The database systems can be scaled with two basic approaches, vertical or horizontal. In the vertical scaling approach, you need to add more capacity to the existing server, for instance, adding more CPU, RAM or storage to the server. It's obvious that adding more CPU or RAM to the existing server is more expensive and difficult to maintain. The following diagram shows you the process of vertical scaling:

In horizontal scaling, the database system can be scaled through different servers instead of only one server, that is, the database is scaled horizontally rather than vertically. This approach is safer, easier, and less expensive than vertical scaling. The process of horizontal scaling is shown in the following diagram: