Create Service Object

Create Service Object

HARD-GATE

TESTS GATE IMPLEMENTATION:
EVERY service object MUST have its test written and validated BEFORE implementation.
  1. Write the spec/test for .call (with contexts for success, error, edge cases)
  2. Run the spec/test — verify it fails because the service does not exist yet
  3. ONLY THEN write the service implementation
The final artifact must include the test command and the failure message
before implementation. Use the observed failure when available; otherwise show
the exact expected failure class/message for the missing service.
See tdd-process for the full gate cycle.

Core Process

1. Write Spec (Test-First): Create the spec/test file at spec/services/<module_name>/<service_name>_spec.rb (or test/services/). Cover success and error paths for .call. Run it to confirm it fails (see HARD-GATE). Tests must assert success: and response: top-level keys and the meaningful payload shape.
2. Define Service Skeleton: Create services/<module_name>/<service_name>.rb with the correct module namespace.
3. Select Pattern: Choose Standard, Batch, Class-only (Pattern 3), or Orchestrator based on requirements. State whether instance state is required — if not, use Pattern 3 (no initialize, no instance variables).
4. Implement Contract: Implement self.call and #call. Response must always be { success: true, response: { ... } } or { success: false, response: { error: { message: '...' } } }. Keep call ≤ 20 lines; extract sub-services if longer. Validate inputs at top of call; return error hash if invalid. Return serialized data only — no raw persistence model objects (e.g. ActiveRecord, ROM) in response.
5. Handle Errors and Logging: Catch StandardError (and domain exceptions). Log with the application logger (e.g., logger.error). Use UPPER_SNAKE_CASE constants for all user-facing error strings — never inline in a rescue. Never re-raise to caller.
6. Add YARD Documentation: Add @param, @return [Hash], and @raise tags to self.call and every other public method. Document self.call separately from #call. For class-only services (Pattern 3), if the class returns a non-standard shape (e.g. nil / error string), document that explicitly in YARD and the README.
7. Write Module README: Generate services/<module_name>/README.md explaining domain context. Required even for single-service modules.

Additional Constraints

Core Patterns

1. The .call Pattern

def self.call(params)
  new(params).call
end

def call
  # ... processing ...
  { success: true, response: { data: result } }
rescue StandardError => e
  logger.error("Processing Error: #{e.message}")
  logger.error(e.backtrace.join("\n"))
  { success: false, response: { error: { message: ERROR_MESSAGE } } }
end

2. Batch Processing + Per-Item Rescue (Partial Success)

def call
  results = @items.each_with_object({ successful: [], failed: [] }) do |item, acc|
    # process...
  rescue StandardError => e
    logger.error("Unexpected item error: #{e.message}")
    acc[:failed] << { sku: item[:sku], error: e.message }
  end
  { success: true, response: results }
end

3. Class-only Services (Static Methods)

When no instance state is needed, use ONLY class methods — no initialize, no instance variables. Suitable for validators, formatters, and argument-only helpers.

class Orders::QuantityValidator
  def self.call(quantity:)
    return { success: false, response: { error: { message: INVALID_QUANTITY } } } unless quantity.positive?

    { success: true, response: { valid: true } }
  end
end

4. Orchestrator Delegation (≤20-line call)

def call
  user_result = UserCreationService.call(@params)
  return user_result unless user_result[:success]
  # ... continue ...
end

Extended Resources (Progressive Disclosure)

Load these files only when their specific content is needed:

• assets/examples.md — Detailed examples of the 4 core patterns (Standard, Batch, Static, Orchestrator).
• assets/service_skeleton.md — Basic starting skeleton.
• assets/module_readme_template.md — Template for the mandatory module README.

Integration