A boolean logic rules engine was the first project I did for Neo4j before I joined the company some five years ago. I was working for some start-up at the time but took a week off to play consultant. I had never built a rules engine before, but as far as I know, ignorance has never stopped anyone from trying. Neo4j shipped me to the client site and put me in a room with a projector and a white board where I live coded with an audience of developers staring at me, analyzing every keystroke and cringing at every typo and failed unit test. I forgot what sleep was, but I managed to figure it out... and I lost all sense of fear after that experience.
The data model chained together fact nodes with crisscrossing relationships, each chain containing the same path ID property that we followed until reaching an end node which triggered a rule. There were a few complications along the way and more complexity near the end for ordering and partial matches. The traversal ended up being some 40 lines of the craziest Gremlin code I ever wrote, but it worked. After the proof of concept, the project was rewritten using the Neo4j Java API because at the time only a handful of people could look at a 40-line Gremlin script and not shudder in horror. I think we're up to two handfuls now.
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