Sharing Applications Between Services

A reader posted a good question regarding my Service Boundary Example post. He asked:

Let's say as a result of our enterprise architecture planning we've decided that we're going to have a Sales Service (Leads, Opportunities, etc) and a Marketing Service (Campaigns, Promotions, etc).

However let's say we currently have one legacy CRM system that supports the marketing department and the sales department.

Which service boundary will contain that system?

The answer to that is that different parts of the legacy CRM would sit behind each service boundary. There is no issue in sharing infrastructure between services. You just need to be aware of the dependencies on that infrastructure from a risk management perspective.

Service boundaries are drawn up around data. There must be no data shared between services except through explicit message exchanges.

At the very least I would suggest that we have process isolation between services, so that data isolation is enforced at the OS level. To be fair however, you don't really require process isolation between two services in order for them to function as services. It is just obviously a good design choice.

So in this case, as long as the lead and opportunity data are never used by the Marketing service, and the campaigns and promotions data are never used by the Sales service, then we have data isolation and we haven't offended the SOA gods too deeply.

When transitioning legacy systems into an SOA, we sometimes have to compromise further on this because we have a single legacy system used by two services that doesn't enforce this data isolation. We can't control the design of this system. And to be fair, it wasn't designed to be used in this way.

The issue is we now have a message sent to one service, affecting the state of the second. This is obviously very bad and not ideal, and I wouldn't consider these services as true isolated services in the context of SOA. However it can be a useful half way point on the way to true SOA.