Magento2

The best approche is to add these files  "checkout_cart_index.xml" and "checkout_index_index.xml" in this directory 

app/design/frontend/{Vendor}/{theme}/Magento_Checkout/layout/

with this content 

checkout_cart_index.xml

- https://www.magezon.com/magezon-page-builder-for-magento-2.html : Build CMS Page, CMS Block, Product Description and Category Description

- https://www.magezon.com/magento-2-blog-extension.html : Empower Your Magento Blog and Grab More Audiences

How many times you tried to install Magento 2 and later you could not see the website working !!, How many times you tried to search for these sentences 

"Magento 2 css not working"

"Magento 2 js not working"

"Magento 2 404 page not found" because of the static files?

Now here is the solution, and easily it is the missed .htaccess (for apache)

by quick look at this path "{magento root}/pub/static", you will see that you missed the .htaccess file

Go to the Nginx  configuration then you need to edit the line similar to this line 

location ~ (index|get|static|report|404|503)\.php$ {

to be like this 

location ~ (index|get|static|report|404|503|phpinfo)\.php$ {

then restart Nginx 

do you have this error always after each upgrade and you are not able to figure out which value cause this issue?

the best way is to debug the issue and find which one by going to this file 

\vendor\magento\framework\Serialize\Serializer\Json.php

go to this function 

You can use the Magento 2 CLI to do it, you can check your current search engine using:

php -f bin/magento config:show catalog/search/engine

if you want to change the engine to Elasticseach version 6 then use this command 

php -f bin/magento config:set catalog/search/engine 'elasticsearch6'

In order to redirect a customer from observer you need to use another way than the Controller 

protected $_response;

protected $_objectManager;

public function __construct(
    \Magento\Framework\App\ResponseInterface $response,
    \Magento\Framework\ObjectManagerInterface $objectManager,

    ) {
        $this->_response = $response;
       $this->_objectManager = $objectManager;

   }

then in your function you can use this 

According to Varnish documentation, “A purge is what happens when you pick out an object from the cache and discard it along with its variants.” A Varnish purge is very similar to a Magento cache clean command (or clicking Flush Magento Cache in the Magento Admin).

In fact, as discussed in this section, when you clean, flush, or refresh the Magento cache, Varnish purges as well.

Configure Magento to purge Varnish

execute this code 

bin/magento setup:config:set --http-cache-hosts=192.0.2.100,192.0.2.155:6081

If you want to see the total loaded time for any page to compare the time between the first and second time you can use this command 

curl -L --output /dev/null --silent --show-error --write-out 'lookup:        %{time_namelookup}\nconnect:    %{time_connect}\nappconnect:    %{time_appconnect}\npretransfer:   %{time_pretransfer}\nredirect:      %{time_redirect}\nstarttransfer: %{time_starttransfer}\ntotal:      %{time_total}\n' 'https://www.example.com/'

you need to increase the memory_limit in php.ini

and as a quick solution you can use these commands directly in the command line

php -dmemory_limit=5G bin/magento setup:di:compile

and 

php -dmemory_limit=5G bin/magento setup:static-content:deploy

another code for the composer update

php -dmemory_limit=5G composer.phar update

php -d memory_limit=-1 /usr/local/bin/composer update