Another problem is data storing and sharing. An effective microservice needs to be independent of other microservices, and ideally, should not share a database. What does this mean for our hotel booking app?

Again, that raises a lot of questions such as the following:

• Do we use the same users' IDs across all databases, or do we have independent IDs in each service and keep it as a hidden implementation detail?
• Once a user is added to the system, do we replicate some of her information in other services databases via strategies like data pumping, or is that overkill?
• How do we deal with data removal?

These are hard questions to answer, and there are many different ways to solve those problems, as we'll learn throughout the book.