Database Systems - NoSQL Key-Value Pair Databases Overview - APPFICIAL

In a key-value database, a unique key refers to a data value. For example, the key 23 can refer to basketball player Michael Jordan. The value in the key-value pair can be any type, such as text, numbers, characters, JSON, or even multiple values per key.

Key-value pair databases have a few queries:

put(key, value) - sets the value to an indexed key.
get(key) - gets the value of the key.
multiGet(key1, ..., keyN) - gets the values of keys 1 through N.
delete(key) - deletes the value of the key

Key-value databases require:
Simple data models – can be managed by an application
Data access from the key – the get() queries specifies the key to access the value data
Large amounts of small data records – terabytes of records as small as kilobytes
Fast reads and writes – not optimized for updates. Some key-value databases are stored in-memory for super fast access.

Key-value databases physical requirements:
- Sharding – can shard data (horizontal partition) across thousands of machines
Replication – replicate values in case of failure
Consistency – its okay to be temporarily inconsistent, but should eventually become consistent
Hashing - keys are typically a hash index, and values are stored in hash buckets

Popular key value databases are Redis, Amazon DynamoDB and Oracle NoSQL

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