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Adapting your database models to an exotic NoSQL may eventually alter your functionality. Users don't like change.

reddit, digg and others were able to make the transition without altering their functionality. I don't see this as a very hard argument for not.

Even if it were the case, careful planning for the transition could mitigate distasteful changes.

Reddit uses Cassandra for a very small part of their functionality - their primary database is Postgre. Digg has yet to transitation over to Cassandra and generally, their transition has been delayed (even if they have a lot more resources than a regular startup). Facebook only uses Cassandra for inbox search, their primary database is MySQL.

The bottom line is to use NoSQL where it makes sense and where it solves the job better than relational databases.

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