Position of Content-Length in middleware stack

1

I wanted to add my remarks to an old thread I just saw (Slim 4 Content Length Middleware position) but I didn’t dare to resurrect a six month topic.

At Content Length Middleware we can read:

This middleware should be placed on the center of the middleware stack so it gets executed last.

Er… The center of a stack?

I’ve always found this extremely confusing and I eventually added this on top of my index.php:

/*
 * ╭── $app->run() ──╮
 * │                 │
 * │                ╲│╱
 */

This is to remind me that middleware, to my eyes, runs twice and in inverse order: one time to process the request (bottom to top) and a second one to process the response (top to bottom). Yes, I know it’s only invoked once but that’s how it feels to my intuition:

public function exampleMiddleware(Request $request, RequestHandler $handler): Response
{
    // Optionally do stuff with $request (first run)
    $response = $handler->handle($request);
    // Optionally do stuff with $response (second run)
    return $response;
}

I’ve made my ContentLengthMiddleware() the last being add()ed so it’s the outer layer of the onion and the last one to process the response. That fixed some rare intermittent bugs where I couldn’t use the output due to incorrect Content-Length upon middleware making changes to response body.

I’ve no idea if this is what docs mean but I understand it doesn’t make sense to calculate response body length until everybody else is finished fiddling with it.

Is my overall reasoning unusual, misguided or wrong in some way?

2

it doesn’t make sense to calculate response body length until everybody else is finished fiddling with it.

I think this is correct.