The CAP triangle
What happens if we have two databases, one in London and one in New York. What if two people try and register the same username in both locations.
In the diagram below we can see that there are three relational databases in three locations.
We would like each of these databases to serve the same information to the client computers attached to each of them.
However, each of the clients may be adding and updating information to the data stores, each of the relational databases will be consistent (they will obey the rules of referential integrity internally) but how can we make sure that each database is updated with information from the other databases.
Before we can do that, we need to understand how each of the databases ensure integrity. How can each database ensure that it is internally consistent?
To do that we must understand transactions.
