cassandra 读写数据一致性(consistency level)

cassandra 提供了高性能的数据写入、读取功能，整个集群形成去中心化的环形结构，没有单点故障，没有主从节点之分，内部节点通过gossip协议进行通讯， 不需要依赖外部（hbase通过zookeeper）

用户可以制定数据读写一致性（指定数据一次成功写入或读取的副本数目，判断一次读写是否成功），这种策略保证了数据读写一致性，同时满足了不同用户的不同需求。

1 读写一致性配置： http://docs.datastax.com/en/cassandra/latest/cassandra/dml/dmlConfigConsistency.html、

最常用的几种

QUORUM	A write must be written to the commit log and memtable on a quorum of replica nodes across all datacenters.	Used in either single or multiple datacenter clusters to maintain strong consistency across the cluster. Use if you can tolerate some level of failure.
LOCAL_QUORUM	Strong consistency. A write must be written to the commit log and memtable on a quorum of replica nodes in the same datacenter as the coordinator. Avoids latency of inter-datacenter communication.	Used in multiple datacenter clusters with a rack-aware replica placement strategy, such as NetworkTopologyStrategy, and a properly configured snitch. Use to maintain consistency locally (within the single datacenter). Can be used withSimpleStrategy.
ONE	A write must be written to the commit log and memtable of at least one replica node.	Satisfies the needs of most users because consistency requirements are not stringent.

2. spark写入cassandra配置，写入数据一致性配置参数 

output.consistency.level

LOCAL_QUORUM

Consistency level for writing

https://github.com/datastax/spark-cassandra-connector/blob/master/doc/reference.md#write-tuning-parameters

3. java 写入cassandra 配置 写入数据一致性

        QueryOptions queryOptions = new QueryOptions();
        queryOptions.setConsistencyLevel( ConsistencyLevel.ONE );  //The default consistency level for queries: ConsistencyLevel.ONE.
        
   cluster = Cluster.builder()
                .addContactPoints( contactPointArray )
                //.withLoadBalancingPolicy(new RoundRobinPolicy())
                .withPoolingOptions(poolingOptions)
                .withQueryOptions( queryOptions )
                .build();

https://github.com/datastax/java-driver/tree/3.x/manual

http://docs.datastax.com/en/drivers/java/3.2/com/datastax/driver/core/QueryOptions.html