Sometimes it is desirable to view a number of distinct databases as one large unified database. One of the ways of doing this, is to create a federation of these databases, which to an external client, looks like one large database. This is a different approach than data warehousing, where the data is copied to a new database.
One important challenge for this requirement is how best to target requests for information to the right data source. In a federation there are a number of places where data can be held. It is not always clear which source to ask. Another challenge is the problem of conflicting data, what do you do if there are multiple responses which do not agree?
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