Source configuration values fallback mechanism, by highest priority

• environment variables (Magento Cloud)
• app/etc/env.php
• app/etc/config.php
• core_config_data database table
• config.xml module files

Configuration values fallback mechanism, by highest priority

• Store view
• Website
• Default
• System

Config values are fully loaded from each sources then merged by priority

vendor/magento/framework/App/Config/ConfigSourceAggregated.php:get()
foreach ($this->sources as $sourceConfig) {
...
$data = array_replace_recursive(is_array($data) ? $data : [], $configData);
...
}

Let’s take an example with the number of product per page in a grid mode on category pages. Config path is catalog/frontend/grid_per_page_values

1- environment variables (Magento Cloud)

Theses variables are set via the project web interface.
Pattern name is: env:CONFIG__[STORE]__[XML_PATH]
In our case it will be: env:CONFIG__DEFAULT__CATALOG__FRONTEND__GRID_PER_PAGE_VALUES

Or in .magento.app.yaml
variables:
    env:
       CONFIG__DEFAULT__PAYPAL_ONBOARDING__MIDDLEMAN_DOMAIN: ‘payment-broker.magento.com’
       CONFIG__STORES__DEFAULT__PAYPAL__NOTATION_CODE: ‘Magento_Enterprise_Cloud’

Magento DevDoc

2- env.php & config.php files

Driven by Magento\Framework\App\Config\InitialConfigSource

Values can be generated with app:config:dump system cli or manually added in files
Magento DevDoc

Example:

# app/etc/config.php
<?php
return [
    'modules' => [
        'Magento_Store' => 1,
        'Magento_AdminNotification' => 1,
        ...
        'Vertex_Tax' => 0,
    ],
    'system' => [
        'default' => [
            'catalog' => [
                'frontend' => [
                    'grid_per_page_values' => '24,48,96'
                ],
            ],
        ],
    ],
];

These configuration values will be locked, you wont be able to edit them from admin panel.
If you want to update a value in these files, you’ll have to:

• edit the file
• or remove it from the file to unlock it

3- core_config_data table

Driven by Magento\Config\App\Config\Source\RuntimeConfigSource

If config value was not found in previous step, then a look is made inside core_config_data table.
Config values are populated in this table when saving config form (driven by system.xml files) from admin panel in Stores > Configuration sections.

4- config.xml files

Driven by Magento\Config\App\Config\Source\ModularConfigSource

Finally if config was not found in core_config_data neither, configuration value is loaded from config.xml module file.

# vendor/magento/module-catalog/etc/config.xml
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
    <default>
        <catalog>
            <frontend>
                <grid_per_page_values>9,15,30</grid_per_page_values>
            </frontend>
        </catalog>
    </default>
</config>

Good to know

Config importer pool

There is a config importer pool: Magento\Deploy\Model\DeploymentConfig\ImporterPool

There are 3 importers by default:

$importers = $this->configImporterPool->getImporters();
array(3) {
  ["scopes"] => string(35) "Magento\Store\Model\Config\Importer"
  ["themes"] => string(35) "Magento\Theme\Model\Config\Importer"
  ["system"] => string(36) "Magento\Config\Model\Config\Importer"
}

invokeSave

At the end of setup:upgrade process, app:config:import will be executed.
The ChangeDetector will compare config values from env.php & config.php with those of the database.

$changedData = array_replace_recursive(
    $this->arrayUtils->recursiveDiff($savedFlag, $data),
    $this->arrayUtils->recursiveDiff($data, $savedFlag)
);

If there are differences then the configurations that need to be updated from the files will be imported into the flag table in the system_config_snapshot row. A new hash will then be generated based on these new values in the config_hash row.

What is the purpose of values stored in system_config_snapshot row?
These values are not taken into account by Magento when building storefront configurations. They are only used by the ChangeDetector which will simulate a save of configuration values (only if config value has been updated) to run before and after save methods.

vendor/magento/module-config/Model/Config/Importer/SaveProcessor.php::invokeSave

foreach ($scopeData as $path) {
    $value = $this->scopeConfig->getValue($path, $scope, $scopeCode);
    $backendModel = $this->valueFactory->create($path, $value, $scope, $scopeCode);

    if ($backendModel instanceof Value) {
        $backendModel->beforeSave();
        $backendModel->afterSave();
    }
}

Convert php array config to xml path config

vendor/magento/framework/Stdlib/ArrayUtils.php:flatten