Magento2

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
 

We can disable Disable Smile-SA / Elasticsuite in Magento by executing this command 

php bin/magento modeul:disable Smile_ElasticsuiteCore Smile_ElasticsuiteCatalog Smile_ElasticsuiteCatalogGraphQl Smile_ElasticsuiteCatalogRule Smile_ElasticsuiteCatalogOptimizer Smile_ElasticsuiteTracker Smile_ElasticsuiteThesaurus Smile_ElasticsuiteSwatches Smile_ElasticsuiteIndices Smile_ElasticsuiteVirtualCategory

to install the needed plugins you need to execute this command 

To change the time please copy this file 

vendor\magento\module-ui\view\frontend\web\js\view\messages.js

to your them to be in this path

app\design\frontend\{vendor}\{theme}\Magento_Ui\web\js\view\messages.js

then modify this function 

You can add this to your code in order to reindex in UTC -5

0 5 * * * cd /home/{your-full-path}/public_html/ && php -f bin/magento indexer:reindex

Create this file "default.xml" in your theme directory in this folder "{vendor}/{theme}Magento_Search/layout"

add this code in the file