The requirement for a system to be scalable means that a system that supports a business now should also be able to support the same business with the same quality of service when it grows. A database can store 1 GB of data and effectively process 100 queries per second. What if the business grows 100 times? Will it be able to support 10,000 queries per second processing 100 GB of data? Maybe not now and in not the same installation. However, a scalable solution should be ready to be expanded to be able to handle the load as soon as it is needed.
Usually, scalability comes together with the distributed architecture of a system. If the database was able to utilize the power of multiple computers, then in order to scale it, one would need to add more computers to the cluster. There are solutions that do this. In fact, many NoSQL databases...
