What is Redis

Redis is an open source (BSD licensed), in-memory data structure store, used as a database, cache, and message broker. Redis provides data structures such as strings, hashes, lists, sets, sorted sets with range queries, bitmaps, hyperloglogs, geospatial indexes, and streams. Redis has built-in replication, Lua scripting, LRU eviction, transactions, and different levels of on-disk persistence, and provides high availability via Redis Sentinel and automatic

Redis simplifies your code by enabling you to write fewer lines of code to store, access, and use data in your applications. For example, if your application has data stored in a hashmap, and you want to store that data in a data store – you can simply use the Redis hash data structure to store the data.

Essentially, Redis is a NoSQL in-memory data structure store that can persist on disk. It can function as a database, a cache, and a message broker. Redis has built-in replication, Lua scripting, LRU eviction, transactions, and different levels of on-disk persistence.

Redis is a data structure server. As a key-value data store, Redis is similar to Memcached, although it has two major advantages over that option: support of additional datatypes and persistence. ... All of the data is stored in RAM, so the speed of this system is phenomenal, often performing even better than Memcached.

It's fully managed. Google fully manages administrative tasks for Redis instances such as hardware provisioning, setup and configuration management, software patching, failover, monitoring and other nuances that require considerable effort for service owners who just want to use Redis as a memory store or a cache

Redis may be slow in scenarios where Not enough memory, generating swapping at the OS level. Too many O(n) operations (like KEYS) executed in the single-threaded engine. Large objects stored in Redis, leading to uncontrolled expansion of the communication buffers. Huge number of simultaneous sessions (>30000)

References 

https://redis.io/