Magento 2: Setup/InstallSchema.php is Required. Sort of.

Although there’s no documentation stating this, based on system behavior it seems like the

Package/Module/Setup/InstallSchema.php

class files are sort of required by a Magento module.

I say sort of required because you can run a module without them. You can use that module to add features to Magento. However, a version number for that module will not be added to the setup_module table unless the InstallSchema class is present.

I discovered this when I tried adding an UpgradeSchema class to a module that didn’t have an InstallSchema. The UpgradeSchema still ran, but calling $context->getVersion()

public function upgrade(
    SchemaSetupInterface $setup, 
    ModuleContextInterface $context
)
{
    $setup->startSetup();

    $version = '[' . $context->getVersion() . ']';
    file_put_contents('/tmp/schema.log',$version . "n",FILE_APPEND);

    $setup->endSetup();
}

returned an empty string, as there were no entries for my module in the setup_module table.