etcd

etcd is a highly available key-value store.

redis, memcached are popular key-value stores. These are general-purpose distributed memory caching system often used to speed up dynamic database-driven websites by caching data and objects in memory.

etcd cannot be stored in memory, they can only be persisted in disk storage, whereas redis can be cached in ram and can also be persisted in disk.

etcd does not have various data types. But redis and other key-value stores have data-type flexibility.

etcd guarantees only high availability, but does not give you the fast querying and indexing. All the nosql key-value stores are built with the goal of fast querying and searching.