After reading about these databases, I see many similarities and I find them very interesting, since the data structures that are saved adapt very well to object-oriented programming as well as dictionaries or lists. In MongoDB, the data is saved and read from the server's hard disk. These data can be downloaded into dictionaries or lists of the programming language that you use and will be operating in the RAM of the computer, where they will be mutating until you decide to save them again. What I can not understand is the usefulness of Redis. A database in RAM ?. In principle it sounds great because of the speed of its operation, but I do not understand the utility well. There will be times when I will have to save the data on the hard disk as well as other times when I will have to extract them for their use. And part of that information I will have to use in certain data structures of my programs, so they will be duplicated in RAM. And what happens, if for example, the server falls? Would it lose all that information? First of all, sorry for my ignorance and thanks in advance. Greetings.