Technical guidelines

About this document


This document lists the fundamental coding and application design principles that guide Magento 2 developer team members.

Magento core developers use this document as a reference during code reviews; some rules have corresponding code checks in the Magento static tests.

These guidelines came from many years of hard work, experience, and discussions. We strongly believe that new technical initiatives should follow these recommendations, and the existing code should be improved to meet them.

Text conventions

Use RFC2119 to interpret keywords like:






  • MAY


Guidelines for Magento 2.2 only

Statements applicable to Magento v.2.2 only are marked with [2.2], like this:

Explicit return types MUST BE used on functions [2.2].

1. Basic programming principles

1.1. Function arguments SHOULD NOT be modified.

1.2. Explicit return types MUST BE declared on functions [2.2].

1.3. Type hints for scalar arguments SHOULD be used [2.2].

2. Class design

2.1. Object decomposition MUST follow the SOLID principles.

2.2. Object MUST be ready for use after instantiation. No additional public initialization methods are allowed.

Not recommended Recommended
class Config
    private $data;

    public function init() // or load()
        $this->data = $this->fileReader->load('cache.xml');

    public function getValue($key)
        return $this->data[$key];
class Config
    private $data;

    public function getValue($key)
        if ($this->data === null) {
            $this->data = $this->fileReader->load('cache.xml');

2.3. Class constructor can have only dependency assignment operations and/or argument validation operations. No other operations are allowed.

2.3.1. Constructor SHOULD throw an exception when validation of an argument has failed.

class Composite
     * @var RendererInterface[]
    private $renderers;

     * @param RendererInterface[] $renderers
     * @throws InvalidArgumentException
    public function __construct(array $renderers)
        foreach ($renderers as $renderer) {
            if (!$renderer instanceof RendererInterface) {
                throw new InvalidArgumentException(
                    sprintf('Instance of the phrase renderer is expected, got %s instead.', get_class($renderer))
        $this->renderers = $renderers;

2.3.2. Events MUST NOT be triggered in constructors.

Not recommended Recommended
class Config
    private $data;

    public function __construct($fileReader, $eventManager)
        $this->data = $fileReader->read('cache.xml');
class Config
    private $fileReader;

    private $eventManager;

    public function __construct($fileReader, $eventManager)
        $this->eventManager = $eventManager;
        $this->fileReader = $fileReader;

    public function getData($key)
        if ($this->data === null) {
            $this->data = $this->fileReader->read('cache.xml');
        return $this->data[$key];

2.4. All dependencies MUST be requested by the most generic type that is required by the client object.

2.5. Proxies and interceptors MUST NEVER be explicitly requested in constructors.

2.6. Inheritance SHOULD NOT be used. Composition SHOULD be used for code reuse.

2.7. All non-public properties and methods SHOULD be private.

2.8. Abstract classes MUST NOT be marked as public @api.

2.9. Service classes (ones that provide behavior but not data, like EventManager) SHOULD NOT have a mutable state.

2.10. Only data objects or entities (Product, Category, etc.) MAY have any observable state.

2.11. “Setters” SHOULD NOT be used. They are only allowed in Data Transfer Objects.

2.12. “Getters” SHOULD NOT change the state of an object.

2.13. Static methods SHOULD NOT be used.

2.14. Temporal coupling MUST be avoided.

2.15. Method chaining in class design MUST be avoided.

2.16. Law of Demeter SHOULD be obeyed.

2.17. Patterns:

2.17.1. Proxies SHOULD be used for lazy-loading optional dependencies.

2.17.2. Composites SHOULD be used when there is a need to work with a tree as a single object.

For example:

You need to read configuration from different sources (like database or filesystem) and want to make the reading process configurable: allow extensions to add more configuration sources. In this case, you can create a ConfigReaderInterface with a composite implementation - ConfigReaderComposite, and configure particular readers as children of a composite reader.

2.17.3. Strategy SHOULD be used when there are multiple algorithms for performing an operation.

3. Dependency injection

3.1. There SHOULD be no circular dependencies between objects.

3.2. The app/etc/di.xml file MUST contain only framework-level Dependency Injection (DI) settings.

3.3. All modular DI settings (except for Presentation layer configuration) SHOULD be stored in <module_dir>/etc/di.xml.

3.4. All modular Presentation layer DI settings SHOULD be stored in <module_dir>/etc/<area_code>/di.xml.

4. Interception

4.1. Around-plugins SHOULD only be used when behavior of an original method is supposed to be substituted in certain scenarios.

4.2. Plugins SHOULD NOT be used within own module.

4.3. Plugins SHOULD NOT be added to data objects.

4.4. Plugins MUST be stateless.

5. Exceptions

5.1. All exceptions that are surfaced to the end user MUST produce error messages in the following format:

  • Symptom

  • Details

  • Solution or workaround

5.2. Exceptions MUST NOT be handled in the same function where they are thrown.

5.3. If a function A calls function B, and function B might throw an exception, this exception MUST be either processed by function A or declared by the @throws annotation in the documentation block of function A.

5.4. Exceptions MUST NOT handle message output. It is the processing code that decides how to process an exception.

5.5. Business logic (both application and domain) MUST NOT be managed with exceptions. Conditional statements SHOULD be used instead.

5.6. The short name of an exception class MUST be clear, meaningful, and state the cause of exception.

5.7. Thrown exceptions SHOULD be as specific as possible. The top generic \Exception SHOULD NOT be thrown anywhere.

5.8. All direct communications with third-party libraries MUST be wrapped with a try/catch statement.

5.9. \Exception SHOULD be caught only in the code that calls third-party libraries, in addition to catching specific exceptions thrown by the library.

5.10. \Exception SHOULD NOT be thrown in Front Controller and Action Controllers.

5.11. A separate exceptions hierarchy SHOULD be defined on each application layer. It is allowed to throw exceptions that are only defined on the same layer.

5.12. If an exception is caught on the application layer that differs from the one where it has been thrown, and it SHOULD be re-thrown, you SHOULD create a new exception instance that is appropriate for the current layer. In this case, the original exception must be passed to a new instance with the “previous” argument.

5.13. It is not allowed to absorb exceptions with no logging or/and any workaround operation executed.

5.14. Any exception SHOULD be logged only in the catch block where it is processed, and SHOULD NOT be re-thrown.

5.15. Exceptions SHOULD NOT be caught in a loop. The loop SHOULD be wrapped with a try/catch construct instead.

5.16. If a method uses system resources (such as files, sockets, streams, etc.), the code MUST be wrapped with a try block and the corresponding finally block. In the finally sections, all resources SHOULD be properly released.

5.17. LocalizedException SHOULD only be thrown in the Presentation layer (Controllers, Blocks).

6. Application layers

6.1. All layers

6.1.1. Application SHOULD be structured in compliance with the CQRS principle.

6.1.2. Every application layer (Presentation, Service Contracts, Data Access) MUST process (handle or re-throw) exceptions of the underlying layer.

6.2. Presentation layer

6.2.1. According to CQRS, the Presentation layer hosts the Command and the Query Infrastructures:

  • Command for Actions

  • Query for Layout and its elements (Blocks and UI Components)

6.2.2. Request, Response, Session, Store Manager and Cookie objects MUST be used only in the Presentation layer.

6.2.3. All actions MUST return the ResultInterface implementation.

6.2.4. Actions MUST NOT reference blocks declared in layout.

6.3. Data Access (Persistence) layer

6.3.1. Entities MAY have fields scoped differently (in product, EAV — per store, options — per website).

6.3.2. Every persistence operation MUST be performed with one scope set.

6.3.3. Entities MUST NOT contain persistence-related logic.

6.3.4. MySQL’s strict_mode variable SHOULD be aligned with the default strict_mode of the latest MySQL release.

6.4. Service Contracts (Application) layer

We are reviewing this section and will publish it soon.

7. Configuration

7.1. An Application Instance consists of:

  • Code

  • Environment Configuration

  • Data

7.2. Code includes:

  • application codebase

  • XML configuration

  • generated code and static files

  • database structure

  • system configuration values

  • configuration scopes (stores/store groups/websites)

  • CMS entities

7.3. Environment Configuration includes information about application services connection.

7.4. Data includes the business entity data.

7.5. Code and Environment Configuration MUST not be stored in Data Storage.

7.6. Installation process MUST NOT modify Code.

7.7. All XML configuration formats MUST be declarative. Imperative nodes are not allowed.

7.8. All Configuration objects MUST use Magento\Framework\Config.

8. Modularity

8.1. The Application Framework (Magento\Framework\*) MUST NOT depend on application modules.

8.2. All dependencies MUST be declared in the component’s composer.json file.

8.3. If component A uses behavior of Component B, such Component B MUST be declared in the require section of Component A’s composer.json file, except for cases where Component B is used in the code that customizes the behavior of Component B.

8.4. If component A extends/customizes the behavior of component B through its customization points (layout handles, plugins, events, etc.), such Component B MUST be declared in the suggest section of Component A.

8.5. Only the @api code of any module can be referenced by other modules.

8.6. A module MUST NOT contain references to theme resources.

8.7. A component MUST NOT rely neither on dependencies of dependencies nor on dependencies of the project it is included in (e.g., Magento application). All component dependencies MUST be stated explicitly.

9. Browser-Server interaction in web application

9.1. All Client-Server calls must follow the HTTP Protocol.

9.2. All customer-agnostic data (Products, Categories, CMS Pages) MUST be rendered on a server and cached in a public cache server (Varnish).

9.3. All customer-specific data MUST be rendered on the browser side using a JavaScript (JS) application.

9.4. HTML markup generated on server MUST NOT contain user-specific data.

9.5. HTML markup generated on server MUST NOT contain session-specific data (e.g. a form element with a CSRF token).

9.6. A JS application MAY receive customer-specific data using the CustomerData JS API.

9.7. All state-modifying requests from a browser SHOULD be performed with AJAX requests.

9.8. If an error occurs during request handling, the server MUST return an appropriate HTTP Status Code and an explanation of an error in the response body.

9.9. All headers MUST be respected.

9.10. The Request, Session, and Cookie objects MUST NOT be injected in an object constructor. They MUST be passed only as method arguments.

9.11. Operation scopes MUST always be explicitly requested by operations (StoreManager SHOULD NOT be used to retrieve the store ID).

10. JavaScript (JS) application

10.1. The Magento 2 UI Component framework MUST be used to build front-end applications.

10.2. Only private content SHOULD be rendered in browser.

10.3. All module dependencies of a RequireJS module MUST be declared in the module’s definition header. No direct calls to require SHOULD be made unless the list of modules to be loaded is dynamic.

10.4. The W3C Content Security Policy MUST be followed.

10.5. The Airbnb JS Style Guide SHOULD BE followed.

10.5.1. ES2015 SHOULD be used as a JS standard.

10.5.2. Language features (closures) MUST be used for scope management. There SHOULD be no _ (underscore) naming convention for private properties.

10.5.3. All asynchronous operations MUST be represented with ES2015 Promises.

10.5.4. Global properties (window.*) MUST NOT be used. A module system SHOULD be used for shared objects.

10.5.5. Modules MUST NOT have external side effects.

10.5.6. Every ES2015 module MUST declare a default value.

10.5.7. Function declarations MUST be used for private functions instead of function expressions.

10.5.8. Re-declaration of function names MUST NOT be used.

11. Testing

11.1. White-box testing (unit, integration, functional)

11.1.1. Only public methods SHOULD be tested. Private and protected behavior SHOULD be tested through public methods.

11.2. Unit testing

11.2.1. All objects SHOULD be tested in isolation.

11.2.2. ObjectManager MUST NOT be used in unit tests.

11.2.3. ObjectManagerHelper MAY BE used to automatically mock all dependencies of the object under test.

12. Web API

12.1. Both REST and SOAP API’s MUST be exposed.

12.2. All Web API GET endpoints MUST return lists of entities.

13. Command line interface (CLI)

13.1. Magento 2 CLI Command Naming Guidelines MUST be followed.

13.2. A CLI command MUST be created for any functionality intended to be used by a system integrator/system administrator/developer (for example: change indexer mode, generate a configuration file, etc.).

13.3. A CLI command MUST always run in a global area. If a command needs a specific area to perform its functions, such area SHOULD be set up before execution.

13.4. Exception in a single CLI command SHOULD NOT break the CLI framework; running other commands SHOULD still be possible.

14. Events

14.1. All values (including objects) passed to an event MUST NOT be modified in the event observer. Instead, plugins SHOULD BE used for modifying the input or output of a function.

class SampleEventObserverThatModifiesInputs
     * @param \Magento\Framework\Event\Observer $observer
    public function execute(\Magento\Framework\Event\Observer $observer)
        /** @var \Magento\Framework\App\DataObject $transport */
        $transport = $observer->getData('transport');

        if ($transport->getData('some_value') === true) {
             * Expecting this value to go back to the original event dispatcher violates
             * this rule. Other observers could change the data, or Magento could make
             * architectural changes always sending immutable objects.
            $transport->setData('output_return_value', true);

14.2. Events used SHOULD be observed as specifically as possible. A global subscription to an event SHOULD NOT be used when the area impacted is just frontend.