---

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    withdrawn = Withdrawn.follow(withdraw)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    withdrawn.processed_time = time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    write.(withdrawn, stream_name)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# Withdrawn event message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class Withdrawn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  apply Withdrawn do |withdrawn|
    account.id = withdrawn.account_id

    amount = withdrawn.amount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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