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# Key-value store
## Abstraction: hash table
A key-value store generally allows for O(1) reads and writes and is often backed by memory or SSD. Data stores can
maintain keys in [lexicographic order ](https://en.wikipedia.org/wiki/Lexicographical_order ) , allowing efficient
retrieval of key ranges. Key-value stores can allow for storing of metadata with a value.
Key-value stores provide high performance and are often used for simple data models or for rapidly-changing data, such
as an in-memory cache layer. Since they offer only a limited set of operations, complexity is shifted to the application
layer if additional operations are needed.
A key-value store is the basis for more complex system systems such as a document store, and in some cases, a graph
database.
## Source(s) and further reading: key-value store
- [Key-value database ](https://en.wikipedia.org/wiki/Key-value_database )
- [Disadvantages of key-value stores ](http://stackoverflow.com/questions/4056093/what-are-the-disadvantages-of-using-a-key-value-table-over-nullable-columns-or )
- [Redis architecture ](http://qnimate.com/overview-of-redis-architecture/ )
- [Memcached architecture ](https://www.adayinthelifeof.nl/2011/02/06/memcache-internals/ )
