1. key-value database
   • Redis and memcache
   • extremely fast
   • in memory
   • limited space
   • use case
     • caching
     • pub/sub – chat or messege queue

2. wide column database
   • hbase and cassandra
   • key and wide columns data for more info
   • decentralize and can be scale
   • use case
     • time-series
     • historical records
     • high write, low read

3. document database
   • mongodb
   • collection -> document -> key-value (can be stack into several layers)
   • schema-less
   • use case
     • app
     • games
     • iot

4. relational database
   • mysql, postgres
   • SQL
   • primary key, foreign key
   • schema required

5. graph database
   • neo4j
   • node – relationship
   • cypher
   • use lot of join -> faster performance than SQL
   • use case
     • graph
     • knowledge graph
     • recommendation engine

6. full-text search database
   • elastic, solr, lucene, meili
   • index – text
   • inverted index to link term(for search) <-> index of documents
   • use case
     • search engine

7. multi-model database
   • fauna
   • combine document database and graph database