A database can be classified as either centralized or distributed. A centralized system manages a single database, while a distributed system manages multiple databases on multiple systems. A distributed system’s components can be classified depending on the autonomy of the component systems into two types: federated (autonomous) or non-federated (non-autonomous). This figure illustrates the difference between centralized and distributed.

Centralized databases have all the data in one system in one place. All users come to the one system to access the data. For certain restricted data, centralization can be ideal, but for data that needs to be widely available, centralized databases have risks. For example, if the centralized system is unavailable, there are no other alternatives for accessing the data.

Distributed databases make possible quick access to data over a large number of nodes. Popular distributed database technologies are based on using commodity hardware servers. They are designed to scale out from single servers to thousands of machines, each offering local computation and storage. Rather than rely on hardware to deliver high-availability, the database management software itself is designed to replicate data amongst the servers, thereby delivering a highly available service on top of a cluster of computers. Database management software is also designed to detect and handle failures. While any given computer may fail, the system overall is unlikely to.

Some distributed databases implement a computational paradigm named MapReduce to further improve performance. In MapReduce, the data request is divided into many small fragments of work, each of which may be executed or re-executed on any node in the cluster. In addition, data is co-located on the compute nodes, providing very high aggregate bandwidth across the cluster. Both the filesystem and the application are designed to automatically handle node failures.