Usage of the LOCAL_QUORUM consistency level in Datastax driver

Question

For some reasons I need to query a particular datacenter within my cassandra cluster. According to the documentation, I can use the LOCAL_QUORUM consistency level:

Returns the record after a quorum of replicas in the current datacenter as the coordinator has reported. Avoids latency of inter-datacenter communication.

Do I correctly understand, that in order to specify a particular datacenter for the current query, I have to build a cluster on the given endpoint belonging to this particular DC?

Say, I have two DC's with the following nodes:

DC1: 172.0.1.1, 172.0.1.2
DC1: 172.0.2.1, 172.0.2.2

So, to work with DC1, I build a cluster as:

Cluster cluster = Cluster.builder().addContactPoint("172.0.1.1").build();
Session session = cluster.connect();
Statement statement = session.prepare("select * from ...").bind().setConsistencyLevel(ConsistencyLevel.LOCAL_QUORUM);
ResultSet resultSet = session.execute(session);

Is it a proper way to do that?

Aaron Aaron · Accepted Answer · 2018-01-30T14:14:27

By itself, DCAwwareRoundRobinPolicy will pick the data center that it finds with the "least network distance" algorithm. To ensure it connects where you want, you should specify the DC as a parameter.

Here is how I tell our dev teams to do it:

Builder builder = Cluster.builder()
  .addContactPoints(nodes)
  .withQueryOptions(new QueryOptions()
    .setConsistencyLevel(ConsistencyLevel.LOCAL_ONE))
  .withLoadBalancingPolicy(new TokenAwarePolicy(
    new DCAwareRoundRobinPolicy.Builder()
      .withLocalDc("DC1").build()))
  .withPoolingOptions(options);

Note: this may or may not be applicable to your situation, but do I recommend using the TokenAwarePolicy with the DCAwareRoundRobin nested inside it (specifying the local DC). That way any operation specifying the partition key will automatically route to the correct node, skipping the need for an extra hop required with a coordinator node.

Usage of the LOCAL_QUORUM consistency level in Datastax driver

2 Answers