Getting Started

Overview

Mezzio allows you to write PSR-7 middleware applications for the web.

PSR-7 is a standard defining HTTP message interfaces; these are the incoming request and outgoing response for your application. By using PSR-7, we ensure that your applications will work in other PSR-7 contexts.

Middleware is any code sitting between a request and a response; it typically analyzes the request to aggregate incoming data, delegates it to another layer to process, and then creates and returns a response. Middleware can and should be relegated only to those tasks, and should be relatively easy to write and maintain.

Middleware is also designed for composability; you should be able to nest middleware and re-use middleware.

With Mezzio, you can build PSR-7-based middleware applications:

• APIs
• Websites
• Single Page Applications
• and more.

Features

Mezzio builds on laminas-stratigility to provide a robust convenience layer on which to build applications. The features it provides include:

• Routing

Stratigility provides limited, literal matching only. Mezzio allows you to utilize dynamic routing capabilities from a variety of routers, providing much more fine-grained matching capabilities. The routing layer also allows restricting matched routes to specific HTTP methods, and will return "405 Not Allowed" responses with an "Allow" HTTP header containing allowed HTTP methods for invalid requests.

Routing is abstracted in Mezzio, allowing the developer to choose the routing library that best fits the project needs. By default, we provide wrappers for Aura.Router, FastRoute, and the laminas-mvc router.

• container-interop

Mezzio encourages the use of Dependency Injection, and defines its Application class to compose a container-interop ContainerInterface instance. The container is used to lazy-load middleware, whether it is piped (Stratigility interface) or routed (Mezzio).

• Templating

While Mezzio does not assume templating is being used, it provides a templating abstraction. Developers can write middleware that typehints on this abstraction, and assume that the underlying adapter will provide layout support and namespaced template support.

• Error Handling

Applications should handle errors gracefully, but also handle them differently in development versus production. Mezzio provides both basic error handling via Stratigility's own FinalHandler implementation, as well as more advanced error handling via two specialized error handlers: a templated error handler for production, and a Whoops-based error handler for development.

Flow Overview

Below is a diagram detailing the workflow used by Mezzio.

The Application acts as an "onion"; in the diagram above, the top is the outermost layer of the onion, while the bottom is the innermost.

The Application dispatches each middleware. Each middleware accepts a request, a response, and the next middleware to dispatch. Internally, it's actually receiving the middleware stack itself, which knows which middleware to invoke next.

Any given middleware can return a response, at which point execution winds its way back out the onion. Additionally, any given middleware can indicate an error occurred, at which point it can call on the next error handling middleware. These act like regular middleware, but accept an additional error argument to act on.

Pipelines

The terminology "pipeline" is often used to describe the onion. One way of looking at the "onion" is as a queue, which is first-in-first-out (FIFO) in operation. This means that the first middleware on the queue is executed first, and this invokes the next, and so on (and hence the "next" terminology). When looked at from this perspective:

• In most cases, the entire queue will not be traversed.
• The innermost layer of the onion represents the last item in the queue.
• Responses are returned through the pipeline, in reverse order of traversal.

The Application allows arbitrary middleware to be injected, with each being executed in the order in which they are attached; returning a response from middleware prevents any middleware attached later from executing.

You can attach middleware manually, in which case the pipeline is executed in the order of attachment, or use configuration. When you use configuration, you will specify a priority integer to dictate the order in which middleware should be attached. Middleware specifying high integer prioritiess are attached (and thus executed) earlier, while those specifying lower and/or negative integers are attached later. The default priority is 1.

Mezzio provides a default implementation of "routing" and "dispatch" middleware, which you either attach to the middleware pipeline manually, or via configuration.

Routing within Mezzio consists of decomposing the request to match it to middleware that can handle that given request. This typically consists of a combination of matching the requested URI path along with allowed HTTP methods:

• map a GET request to the path /api/ping to the PingMiddleware
• map a POST request to the path /contact/process to the HandleContactMiddleware
• etc.

Dispatching is simply the act of calling the middleware mapped by routing. The two events are modeled as separate middleware to allow you to act on the results of routing before attempting to dispatch the mapped middleware; this can be useful for implementing route-based authentication or validation.

The majority of your application will consist of routing rules that map to routed middleware.

Middleware piped to the application earlier than routing should be middleware that you wish to execute for every request. These might include:

• bootstrapping
• parsing of request body parameters
• addition of debugging tools
• embedded Mezzio applications that you want to match at a given literal path
• etc.

Such middleware may decide that a request is invalid, and return a response; doing so means no further middleware will be executed! This is an important feature of middleware architectures, as it allows you to define application-specific workflows optimized for performance, security, etc.

Middleware piped to the application after the routing and dispatch middleware will execute in one of two conditions:

• routing failed
• routed middleware called on the next middleware instead of returning a response.

As such, the largest use case for such middleware is for error handling. One possibility is for providing custom 404 handling, or handling application-specific error conditions (such as authentication or authorization failures).

Another possibility is to provide post-processing on the response before returning it. However, this is typically better handled via middleware piped early, by capturing the response before returning it:

function ($request, $response, $next)
{
    $response = $next($request, $response);
    return $response->withHeader('X-Clacks-Overhead', 'GNU Terry Pratchett');
}

The main points to remember are:

• The application is a queue, and operates in FIFO order.
• Each middleware can choose whether to return a response, which will cause the queue to unwind, or to traverse to the next middleware.
• Most of the time, you will be defining routed middleware, and the routing rules that map to them.
• You get to control the workflow of your application by deciding the order in which middleware is queued.