[meritt]

Agreed. If Redis is an acceptable alternative to MongoDB you're using one of the two very incorrectly.

[lmm]

Use case: I want to store some json values against some string keys. Which would be very incorrect to use for this?

[jjirsa]

How often are you updating the json values? 10/s? 100/s? 1M/s?

How many string keys? Millions? Billions? What happens if you lose an update?

Redis great for lots of rapid reads and moderate write speed for data that fits on a single server or can be manually sharded well (if it's straight k=>v, as you describe, that basically means your json fits in RAM; if you're using larger objects like sets/zsets/etc, it becomes a slightly different discussion), as long as your application can lose a few seconds without killing you (BGSAVE isn't instantaneous, of course).

[StavrosK]

If the data is critical and you must keep them, both. If not, either.

[webtrill]

MongoDB is not significantly better at any particular thing so yes, Redis can be used as an alternative for the things I may use MongoDB for. Keep a close eye on http://www.amisalabs.com