I have noticed that from time to time my Magento installation shows 404 on all pages. Panic!

After doing some research I found out that running:
chown -R root:www-data . in the document folder solved the problem temporarily.

Until the next day and the problem is back.

I have since then been able to recreate the problem with that if I run:

php bin/magento setup:di:compile

it crashes and requires me to run the: chown -R root:www-data .
to fix the problem.

Why is this? Why do I always get these errors?

The error log shows problems with the cache folder:

`#0
/var/www/html/vendor/magento/zendframework1/library/Zend/Cache/Backend/File.php(180):
Zend_Cache::throwException('cache_dir "/var...')

1 /var/www/html/vendor/colinmollenhour/cache-backend-file/File.php(87):

Zend_Cache_Backend_File->setCacheDir('/var/www/html/v...')

2 /var/www/html/vendor/magento/zendframework1/library/Zend/Cache.php(153):

Cm_Cache_Backend_File->__construct(Array)

3 /var/www/html/vendor/magento/zendframework1/library/Zend/Cache.php(94):

Zend_Cache::_makeBackend('Cm_Cache_Backen...', Array, true, true)

4 /var/www/html/vendor/magento/framework/App/Cache/Frontend/Factory.php(156):

Zend_Cache::factory('MagentoFramewo...', 'Cm_Cache_Backen...',
Array, Array, true, true, true)

5 /var/www/html/vendor/magento/framework/Cache/Frontend/Adapter/Zend.php(38):

MagentoFrameworkAppCacheFronte" while reading response header from
upstream, client: 172.68.182.104, server: _, request: "GET /
HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host:
"www.kickacc.se" ^C`

Execute these commands as a root user

find var generated vendor pub/static pub/media app/etc -type f -exec chmod g+w +
find var generated vendor pub/static pub/media app/etc -type d -exec chmod g+ws +

Set the root ownership as sudo chown -R :<web server group> . after executing this.

Try once again and say if you are facing the issue. Please also share the var and generated ownership file permissions to analyze the cause of the issue.

So don't you have a Magento user other than the root user

There might be 2 case 

1. Cache issue

rm -rf var/cache/ var/generation/ var/page_cache/ var/view_preprocessed/ 
rm -rf pub/static/frontend/ pub/static/adminhtml/ pub/static/_requirejs/
php bin/magento setup:upgrade
php bin/magento setup:static-content:deploy

2. Permission Issue

find . -type f -exec chmod 644 ;
find . -type d -exec chmod 755 ;
find ./var -type d -exec chmod 777 ;
find ./pub/media -type d -exec chmod 777 ;
find ./pub/static -type d -exec chmod 777 ;
chmod 777 ./app/etc
chmod 644 ./app/etc/*.xml