Relational Databases

RDBMS

Relational database management system (RDBMS)
A relational database like SQL is a collection of data items organized in tables.
ACID is a set of properties of relational database transactions.
- Atomicity - Each transaction is all or nothing
- Consistency - Any transaction will bring the database from one valid state to another
- Isolation - Executing transactions concurrently has the same results as if the transactions were executed serially
- Durability - Once a transaction has been committed, it will remain so

Denormalization attempts to improve read performance at the expense of some write performance.
Redundant copies of the data are written in multiple tables to avoid expensive joins.
Once data becomes distributed with techniques such as federation and sharding, managing joins across data centers further increases complexity.
Denormalization might circumvent the need for such complex joins.
To avoid expensive joins, Denormalize where performance demands it.

Disadvantages

Data is duplicated.
Constraints can help redundant copies of information stay in sync, which increases complexity of the database design.
A denormalized database under heavy write load might perform worse than its normalized counterpart.