magento-module

Magento 2 / Mage-OS Module Development

Expertise for writing Magento 2 modules that compile, pass phpcs --standard=Magento2, and follow current (2.4.x / Mage-OS) conventions. Magento has accumulated a decade of outdated tutorials; this skill exists because the obvious approach found in old blog posts is often wrong today. When in doubt, prefer the conventions here over patterns seen in older code.

Non-negotiable conventions (why they matter)

These are the mistakes that get extensions rejected from the marketplace and break upgrades:

• Never use ObjectManager::getInstance() in your own code. Constructor injection only. ObjectManager hides dependencies, breaks compilation analysis, and fails code review. (Exceptions: factories/proxies generated by Magento may use it internally — that's fine.)
• Declarative schema (db_schema.xml), never InstallSchema/UpgradeSchema scripts. Install scripts have been deprecated since 2.3 and make schema state unauditable.
• Plugins over preferences. A preference (class rewrite) conflicts with every other module that rewrites the same class. A plugin composes. See the decision tree before choosing any extension mechanism.
• View models, not block classes, for template logic. Custom blocks are legacy; a view model is a plain class injected into a template via layout XML.
• Escape all template output with $escaper->escapeHtml() / escapeHtmlAttr() / escapeUrl(). Unescaped echo in .phtml is an XSS finding.
• Service contracts first: depend on Api/ interfaces (e.g. ProductRepositoryInterface), not concrete Model classes, when consuming other modules.
• Area-scope your di.xml: global etc/di.xml vs etc/frontend/di.xml vs etc/adminhtml/di.xml. A frontend-only plugin registered globally slows down everything.

Workflow

1. Identify the task type and read the matching reference before writing code:

   Task	Read first
   Change/intercept core behavior	references/extension-mechanisms.md
   New module from scratch	this file + run scripts/scaffold.py
   Database tables / columns	references/declarative-schema.md
   DI wiring, virtual types, factories, proxies	references/di-patterns.md
   Admin settings, grids, menus, ACL	references/admin-ui.md
   Frontend pages, blocks, templates, layout	references/frontend.md
   REST / GraphQL / web APIs	references/api.md
   CLI commands, cron jobs, message queues	references/cli-cron.md
   Writing unit / integration tests (PHPUnit)	references/testing.md
   Errors, "not working", compile failures	references/debugging.md

2. For a new module, scaffold the boilerplate with the script — it is deterministic and avoids typos in XML namespaces that cost a compile cycle to discover:

   python scripts/scaffold.py Vendor_Module --path app/code [--description "..."] \
       [--sequence Magento_Catalog,Magento_Checkout]
   

   This emits registration.php, etc/module.xml, and composer.json. Everything else (di.xml, plugins, schema, layout) you write by hand following the references — those parts need judgment, the boilerplate doesn't.

3. Implement the business logic. Keep each class small; one responsibility per plugin or observer. Name plugins descriptively (<type name="..."><plugin name="acme_add_gift_label" .../></type> — the name is global, so prefix with the vendor).

4. Verify before declaring done. From the Magento root:

   bin/magento module:enable Vendor_Module
   bin/magento setup:upgrade            # registers module, applies db_schema
   bin/magento setup:di:compile         # catches DI mistakes; must pass
   vendor/bin/phpcs --standard=Magento2 app/code/Vendor/Module   # if installed
   bin/magento cache:flush
   

   If setup:di:compile fails, go to the debugging reference — the error messages are cryptic but mechanical to resolve. Do not hand unverified code back to the user when a Magento installation is available to compile against.

Decision shortcuts

• "Override what a core method returns/receives" → plugin (after/before).
• "React to something happening (order placed, product saved)" → observer, or a plugin on the service contract if you need to alter the result.
• "Replace an entire class implementation" → almost never; re-read references/extension-mechanisms.md — there is usually a plugin- or di-argument-based alternative that composes better.
• "Add a column to a core table" → don't; use an extension attribute or a satellite table (references/declarative-schema.md).
• "Template needs data" → view model (references/frontend.md).
• "Expose data to REST/GraphQL/headless" → service contract (Api/ interface) first, then webapi.xml or schema.graphqls (references/api.md) — never expose a Model.
• "Run code from CLI / on a schedule / async" → console command, cron job, or message queue (references/cli-cron.md); keep the entry class thin, work in a service.

Final checklist

Before finishing any task, run through references/checklists.md — it covers cache tags, ACL coverage, i18n (__() + i18n/en_US.csv), escaping, and the composer/module.xml consistency checks that reviewers look for.

Mage-OS notes

Mage-OS is a community fork, drop-in compatible with Magento 2.4.x. Code targeting Magento 2.4 works unchanged. In composer.json, depend on magento/framework version ranges (the Mage-OS packages provide/replace them) rather than pinning magento/product-community-edition.

Pairing with live data

If the elgentos magento2-dev-mcp MCP server is connected, prefer it for reading merged configuration (effective di.xml, layout) instead of reasoning from single files — Magento merges XML across modules and the single-file view misleads.