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+# 数据库
+
+```
+KISS - Keep It Simple & Stupid；
+```
+
+数据被快速制造和消费，数据看上去更加无秩序，或叫非结构化，据说超过80%，且持续加速增加，于是新的非关系型DB/noSQL快速崛起
+
+
+#### :question:
+- 类型
+- 特点
+- 选型
+
+
+#### 数据库类型
+```
+- Relational DBMS
+  - oracle,mysql,postgreSQL,hive
+- Key-value stores
+  - redis,memcached
+- Document stores/document-oriented database
+- Graph DBMS
+  - neo4j,Microsoft Azure Cosmos DB
+- Time Series DBMS
+  - InfluxDB,Kdb+,Prometheus,Graphite,RRDtool,OpenTSDB,Druid
+- Object oriented DBMS
+- RDF stores
+- Search engines
+  - Elasticsearch,Splunk
+- Wide column stores
+ - Cassandra,hbase
+- Multivalue DBMS
+- Native XML DBMS
+- Event Stores
+- Content stores
+- Navigational DBMS
+```
+
+#### Document stores database
+also called document-oriented database systems, are characterized by their schema-free organization of data.
+Records do not need to have a uniform structure, i.e. different records may have different columns.
+The types of the values a€?a€?of individual columns can be different for each record.
+Columns can have more than one value (arrays).
+Records can have a nested structure.
+Document stores often use internal notations, which can be processed directly in applications, mostly JSON. 
+JSON documents of course can also be stored as pure text in key-value stores or relational database systems. 
+That would, however, require client-side processing of the structures, which has the disadvantage that the features offered by document stores (such as secondary indexes) are not available.
+<br>存储文档数据库，也叫面向文档数据库，主要特点是非结构化、数据的自由组织
+纪录可以非结构化，比如不同纪录中的可以有不同的列，不同列的数据类型也可以不同，列的数据可以是多个值，比如一个array，记录还可以是嵌套结构。
+比如方便直接存储JSON数据，key-value数据库也可以存储JSON数据，但客户需要自己处理数据结构，这样就容易失去JSON数据组织的优点，比如第二个索引
+
+#### kafka & redis
+
+## 消息系统
+:question:
+- 异步
+- 解耦
+- 顺序
+
+kafka通过zookeeper来存储集群的meta信息
+
+[Redis哨兵模式（sentinel）学习总结及部署记录（主从复制、读写分离、主从切换）](https://www.cnblogs.com/kevingrace/p/9004460.html)
+
+
+
+![db trend](imgs/db/dbtrend.png)
+![db trend 24 months](imgs/db/dbtrend24m.png)
+
+
+
+#### references
+1. [db ranking](https://db-engines.com/en/ranking)
+1. [influxDB vs. openTSDB](http://blog.fatedier.com/2016/07/06/test-influxdb-and-opentsdb/)
+1. [openTSDB](http://blog.fatedier.com/2016/07/04/research-of-time-series-database-opentsdb/)
+1. [恒丰银行——大数据实时流处理平台](http://www.sohu.com/a/148106853_400678)