

```php
'urlManager' => [
    'enablePrettyUrl' => true,
    'enableStrictParsing' => true,
    'showScriptName' => false,
    'rules' => [
		['class' => 'yii\rest\UrlRule', 'controller' => 'user'],
    ],
]
```

With the above minimal amount of effort, you have already finished your task of creating the RESTful APIs
for accessing the user data. The APIs you have created include:

* `GET /users`: list all users page by page;
* `HEAD /users`: show the overview information of user listing;
* `POST /users`: create a new user;
* `GET /users/123`: return the details of the user 123;
* `HEAD /users/123`: show the overview information of user 123;
* `PATCH /users/123` and `PUT /users/123`: update the user 123;
* `DELETE /users/123`: delete the user 123;
* `OPTIONS /users`: show the supported verbs regarding endpoint `/users`;
* `OPTIONS /users/123`: show the supported verbs regarding endpoint `/users/123`.

You may access your APIs with the `curl` command like the following,

```
curl -i -u "Your-API-Key:" -H "Accept:application/json" -d "_method=GET" "http://localhost/users"
```

> Tip: You may also access your API via Web browser. You will be asked
> to enter a username and password. Fill in the username field with the API key you obtained
> previously and leave the password field blank.

Try changing the acceptable content type to be `application/xml`, and you will see the result
is returned in XML format.

Using the `fields` and `expand` parameters, you can request to return a subset of the fields in the result.
For example, the following URL will only return the `id` and `email` columns in the result:

```
http://localhost/users?fields=id,email
```

You may have noticed that the result of `http://localhost/users` includes some sensitive columns,
such as `password_hash`, `auth_key`. You certainly do not want these to appear in your API result.
To filter these data out, modify the `User` class as follows,

```php
class User extends ActiveRecord
{
	public function fields()
	{
		$fields = parent::fields();
		unset($fields['auth_key'], $fields['password_hash'], $fields['password_reset_token']);
		return $fields;
	}
}
```

In the following subsections, we will explain in more details about implementing RESTful APIs.

TBD