How do you design and maintain custom Craft CMS plugins?

Long Answer

Designing custom Craft CMS plugins that survive future platform updates means treating a plugin like a product: clear public APIs, automated tests, and an opinionated release process. The target is a plugin that cleanly extends Craft, minimizes coupling, and communicates change to integrators and content teams.

1) Architecture & boundaries
Model the plugin around Craft’s conventions: a Plugin class as the entry point, Services for business logic, Controllers for HTTP, and Components for integration (elements, fields, utilities). Expose a small, deliberate public surface via services (e.g., MyPlugin::getInstance()->service->method()), keeping controllers thin and internal classes private. Prefer composition over inheritance; subscribe to Craft events (element save/delete, queue jobs) instead of overriding behavior, so upstream changes don’t break your logic.

2) Data model & migrations
Separate content from configuration. Store settings in a Model validated by rules, persist defaults in config/project/*.yaml so environments stay reproducible. For DB changes, write incremental migrations (mYYMMDD_HHMM...) that are forward-only and idempotent. Use safeUp()/safeDown() carefully; for production, plan expand-then-contract changes (add columns, backfill with queues, switch reads, then remove). Avoid altering Craft core tables unless absolutely necessary; prefer your own namespaced tables with foreign keys and indexes tuned from query plans.

3) Extensibility & integration points
Expose plugin events and interfaces so other plugins/sites can extend yours without forking. For UI, build CP widgets and field types with accessible markup and Vue/Alpine patterns Craft favors. Keep asset bundles lean; defer heavy JS and load only within the CP routes that need it. For front-end helpers, publish Twig extensions and tags that map to service calls rather than embedding logic in templates.

4) Configuration strategy
Support both CP settings and config/project.php/environment variables. Mark sensitive values (API keys) as ENV-driven, document required scopes, and validate at runtime. Provide per-site overrides for multisite installs. Respect Craft’s project config: write changes through services so they serialize correctly; include a schemaVersion and handle afterInstall()/afterUninstall() cleanups reliably.

5) Performance & queues
Never block requests with heavy work. Use Craft’s queue for imports, backfills, webhooks, and image processing. Provide retryable jobs with idempotent operations. Cache expensive results with Craft’s cache component; namespace keys by site/locale and invalidate via events. Profile queries with debug tools; add compound indexes and avoid N+1 when loading elements (use eager-loading and criteria).

6) Testing & CI
Add PHPUnit/Pest suites with Craft’s test harness. Cover services, migrations (up/down on a scratch DB), and controllers (CSRF, permissions). Snapshot project config to prevent drift. In CI, matrix test against multiple Craft versions and supported PHP versions; run static analysis (Psalm/PHPStan) and linters. Add Playwright/Codeception for CP UI flows if you ship complex widgets.

7) Versioning & deprecation
Use semantic versioning and publish a changelog. For API changes, ship deprecation layers: mark methods @deprecated, keep old signatures calling the new internals, and log deprecation notices so site logs guide upgrades. Avoid breaking changes in minors; if unavoidable, provide an upgrader command that fixes config or data in place and prints a report.

8) Security & permissions
Integrate Craft’s permission system; gate CP controllers and actions with granular capabilities. Sanitize input in controllers/models; escape output in Twig helpers. For external calls, time-out and retry with back-off; store secrets outside DB when possible (ENV). Add a security policy and respond to reports with patched releases.

9) Distribution & supportability
Publish to Packagist with a clear composer.json (PHP and Craft version constraints). Keep dependencies minimal and pinned to ranges you test. Provide README/setup docs, migration guides, example templates, and CP screenshots. Include telemetry opt-in to understand adoption (never collect content).

10) Upgrade playbook
Before Craft upgrades, track RC notes, run your matrix CI, and smoke-test CP flows. If Craft deprecates events or APIs you use, map replacements and ship your deprecations ahead of time. Maintain a compatibility table (Craft version → plugin version). For breaking Craft changes, prepare a migration branch and a beta tag so early adopters can validate.

With these practices, your custom Craft CMS plugins remain predictable: they integrate cleanly, scale under load, and upgrade smoothly. The payoff is fewer regression surprises and a shorter path to supporting the next Craft release.

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Table

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Common Mistakes

Overriding core classes instead of using events and services, creating tight coupling that breaks on Craft updates. Placing heavy logic in controllers/Twig, making it untestable. Editing Craft tables directly or shipping destructive migrations without expand/contract. Ignoring project config, causing “works on my machine” drift. No matrix CI—plugin works on your PHP/Craft combo only. Breaking APIs in minor versions, with no deprecation path or changelog. Blocking requests with long imports instead of queue jobs. Storing secrets in DB or code. Bundling large JS/CSS globally in CP, slowing unrelated screens. Skipping permission checks on CP routes.

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Sample Answers

Junior:
“I follow Craft conventions: services for logic, controllers thin, settings in a model. I write migrations and avoid touching core tables. I add unit tests and document installs.”

Mid:
“I expose a small service API and subscribe to Craft events. Migrations use expand/contract with queue backfills. I support project config, ENV-driven secrets, and run CI against two Craft versions and PHP ranges. Releases follow SemVer with a changelog.”

Senior:
“My custom Craft CMS plugins ship deprecation layers and a compatibility table. Matrix CI runs PHPUnit, static analysis, and CP smoke tests. Performance uses cache and queues; CP UI loads scoped assets only. Before Craft upgrades, I run RC tests, publish a beta, and provide migration guides. Permissions are granular; security and observability are first-class.”

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Evaluation Criteria

Strong answers frame custom Craft CMS plugins as products: clear public services, event-based integration, and minimal surface area. They include migrations with expand/contract, project config alignment, and queue-based backfills. Testing via matrix CI (Craft/PHP), static analysis, and CP smoke flows shows maturity. Versioning discipline—SemVer, deprecations, changelog—and dependency constraints signal upgrade readiness. Security (permissions, ENV secrets) and performance (cache, eager-loading, queues) round it out. Weak answers rely on overriding core, ad-hoc SQL, or ignore project config and tests, guaranteeing breakage on the next Craft update.

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Preparation Tips

Set up a demo plugin: one service, one CP settings page, a field type, and a migration. Add project config support and ENV-based secrets. Implement a queue-driven backfill to practice expand/contract. Create PHPUnit tests for services and a migration test that spins a temp DB. Configure GitHub Actions to run a matrix: Craft LTS + latest, PHP supported versions. Add Psalm/PHPStan and coding standards. Publish to a private Packagist to practice versioning, then simulate a Craft minor upgrade and fix deprecations with shims. Write a one-page migration guide and verify teammates can update without manual steps.

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Real-world Context

A content network built a translation plugin using events on element save and a queued export pipeline. When Craft added project config improvements, the team already stored settings in models and ENV, so upgrades were painless. Another team shipped destructive migrations and heavy controller logic—updates to Craft changed element events and broke everything. They refactored to services + events, added expand/contract migrations with backfills, and introduced matrix CI; future Craft upgrades became routine. A SaaS shop reduced CP slowdowns by scoping asset bundles per route and caching API results, cutting CP TTFB by 40% while keeping custom Craft CMS plugins compatible.

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Key Takeaways

• Keep plugin APIs small; extend Craft via events/services, not overrides.
  
• Use forward-only migrations with expand/contract and queue backfills.
  
• Align with project config; validate settings via models and ENV.
  
• Ship tests and matrix CI (Craft/PHP); follow SemVer with deprecations.
  
• Secure, performant plugins: permissions, caching, eager-loading, scoped assets.
  
  

Practice Exercise

Scenario:
You must add a “Content Audit” feature via a custom Craft CMS plugin that scans entries, flags stale content, and exposes a CP utility with export.

Tasks:

1. Architecture: Create a Plugin with a AuditService (scan logic), Utility (CP UI), and a settings model (age threshold, sections).
   
2. Data: Add a namespaced table for audit results via a forward-only migration. Plan expand/contract for future columns (e.g., owner, risk score).
   
3. Integration: Subscribe to element save/delete to refresh audit state. Expose a Twig tag {% audit_count %} that delegates to the service.
   
4. Performance: Run scans in queue jobs; cache counts per site/section with namespaced keys; invalidate via events.
   
5. Config: Support ENV for thresholds; ensure project config serialization works; set schemaVersion.
   
6. Testing & CI: Write unit tests for the service and a migration test; configure a Craft/PHP matrix in CI with static analysis.
   
7. Release: Publish v1.0.0 with a changelog and SemVer. Add deprecation stubs for any renamed methods. Provide docs and screenshots.
   
   

Deliverable:
Repo + CI badge, installation guide, and a short upgrade note proving the plugin survives Craft updates without manual fixes.