Add plugin documentation

docs/plugins.md

@@ -1,5 +1,118 @@
 # Plugin System
 
-The plugin system will allow to manipulate requests and responses inside a `HttpClient`.
+The plugin system allows to look at requests and responses and replace them if needed, inside an `HttpClient`.
 
-TODO: finalize system and document.
+Using the `Http\Client\Plugin\PluginClient`, you can inject an `HttpClient`, or an `HttpAsyncClient`, and an array
+of plugins implementing the `Http\Client\Plugin\Plugin` interface.
+
+Each plugin can replace the `RequestInterface` sent or the `ResponseInterface` received. It can also change the behavior of a call,
+like retrying the request or emit another one when a redirection response was received.
+
+## Install
+
+Install the plugin client in your project with composer:
+
+``` bash
+composer require "php-http/plugins:^1.0"
+```
+
+## Usage
+
+First you need to have some plugins:
+
+```php
+use Http\Client\Plugin\RetryPlugin;
+use Http\Client\Plugin\RedirectPlugin;
+
+$retryPlugin = new RetryPlugin();
+$redirectPlugin = new RedirectPlugin();
+```
+
+Then you can create a `PluginClient`:
+
+```php
+use Http\Discovery\HttpClientDiscovery;
+use Http\Client\Plugin\PluginClient;
+
+...
+
+$pluginClient = new PluginClient(HttpClientDiscovery::find(), [
+    $retryPlugin,
+    $redirectPlugin
+]);
+```
+
+You can use the plugin client like a classic `Http\Client\HttpClient` or `Http\Client\HttpAsyncClient` one:
+
+```php
+// Send a request
+$response = $pluginClient->sendRequest($request);
+
+// Send an asynchronous request
+$promise  = $pluginClient->sendAsyncRequest($request);
+```
+
+Go to the [tutorial](tutorial.md) to read more about using `HttpClient` and `HttpAsyncClient`
+
+## Available plugins
+
+Each plugin has its own configuration and dependencies, check the documentation for each of the available plugins:
+
+ - [Authentication](plugins/authentication.md): Add authentication header on a request
+ - [Cookie](plugins/cookie.md): Add cookies to request and save them from the response
+ - [Encoding](plugins/encoding.md): Add support for receiving chunked, deflate or gzip response
+ - [Error](plugins/error.md): Transform bad response (400 to 599) to exception
+ - [Redirect](plugins/redirect.md): Follow redirection coming from 3XX responses
+ - [Retry](plugins/retry.md): Retry a failed call
+ - [Stopwatch](plugins/stopwatch.md): Log time of a request call by using [the Symfony Stopwatch component](http://symfony.com/doc/current/components/stopwatch.html)
+
+## Order of plugins
+
+When you inject an array of plugins into the `PluginClient`, the order of the plugins matters.
+
+During the request, plugins are called in the order they have in the array, from first to last plugin. Once a response has been received,
+they are called again in inverse order, from last to first.
+
+i.e. with the following code:
+
+```php
+use Http\Discovery\HttpClientDiscovery;
+use Http\Client\Plugin\PluginClient;
+use Http\Client\Plugin\RetryPlugin;
+use Http\Client\Plugin\RedirectPlugin;
+
+$retryPlugin = new RetryPlugin();
+$redirectPlugin = new RedirectPlugin();
+
+$pluginClient = new PluginClient(HttpClientDiscovery::find(), [
+    $retryPlugin,
+    $redirectPlugin
+]);
+```
+
+The execution chain will look like this:
+
+```
+Request  ---> PluginClient ---> RetryPlugin ---> RedirectPlugin ---> HttpClient ----
+                                                                                   | (processing call)
+Response <--- PluginClient <--- RetryPlugin <--- RedirectPlugin <--- HttpClient <---
+```
+
+In order to have correct behavior over the global process, you need to understand well each plugin used,
+and manage a correct order when passing the array to the `PluginClient`
+
+`RetryPlugin` will be best at the end to optimize the retry process, but it can also be good
+to have it as the first plugin, if one of the plugins is inconsistent and may need a retry.
+
+The recommended way to order plugins is the following rules:
+
+ 1. Plugins that modify the request should be at the beginning (like the `AuthenticationPlugin` or the `CookiePlugin`)
+ 2. Plugins which intervene in the workflow should be in the "middle" (like the `RetryPlugin` or the `RedirectPlugin`)
+ 3. Plugins which log information should be last (like the `LoggerPlugin` or the `HistoryPlugin`)
+
+However, there can be exceptions to these rules. For example, for security reasons you might not want to log the authentication header
+and chose to put the AuthenticationPlugin after the LoggerPlugin.
+
+## Implementing your own Plugin
+
+Read this [documentation](plugins/plugin-implementation.md) if you want to create your own Plugin.

docs/plugins/authentication.md

@@ -0,0 +1,3 @@
+# Authentication Plugin
+
+TODO: explain the authentication plugin

docs/plugins/cookie.md

@@ -0,0 +1,3 @@
+# Cookie Plugin
+
+TODO: explain the cookie plugin

docs/plugins/encoding.md

@@ -0,0 +1,3 @@
+# Encoding Plugin
+
+TODO: explain the encoding plugin

docs/plugins/error.md

@@ -0,0 +1,3 @@
+# Error Plugin
+
+TODO: explain the error plugin

docs/plugins/plugin-implementation.md

@@ -0,0 +1,79 @@
+# Implementing your own Plugin
+
+When writing your own Plugin, you need to be aware that the `PluginClient` is async first. This means that every plugin must
+be written by respecting the `HttpAsyncClient` contract and returns a `Promise`.
+
+Each plugin must implement the `Http\Client\Plugin\Plugin` interface.
+
+This interface defines the `handleRequest` method that allows to modify behavior of the call:
+
+```php
+/**
+ * handle the request and return the response coming from the next callable
+ *
+ * @param RequestInterface $request Request to use
+ * @param callable         $next    Callback to call to have the request, it muse have the request as it first argument
+ * @param callable         $first   First element in the plugin chain, used to to restart a request from the beginning
+ *
+ * @return Promise
+ */
+public function handleRequest(RequestInterface $request, callable $next, callable $first);
+```
+
+The `$request` comes from upstream. You can replace it and pass a new version downstream if you need. Always be aware that
+the request is immutable.
+
+The `$next` callable is the next plugin in the execution chain. When you need to call it, you must pass the `$request`
+as the first argument of this callable.
+
+```
+public function handleRequest(RequestInterface $request, callable $next, callable $first)
+{
+    $newRequest = $request->withHeader('MyHeader', 'MyValue');
+
+    return $next($newRequest);
+}
+```
+
+
+The `$first` callable is the first plugin in the execution. It allows you to completely reboot the execution chain, or send
+other request if needed, while still going through all the defined plugins. Like the `$next` callable, you must pass the `$request`
+as the first argument of this callable.
+
+```
+public function handleRequest(RequestInterface $request, callable $next, callable $first)
+{
+    if ($someCondition) {
+        $newRequest = new Request();
+        $promise    = $first($newRequest);
+
+        // Use the promise do some jobs ...
+    }
+
+    return $next($request);
+}
+```
+
+In this example the condition is not superfluous, you need to have some way to not calling the $first callable each time or
+you will end up with a infinite execution loop.
+
+The `$next` and `$first` callable will return a `Promise`. You can manipulate the `ResponseInterface` or the `Exception`
+by using the `then` method of the promise.
+
+```
+public function handleRequest(RequestInterface $request, callable $next, callable $first)
+{
+    $newRequest = $request->withHeader('MyHeader', 'MyValue');
+
+    return $next($request)->then(function (ResponseInterface $response) {
+        return $response->withHeader('MyResponseHeader', 'value');
+    }, function (Exception $exception) {
+        echo $exception->getMessage();
+
+        throw $exception;
+    });
+}
+```
+
+Anyway it is always a good practice to read existing implementation inside the [plugin repository](https://github.com/php-http/plugins) to better understand the whole
+process.

docs/plugins/redirect.md

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+# Redirect Plugin
+
+TODO: explain the redirect plugin

docs/plugins/retry.md

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+# Retry Plugin
+
+TODO: explain the retry plugin

docs/plugins/stopwatch.md

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+# Stopwatch Plugin
+
+TODO: explain the stopwatch plugin