[dagheti]

The issue with key-value store isn't that you can't create an application, its that the tools to control data integrity and access your data in unexpected ways are missing or will be alot of work to create. A simple example is that two way key-value pairs for the UID and username in this article. Now any update to one value (or is it key?) must update the other.

If your main concern is scalability, then Redis and the like can look very attractive, but we should be clear on what is being given up: data coherence and query flexibility. That may be fine on your app, or even many apps, but its important to point out what is missing.

[antirez]

I agree in general, SQL is like magic (this is why it can't scale). You write abstract tables and then automatically you can do incredible queries against it, and this is very cool indeed.

But there are some kind of problems that Redis can fix in a very natural way. For example the idea of PUSH and LRANGE solves the get-last-N-items problem in a trivial way compared to MySQL.

Also Set intersection is able to solve tagging easily compared to the complex queries needed in a SQL db.

So SQL is very flexible, but strangely enough some trivial data access pattern is hard to model and slow even if it is as direct as take the last N items from a list.