SQL provides a relational database. There are several excellent implementations of this database, that are designed for extremely demanding applications.
They offer a good compromise between the distributedness, and the
cost of query operations.
In particular the database can be replicated in order to improve
scalability and fault tolerance, as long as the number of
queries operations dominates the number of
writes.
The R-GMA architecture was developed to address
this problem: the scalability of the architecture is further improved
introducing components that combine data from the database and cache
the results (similar to NWS forecasters).
next: Scalability of an end-to-end architecture
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