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A Brief Example...

# Account command handler with withdrawal implementation # Business logic for processing a withdrawal class Handler include Messaging::Handle handle Withdraw do |withdraw| account_id = withdraw.account_id account = store.fetch(account_id) unless account.sufficient_funds?(withdraw.amount) logger.info('Withdrawal rejected') return end withdrawn = Withdrawn.follow(withdraw) time = clock.iso8601 withdrawn.processed_time = time stream_name = stream_name(account_id) write.(withdrawn, stream_name) end end # Withdraw command message # Send to the account component to effect a withdrawal class Withdraw include Messaging::Message attribute :account_id, String attribute :amount, Numeric attribute :time, String end # Withdrawn event message # Event is written by the handler when a withdrawal is successfully processed class Withdrawn include Messaging::Message attribute :account_id, String attribute :amount, Numeric attribute :time, String attribute :processed_time, String end # Account entity # The account component's model object class Account include Schema::DataStructure attribute :id, String attribute :balance, Numeric, default: -> { 0 } def withdraw(amount) self.balance -= amount end def sufficient_funds?(amount) balance >= amount end end # Account entity projection # Applies account events to an account entity class Projection include EntityProjection entity_name :account apply Withdrawn do |withdrawn| account.id = withdrawn.account_id amount = withdrawn.amount account.withdraw(amount) end end # Account entity store # Projects an account entity and keeps a cache of the result class Store include EntityStore entity Account projection Projection end 

This code is not a complete implementation and not production-ready. It's intended only to provide a very high-level introductory overview.

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