Distributed relational databases
As we've discussed before, relational databases weren't designed with scalability in mind. They are great for enforcing strong data assurances, including ACID transactions, but their preferred way of operating is through a single server.
This can impose limitations in terms of how big an application can be using relational databases.
It is worth noting that a database server can grow vertically, which means using better hardware. Increasing the capacity of a server or replacing it with a bigger one is an easier solution for high demand than applying some of these techniques, but there's a limit. In any case, please double-check that the expected size is big enough. These days, there are servers in cloud providers that reach 1 terabyte of RAM or more. That's enough to cover a huge number of cases.
Note that these techniques are useful to grow a system after it is up and running, and can be added to most usages...