Magento 2 Upgrade Strategy: What Your Architecture Tells You

Table of Contents:

• Mapping Module Dependencies
• Core vs. Custom Code Separation
• API Compatibility Planning
• Planning and Phasing the Upgrade
• Final Thoughts

Mapping Module Dependencies

Magento modules often depend on each other—directly through declared dependencies (module.xml) or indirectly through preferences, plugins, and shared services. Before upgrading Magento, understanding this web of dependencies is essential.

What to audit:

• Explicit dependencies: Review module.xml and composer.json for sequence and require declarations.
• Service contracts: Identify where custom modules consume Magento core interfaces or classes that may have changed in the target version.
• Inter-module coupling: Use tools like magento-dependency-graph or manual diagrams to visualize relationships.

Risks during upgrade:

• Cascading failures: A change in one core service can cascade across multiple modules if they override the same behavior.
• Silent breakage: Undeclared dependencies may break silently if a module is removed or renamed in the core.

Best practice: Treat your module layer like infrastructure. Each module should declare what it depends on, expose stable APIs internally, and avoid assumptions about other modules' behavior unless explicitly required.

Core vs. Custom Code Separation

Upgrades are easier when custom logic is encapsulated and minimally dependent on Magento's internal classes. The more tightly customizations are interwoven with core code (via preferences, overwritten templates, or subclassing core models), the more upgrade friction you'll face.

Indicators of healthy separation:

• Customizations use plugins or observers, not preferences.
• Business logic is moved into standalone service classes, not buried in controllers or models.
• Templates override selectively (e.g., only for layout changes), not full blocks or templates unnecessarily.

Problematic patterns:

• Direct modification of vendor or core files, still seen in legacy builds.
• Custom modules that override core logic wholesale instead of layering on top.
• Theme-level template overrides that duplicate outdated block or helper logic.

Upgrade strategy:

• Identify overridden core files and templates in your custom theme or modules.
• Refactor preference-based overrides to plugins or service extensions where possible.
• Confirm compatibility of your custom modules with newer core parameters or contracts.

API Compatibility Planning

If your site integrates with third-party services or frontend clients (e.g., headless storefronts), upgrading Magento means ensuring API contract continuity.

What to evaluate:

• REST and GraphQL schema changes: Some endpoints or fields may be added, deprecated, or renamed.
• Payload structure: Field formatting, nesting, or nullability can shift across versions.
• Authentication and token behavior: Customer token generation, expiration, and guest cart handling may change subtly.

Steps to mitigate impact:

• For GraphQL: Run introspection queries against the upgraded environment to validate your schema assumptions.
• For REST: Compare responses using automated regression tests or saved examples from production.
• Validate middleware or data mapping services for compatibility with Magento version-specific data.

Tip: Keep a changelog or test suite that exercises critical external APIs (cart, product, order, customer) across environments. This gives early warning of schema-level changes that could affect storefront behavior or system syncs.

Planning and Phasing the Upgrade

Once architectural assessment is complete, approach the upgrade as a phased project.

Pre-upgrade audit:

• Run bin/magento setup:upgrade --keep-generated in a cloned environment.
• Scan var/log/, var/report/, and dependency injection logs for breakpoints.
• Capture third-party module version compatibility (and whether updates are available).

Environment setup:

• Use a staging branch/environment with Composer version locks (composer.lock) separated from production.
• Document baseline behavior (checkout flow, admin tasks, search queries) for before/after comparison.

Isolate and test:

• Re-enable modules and test incrementally—core first, then custom, then third-party.
• Run integration and smoke tests after every set of module activation