Keep hashed directories consistent in a load balanced server environment with Craft CMS 3

load-balancer-app.php

<?php
/**
 * Yii Application Config
 *
 * Edit this file at your own risk!
 *
 * The array returned by this file will get merged with
 * vendor/craftcms/cms/src/config/app/main.php and [web|console].php, when
 * Craft's bootstrap script is defining the configuration for the entire
 * application.
 *
 * You can define custom modules and system components, and even override the
 * built-in system components.
 */

/**
 * AdminCP resources in Craft CMS 3 are published in the public @webroot/cpresources 
 * with a directory name that's a hashed from the directory path and the timestamp
 * of when the resource was published.
 *
 * On load balanced environments, the addition of a timestamp can cause 404s as
 * the timestamp will vary from server to server. You can work around this problem
 * by providing your own `hashCallback` function to generate a hash of the directory
 * path without a timestamp.
 *
 * The `hashCallback` can actually be any PHP callable, but here it's presented
 * as an anonymous function. c.f.:
 *
 * vendor/craftcms/cms/src/config/app.web.php
 * vendor/craftcms/cms/src/web/AssetManager.php
 *
 * @author    nystudio107
 * @package   n/a
 * @since     1.0.0
 */
return [
    // All environments
    '*' => [
    ],

    // Live (production) environment
    'live'  => [
        'components' => [
            'assetManager' => function() {
                $generalConfig = Craft::$app->getConfig()->getGeneral();
                $config = [
                    'class' => craft\web\AssetManager::class,
                    'basePath' => $generalConfig->resourceBasePath,
                    'baseUrl' => $generalConfig->resourceBaseUrl,
                    'fileMode' => $generalConfig->defaultFileMode,
                    'dirMode' => $generalConfig->defaultDirMode,
                    'appendTimestamp' => true,
                    'hashCallback' => function ($path) {
                        return hash('md4', $path);
                    }
                ];
                return Craft::createObject($config);
            },
        ],
    ],

    // Staging (pre-production) environment
    'staging'  => [
    ],

    // Local (development) environment
    'local'  => [
    ],
];

@khalwat

With a little less code:

 // Yes, these bootstrappers can be Callables and 
 // don't require to implement BootstrapInterface

 'bootstrap' => [function () {
       \yii\base\Event::on(\craft\web\Controller::class, \craft\web\Controller::EVENT_BEFORE_ACTION, function () {
            \Craft::$app->assetManager->hashCallback = function ($path) {
                return hash('md4', $path);
            };
        });
  }],

This way we touch just a single property ($hashCallback ), instead of repeating the whole config for AssetManager.

It must happen after Application::ensureResourcePathExists() is called, that's why I wrapped it in an EVENT_INIT handler.
Controller::EVENT_BEFORE_ACTION handler

However, both approaches do not fully mitigate the Problem. The paths are consistent, but:

If the first request (e.g. GET /admin) is handled by Webserver1, all /cpresources/* get generated there.
But if the next request goes to a static resource (e.g. GET /cpresources/f3d68a1b74ef1a6133bc59d0e4d8b87c/js/Craft.min.js?v=1520600) on Webserver2, the file does not exist until they are generated by a dynamic request.

Something like this might work as a command in the build process:

foreach(array_keys($classLoader->getClassMap()) as $class) {
 	if ($class instanceof AssetBundle)) {
 	  \Craft::$app->getView()->registerAssetBundle($class);
 	}
 } 

this way we don't need to set $hashCallback - I'll try if it works.

Right @ostark, the supposition is that we have the /cpresources/ in a shared NFS (or whatever) volume

@ostark you ever get anywhere with this? I'm trying to avoid having a shared volume (performance sucks) in a LB setup.

Posted issue here: craftcms/cms#2712