Magento 2's module system is one of the most powerful extensibility frameworks in eCommerce. When you understand plugins, preferences, and service contracts, you can customize any Magento behavior without modifying core files. Here's a comprehensive guide to building modules the right way.
Module Structure
Every Magento module lives in app/code/Vendor/ModuleName/ or in a Composer package. The minimum required files: registration.php (registers the module with Magento), module.xml (declares the module name, version, and dependencies), and composer.json if distributing via Composer. Add additional directories as needed: Model/, Controller/, Block/, view/, etc/.
Plugins vs. Preferences
Plugins (interceptors) are the preferred customization mechanism — they hook before, after, or around a method without replacing the entire class. This lets multiple modules extend the same method without conflict. Preferences replace an entire class with your custom version — use them only when plugins can't achieve what you need, as they conflict with other modules trying to do the same.
Service Contracts
Service contracts are PHP interfaces that define the public API of a module. When you extend a module that uses service contracts, you implement against the interface rather than the concrete class — your code will work correctly even if the underlying implementation changes in future Magento versions. Always use service contracts when available; never call repository methods on concrete model classes.
Magento Marketplace Submission
Marketplace submission requires: coding standards compliance (PHPCS with Magento standards), unit test coverage >70%, security review (no SQL injection, XSS, or CSRF vulnerabilities), and a quality review by Adobe's team. The technical review typically takes 2-4 weeks. Prepare detailed installation and configuration documentation — this is a common rejection reason.
