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rouble shooting is an essential part of any software system. As a Drupal developer it is important to have knowledge about the most important tips about troubleshooting.

Install/Uninstall module error

When you install/uninstall a module there is usually an error if the module code removed but its entry exist in DB. Some time both exists but still this error. The full error is : 

the following module is marked as installed in the core.extension configuration, but it is missing:


Using PMA run the following SQL to fix it.

delete from `key_value` where collection = 'system.schema' and name = 'image_downloader';


The context is not valid

This error occurs in ctools.

Converting back to version 3.9.0 solved it.

composer require 'drupal/ctools:3.9.0'


Solved: Drupal - Exception: Serialization of Symfony is not allowed in serialize line 14

During my earlier days with Drupal module development, the instructor gave these instructions  with a smiley face that more than 90% of drupal issue are related to Drupal cache.

Since then I was paying special attention to it while module development but today I spent a couple of hours & googling around, looking at different logs & debugging & turning stones here and there, beating around the bushes to solve the following problem:

Exception: Serialization of 'Symfony\Component\HttpFoundation\File\UploadedFile' is not allowed in serialize() (line 14 of /opt/web/core/lib/Drupal/Component/Serialization/PhpSerialize.php).

And a million dollar solution is to add the following in the build method.

 // Don't cache


The module mysql is not installed

This error occurs while enabling a module in extend.

The solution is :


docker exec -ti container_name composer update
docker exec -ti container_name chmod 777 /opt -Rf
docker exec -ti container_name /opt/vendor/drush/drush/drush cr


Composer update memory limit

While updating composer packages (specially using docker) you may encounter memory limit reached issue. The solution is to simply run the composer update command on localhost (without docker container), and may be install (composer.json packages) it in docker then.

Twig chainloader twig is not defined

Enable components module

Workflow new status not working

Check for user permissions

Composer - file_get_contents(): read of 8192 bytes failed with errno=21 Is a directory

The composer.lock is a directory, remove it an run composer install again.