redis or mongodb?

  mongodb, question

1. What scene is suitable for redis?
2. What scene is suitable for mongodb?
3. What scenario is not suitable for kv database?
4. Can 4.SNS-based websites completely abandon relational databases?
5. Can 5.B2C websites try to use kv database?

1.redis is similar to membercache and can be used as a sustainable cache. It is not used as a database alone.

2.mongodb has good expansibility similar to mysql

3. Systems involving associative operations

4. Yes, but they will all be used together.

5. Yes, in some data