What goes in the global di.xml and what goes into the area specific di.xml?

Question

I'm creating a Magento 2 module that adds some functionality to the backend. It does not touch the frontend at all.
But if I place some virtual type in the etc/adminhtml/di.xml (the admin di file) file I get an error that the class is not found.
If I add the same think in etc/di.xml (the global di file) everything works properly.
Other things like form modifiers work properly when added to the adminhtml di file.
From what I understood, the etc/di.xml file and etc/adminhtml/di.xml file should both be loaded on the backend pages.
Apparently this does not always work.

Here is the markup that doesn't work in the adminhtml di file.

<virtualType name="UmcConfigClassConfigSchemaLocator" type="Umc\Base\Model\Config\SchemaLocator\SchemaLocator">
    <arguments>
        <argument name="fileSchema" xsi:type="string">class</argument>
        <argument name="mergedSchema" xsi:type="string">class</argument>
    </arguments>
</virtualType>
<virtualType name="UmcConfigClassConfigMapperFactory" type="Umc\Base\Model\Config\Mapper\Factory">
    <arguments>
        <argument name="typeMap" xsi:type="array" />
    </arguments>
</virtualType>
<virtualType name="UmcConfigClassConfigConverter" type="Umc\Base\Model\Config\Converter\Converter">
    <arguments>
        <argument name="mapperFactory" xsi:type="object">UmcConfigClassConfigMapperFactory</argument>
        <argument name="mapperList" xsi:type="array" />
        <argument name="idNodes" xsi:type="array">
            <item name="0" xsi:type="string">class</item>
        </argument>
    </arguments>
</virtualType>
<virtualType name="UmcConfigClassConfigReader" type="Magento\Framework\Config\Reader\Filesystem">
    <arguments>
        <argument name="schemaLocator" xsi:type="object">UmcConfigClassConfigSchemaLocator</argument>
        <argument name="converter" xsi:type="object">UmcConfigClassConfigConverter</argument>
        <argument name="fileName" xsi:type="string">class.xml</argument>
        <argument name="idAttributes" xsi:type="array">
            <item name="/classes/class" xsi:type="string">id</item>
        </argument>
        <argument name="defaultScope" xsi:type="string">umc</argument>
    </arguments>
</virtualType>
<type name="Umc\Base\Model\Config\ClassConfig">
    <arguments>
        <argument name="reader" xsi:type="object">UmcConfigClassConfigReader</argument>
    </arguments>
</type>

Again, all the classes I have are used only in the backend.
I get this error:

RuntimeException: Source class "\UmcConfigClassConfig" for "\UmcConfigClassConfigMapper" generation does not exist.....

But this one works.

<virtualType name="UmcBaseUiDataProviderModuleFormModifierPool" type="Magento\Ui\DataProvider\Modifier\Pool">
    <arguments>
        <argument name="modifiers" xsi:type="array">
            <item name="system" xsi:type="array">
                <item name="class" xsi:type="string">Umc\Base\Ui\DataProvider\Module\Form\Modifier\System</item>
                <item name="sortOrder" xsi:type="number">1</item>
            </item>
        </argument>
    </arguments>
</virtualType>
<type name="Umc\Base\Ui\DataProvider\Module\Form\DataProvider">
    <arguments>
        <argument name="pool" xsi:type="object">UmcBaseUiDataProviderModuleFormModifierPool</argument>
    </arguments>
</type>

What should be placed in the general di.xml and what should be placed in the area specific di.xml?

Note: I've cleared the cache and the generation folder.

Answer 1

The common rule is:

• All modular DI settings (except for presentation layer configuration) SHOULD be put in <module_dir>/etc/di.xml
• All modular presentation layer DI settings SHOULD be put to <module_dir>/etc/<area_code>/di.xml

For you concrete case, also need to add how to configuration merged:

• the configuration in modules dimension is merged values

• the configuration in areas dimension is overwritten values of global scope

So, I don't catch what the concrete problem in your case, but it looks like problem with values overwriting