Extract Engine

Use this skill when the task is to move existing code out of a Rails app and into an engine. Prefer incremental extraction over big-bang rewrites. Preserve behavior first, then improve design.

Quick Reference

Phase	Focus
Prep	Identify bounded feature and host dependencies
Logic	Move stable domain logic (POROs, services) first
Seams	Add adapters/config for host dependencies
Web	Move controllers, routes, views last

HARD-GATE

DO NOT extract and change behavior in the same step.
Extraction must preserve existing behavior; refactoring and improvements belong in a separate step after the move is complete and verified.

Core Process

1. Identify the bounded feature to extract — one coherent responsibility.
2. List hard dependencies on the host app (models, services, config).
3. Define the future engine boundary and host contract.
4. Move stable domain logic first: POROs, services, value objects, policies, query objects. Delay direct host model references, authentication, route ownership, and asset integration.
5. Add adapters or configuration seams for host-owned dependencies. Replace hardcoded host references with config values, adapter objects, or service interfaces.
6. Move controllers, routes, views, or jobs only after seams are clear.
7. Keep regression coverage green throughout each slice.

Each slice must have: one coherent responsibility, minimal new public API, passing regression tests, and a clear next step.

Extended Resources

Pitfalls

Pitfall	What to do
Extracting too much at once	One bounded slice per step
Direct host references in engine	Use adapters or config
Behavior changes mixed with extraction	Preserve behavior first; refactor after
Circular dependencies introduced	Verify import graph before each slice
Implicit host contract	Explicitly document and test the host app contract

Examples

First slice (move PORO, no host model yet):

mkdir -p my_engine/app/services/my_engine
mv app/services/pricing/calculator.rb my_engine/app/services/my_engine/pricing_calculator.rb

# Before (in host app): module Pricing; class Calculator
# After (in engine):
module MyEngine
  class PricingCalculator
    def initialize(line_items)
      @line_items = line_items
    end

    def total
      @line_items.sum { |item| item.price * item.quantity }
    end
  end
end

Verify regression coverage still passes before proceeding:

bundle exec rspec spec/services/pricing/ spec/requests/orders/

Adapter for host dependency:

module MyEngine
  def self.current_user_for(request)
    config.current_user_provider.call(request)
  end
end

# usage
OrderCreator.for_request(request) # resolves via MyEngine.current_user_for(request)

Output Style

1. Propose one small, bounded extraction slice at a time.
2. Outline the files moving, the new boundaries, and the regression tests to run.
3. Language — Must be in English unless explicitly requested otherwise.

Integration

Skill	When to chain
create-engine	Engine structure, host contract, namespace design after extraction
test-engine	Dummy app, regression tests, integration verification
refactor-code	Behavior-preserving refactors before or after extraction slices