What are the scenarios for memcache Redis Mogodb respectively?

Memcache and Redis are key-value storage systems, whereas Redis may be more adaptable than memcache, storing more value types, and Redis supports master-slave synchronization. And MONGO is a document-based database.
1. Memcache can be used primarily as a caching system for caching files, database query results, etc. because it reads in memory and improves the performance of Web services. can be used for Web site data caching, to reduce the direct query of the database, reduce stress.
2, Redis Of course can also be used as a cache, but you look at the Weibo, many scenarios are applications of Redis.
3, MongoDB is a document-based database. Its query speed is very fast, it is very suitable for real-time inserts, updates and queries, such as logging, message logging and so on.

What are the scenarios for memcache Redis Mogodb respectively?