221 lines
7.9 KiB
Markdown
221 lines
7.9 KiB
Markdown
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[<< previous](12-refactoring.md) | [next >>](14-invoker.md)
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### Middleware
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In the last chapter we wrote our RouterClass to implement the middleware interface, and in this chapter I want to explain
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a bit more about what this interface does and why it is used in many applications.
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The Middlewares are basically a number of wrappers that stand between the client and your application. Each request gets
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passed through all the middlewares, gets handled by our controllers and then the response gets passed back through all
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the middlewars to the client/emitter.
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So every Middleware can modify the request before it goes on to the next middleware (and finally the handler) and the
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response after it gets created by our handlers.
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So lets take a look at the middleware and the requesthandler interfaces
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```php
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interface MiddlewareInterface
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{
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public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface;
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}
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interface RequestHandlerInterface
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{
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public function handle(ServerRequestInterface $request): ResponseInterface;
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}
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```
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The RequestHandlerInterface gets only a request and returns a response, the MiddlewareInterface gets a request and a
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requesthandler and returns a response. So the logical thing for the Middleware is to use the handler to produce the
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response.
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But the middleware could just ignore the handler and produce a response on its own as the interface just requires us
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to produce a response.
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A simple example for that would be a caching middleware. The basic idea is that we want to cache all request from users
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that are not logged in. This way we can save a lot of processing power in rendering the html and fetching data from the
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database.
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In this scenario we assume that we have an authentication middleware that checks if a user is logged in and decorates
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the request with an 'isAuthenticated' attribute.
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If the 'isAuthenticated' attribute is set to false, we check if we have a cached response and return that, if that
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response is not already cached, than we let the handler create the response and store that in the cache for a few
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seconds
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```php
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interface CacheInterface
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{
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public function get(string $key, callable $resolver, int $ttl): mixed;
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}
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```
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The first parameter is the identifier for the cache, the second is a callable that produces the value and the last one
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defines the seconds that the cache should keep the item. If the cache doesnt have an item with the given key then it uses
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the callable to produce the value and stores it for the time specified in ttl.
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so lets write our caching middleware:
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```php
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final class CachingMiddleware implements MiddlewareInterface
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{
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public function __construct(private CacheInterface $cache){}
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public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface
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{
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if ($request->getAttribute('isAuthenticated', false)) {
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$key = $request->getUri()->getPath();
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return $this->cache->get($key, fn() => $handler->handle($request), 10);
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}
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return $handler->handle($request);
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}
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}
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```
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we can also modify the response after it has been created by our application, for example we could implement a gzip
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middleware, or for more simple and silly example a middleware that adds a Dank Meme header to all our response so that the browser
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know that our application is used to serve dank memes:
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```php
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final class DankMemeMiddleware implements MiddlewareInterface
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{
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public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface
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{
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$response = $handler->handle($request);
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return $response->withAddedHeader('Meme', 'Dank');
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}
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}
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```
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but for our application we are going to just add two external middlewares:
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* [Trailing-slash](https://github.com/middlewares/trailing-slash) to remove the trailing slash from all routes.
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* [whoops middleware](https://github.com/middlewares/whoops) to wrap our error handler into a nice middleware
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```bash
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composer require middlewares/trailing-slash
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composer require middlewares/whoops
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```
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The whoops middleware should be the first middleware to be executed so that we catch any errors that are thrown in the
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application as well as the middleware stack.
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Our desired request -> response flow looks something like this:
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Client
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v |
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Kernel
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| ^
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v |
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Whoops Middleware
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| ^
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v |
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TrailingSlash
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| ^
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v |
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Routing
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| ^
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v |
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ContainerResolver
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| ^
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v |
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Controller/Action
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As every middleware expects a RequestHandlerInterface as its second argument we need some extra code that wraps every
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middleware as a RequestHandler and chains them together with the ContainerRouteDecoratedResolver as the last Handler.
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```php
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interface Pipeline
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{
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public function dispatch(ServerRequestInterface $request): ResponseInterface;
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}
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```
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And our implementation looks something like this:
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```php
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final class SimplePipeline implements Pipeline
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{
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/**
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* @param MiddlewareInterface[] $middlewares
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* @param RequestHandlerInterface $tip
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*/
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public function __construct(
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private readonly array $middlewares,
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private RequestHandlerInterface $tip
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){}
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public function dispatch(ServerRequestInterface $request): ResponseInterface
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{
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$this->buildStack();
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return $this->tip->handle($request);
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}
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private function buildStack(): void
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{
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foreach (array_reverse($this->middlewares) as $middleware) {
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$next = $this->tip;
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$this->tip = new class ($middleware, $next) implements RequestHandlerInterface {
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public function __construct(
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private readonly MiddlewareInterface $middleware,
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private readonly RequestHandlerInterface $next
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){}
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public function handle(ServerRequestInterface $request): ResponseInterface
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{
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return $this->middleware->process($request, $this->next);
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}
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};
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}
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}
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}
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```
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Here we define our constructor to require two arguments: an array of middlewares and a requesthandler as the final code
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that should produce our response.
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In the buildStack() method we wrap every middleware as a RequestHandler with the current tip property as the $next argument
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and store that itself as the current tip.
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There are of course a lot of more sophisticated ways to build a pipeline/dispatcher that you can check out at the [middlewares github](https://github.com/middlewares/awesome-psr15-middlewares#dispatcher)
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Lets add a simple factory to our dependencies.php file that creates our middlewarepipeline:
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```php
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Pipeline::class => function (
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RouteDecoratedRequestHandler $tip,
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RouteDecorationMiddleware $router,
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) {
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$middlewares = require __DIR__ . '/middlewares.php';
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$middlewares[] = $router;
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return new SimplePipeline($middlewares, $tip);
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},
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```
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And of course a new file called middlewares.php in our config folder:
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```php
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<?php
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declare(strict_types=1);
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return [
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new \Middlewares\Whoops(),
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new \Middlewares\TrailingSlash(),
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];
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```
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And we need to add the pipeline to our Kernel class. I will leave that as an exercise to you, a simple hint that i can
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give you is that the handle()-method of the Kernel should look like this:
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```php
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public function handle(ServerRequestInterface $request): ResponseInterface
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{
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return $this->pipeline->dispatch($request);
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}
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```
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Lets try if you can make the kernel work with our created Pipeline implementation. For the future we could improve our
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pipeline a little bit, so that it can accept a class-string of a middleware and resolves that with the help of a
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dependency container, if you want you can do that as well.
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[<< previous](12-refactoring.md) | [next >>](14-invoker.md)
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