I suggest you look at the CQRS pattern. A good Java implementation is http://www.axonframework.org/. The advantage is the CQRS pattern that it is fairly simple, but highly scalable. So you can start small and simple with the confidence that you can tweak and optimise in the future to scale as required. There are good tutorials and support too. My team is using it for an industrial application and we have found that it has been very robust. It might take a bit of work to get your head around the concepts, but it is worthwhile in the end.