The travel domain is interesting in terms of data modeling challenges. Throughout this book, we will be modeling systems that work together in a website that can be used for planning flight travel. Travelers would like to look at the options for an itinerary before booking any particular set of flights, especially if there is no direct flight from the traveler's current city to the destination city. Normally, an itinerary includes total duration, layover duration, and the number of hops it takes to reach the destination. We cannot, however, derive the itinerary without modeling cities and flights, which brings us to our first data modeling problem.