The integration layer is usually a person

Ask a team how data gets from the operations system into finance and you will often get a name rather than a diagram.

That person is the integration layer. They know which fields matter, which records to skip, and what to do when something looks wrong. None of it is written down.

This works until it does not: leave, turnover, or volume growth. Then the process fails in ways nobody can debug, because the logic never existed outside one person's head.

The first step is not to automate. It is to write down what that person actually does, including the exceptions, which is where the real logic lives. Most of the value in an integration project comes from that conversation.

Then you build. The system encodes the rules, surfaces the exceptions instead of silently resolving them, and gives the same person a queue instead of a full-time relay job.