DDD querying across domains

Question

This is my first shot at trying to develop an application using some of the ideas behind DDD. It's a Rails monolith application (at-least for now). I know i'm not being strict with DDD, but I'm trying to focus on the separation of my domains which seems like the most important concept.

I think this is probably quite a rudimentary question and I'm sure I'll use the wrong DDD terminology, but here we go...

I have two domains:

1. CRM (contacts, companies, branches, people, etc..)
2. Quoting (quotes, quote items, currency, etc..)

In the Quoting domain, my Quote model links to a contact_id and company_id in the CRM domain.

On a page in the CRM domain I want to list all the Quotes that belong to a company.

The code is something like this:

  module CRM
    class QuotesController
      def index 
        @company = Company.find params[:company_id]
        @quotes = Quoting::Quote.for_company(@company.id)
      end

On this page I want to show who created each quote and I'm not sure what the best way is. The contact_id column in Quote links to the CRM::Contact model, and the CRM::Contact model links to the CRM::Person model which includes the name.

Without thinking of DDD, I would've done something like this (this is simplified) (yes, I know I'm not using associations here):

  module Quoting
    class Quote < ApplicationRecord
      belongs_to :company, class_name: "CRM::Company"
      belongs_to :contact, class_name: "CRM::Contact"

      class << self
        def for_company(company_id) 
          Quote.where(company_id: company_id)
               .includes(contact: :person)    <--- feels like it shouldn't be here
        end 
      end
    end 
  end

In the view:

@quotes.each do |quote|
  %tr  
    %td= quote.contact.person.name   (or quote.contact_name)

My issue with that is the line .includes(contact: :person) which includes knowledge about another domain, i.e. it knows that the name is within the person record. If my internal implementation of Contacts/Person changes then obviously this Quoting domain will fail too.

How could this be done in a more DDD style?

Would I have a new Quote model in my CRM domain that would run 2 queries and collate them into a new model?

Thanks!

Answer 1

I’m not a fan of querying across domains in real time because it can cause cascading failures. If the Quoting domain is down and the CRM domain needs information on a quote, the CRM domain can fail through no fault of its own. Imagine the same scenario where the Quoting domain is not down but just slow – the CRM domain gets user complaints because it seems to be the slow one.

The best way to deal with this is to cache supporting information in the domains that need it. For example, the CRM domain could cache just the QuoteID and Name from the Quoting domain. Such caches are prone to stale data unless you also implement a messaging system allowing the source domain to tell all listeners that particular data has changed and they should update their caches. Each time a quote is added or updated, it posts a message for all subscribers with the now-current data. Subscribers cache just the fields they care about locally and use that when necessary.

With the cacheing approach, if the Quoting domain is down the CRM can still function. The CRM function is also free to change how much of the Quoting information it caches, without changing any requirements for the CRM Domain. It also scales well since the Quoting domain does not have to scale up just to satisfy the needs of the unexpectedly successful CRM domain.