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As a corollary, for what should I avoid using Redis?

I've heard wonderful things, but I've also heard that you can easily abuse Redis for things that aren't appropriate.

Data too large to fit in memory; anything that you're going to want to query relationally (as opposed to just key/value) in the future.

Not entirely true. Redis sets are quite powerful.

When you find yourself doing things too often you should be doing in a relational db (i.e. lots of foreign key relations).