Middleware Types

Expressive allows you to compose applications out of pipeline and routed middleware.

Pipeline middleware is middleware that defines the workflow of your application. These generally run on every execution of the application, and include such aspects as:

  • Error handling
  • Locale detection
  • Session setup
  • Authentication and authorization

Routed middleware is middleware that responds only to specific URI paths and HTTP methods. As an example, you might want middleware that only responds to HTTP POST requests to the path /users.

Expressive allows you to define middleware using any of the following:

  • http-interop/http-middleware instances.
  • Callable middleware that implements the http-interop/http-middleware signature.
  • Callable "double-pass" middleware (as used in Expressive 1.X, and supported in Expressive 2.X).
  • Service names resolving to one of the above middleware types.
  • Middleware pipelines expressed as arrays of the above middleware types.

http-interop/http-middleware

The http-interop/http-middleware project is the basis for the proposed PSR-15 specification, which covers HTTP Server Middleware that consumes PSR-7 HTTP messages. The project defines two interfaces, Interop\Http\ServerMiddleware\MiddlewareInterface and Interop\Http\ServerMiddleware\DelegateInterface. Expressive accepts middleware that implements the MiddlewareInterface. As an example:

use Interop\Http\ServerMiddleware\DelegateInterface;
use Interop\Http\ServerMiddleware\MiddlewareInterface;
use Psr\Http\Message\ServerRequestInterface;

class SomeMiddleware implements MiddlewareInterface
{
    public function process(ServerRequestInterface $request, DelegateInterface $delegate)
    {
        // do something and return a response, or
        // delegate to another handler capable of
        // returning a response via:
        //
        // return $delegate->process($request);
    }
}

If you are using PHP 7 or above, you could also implement such middleware via an anonymous class.

Callable http-middleware

Sometimes you may not want to create a class for one-off middleware. As such, Expressive allows you to provide a PHP callable that uses the same signature as Interop\Http\ServerMiddleware\MiddlewareInterface:

use Interop\Http\ServerMiddleware\DelegateInterface;
use Interop\Http\ServerMiddleware\MiddlewareInterface;
use Psr\Http\Message\ServerRequestInterface;

function (ServerRequestInterface $request, DelegateInterface $delegate)
{
    // do something and return a response, or
    // delegate to another handler capable of
    // returning a response via:
    //
    // return $delegate->process($request);
}

One note: the $request argument does not require a typehint, and examples throughout the manual will omit the typehint when demonstrating callable middleware.

Double-pass middleware

Expressive 1.X was based on Stratigility 1.X, which allowed middleware with the following signature:

<?php
use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;

function(
    ServerRequestInterface $request,
    ResponseInterface $response,
    callable $next
) {
    // Process the request and return a response,
    // or delegate to another process to handle
    // the request via:
    //
    // return $next($request, $response);
}

This middleware is called "double-pass" due to the fact that it requires both the request and response arguments.

In such middleware, no typehints are required, but they are encouraged. Additionally, we encourage users to never use the provided $response argument, but instead create a concrete response to return, or manipulate the response returned by $next; this prevents a number of potential error conditions that may otherwise occur due to incomplete or mutated response state.

This middleware is still supported in Expressive 2.X, but we encourage users to adopt http-interop/http-middleware signatures, as we will be deprecating double-pass middleware eventually.

Service-based middleware

We encourage the use of a dependency injection container for providing your middleware. As such, Expressive also allows you to use service names for both pipeline and routed middleware. Generally, service names will be the specific middleware class names, but can be any valid string that resolves to a service.

When Expressive is provided a service name for middleware, it internally decorates the middleware in a Zend\Expressive\Middleware\LazyLoadingMiddleware instance, allowing it to be loaded only when dispatched.

Middleware pipelines

Expressive allows any pipeline or routed middleware to be self-contained middleware pipelines. To prevent the need for instantiating a Zend\Stratigility\MiddlewarePipe or Zend\Expressive\Application instance when defining the pipeline, Expressive allows you to provide an array of middleware:

// Pipeline middleware:
$app->pipe([
    FirstMiddleware::class,
    SecondMiddleware::class,
]);

// Routed middleware:
$app->get([
    FirstMiddleware::class,
    SecondMiddleware::class,
]);

The values in these arrays may be any valid middleware type as defined in this chapter.

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