MongoDB

Advantage:

1. using embedded document, mongo db can represent complex hierarchical relationships with a single record.(data can be represneted in object oriented way)

2. no predefined schemas, documents' keys and values are not of fixed types or sizes. adding and removing fields are easier so development can be fast iterated.

3. scale out: add server into cluster. 

Feature:

　　1. indexing

　　2. aggregation (map-reduce)

　　3. no muti-row transaction, no joins

　　4. do not enforce schema