Round Robin Policy

The Round Robin load balancing policy dispatches requests evenly on cluster nodes.

The effects of the policy can be seen by enabling requests tracing. The coordinator node that served every request is the last host in execution info.

Background

Given

a running cassandra cluster in 2 datacenters with 2 nodes in each

And

the following schema:

CREATE KEYSPACE simplex WITH replication = {'class': 'NetworkTopologyStrategy', 'dc1': '2', 'dc2': '2'};
CREATE TABLE simplex.songs (
  id uuid PRIMARY KEY,
  title text,
  album text,
  artist text,
  tags set<text>,
  data blob
);
INSERT INTO simplex.songs (id, title, album, artist, tags)
VALUES (
   756716f7-2e54-4715-9f00-91dcbea6cf50,
   'La Petite Tonkinoise',
   'Bye Bye Blackbird',
   'Joséphine Baker',
   {'jazz', '2013'})
;
INSERT INTO simplex.songs (id, title, album, artist, tags)
VALUES (
   f6071e72-48ec-4fcb-bf3e-379c8a696488,
   'Die Mösch',
   'In Gold',
   'Willi Ostermann',
   {'kölsch', '1996', 'birds'}
);
INSERT INTO simplex.songs (id, title, album, artist, tags)
VALUES (
   fbdf82ed-0063-4796-9c7c-a3d4f47b4b25,
   'Memo From Turner',
   'Performance',
   'Mick Jager',
   {'soundtrack', '1991'}
);

Configuring Round Robin load balancing policy

Given

the following example:

require 'cassandra'

policy  = Cassandra::LoadBalancing::Policies::RoundRobin.new
cluster = Cassandra.cluster(load_balancing_policy: policy)
session = cluster.connect('simplex')

coordinator_ips = 4.times.map do
  info = session.execute("SELECT * FROM songs").execution_info
  info.hosts.last.ip
end

puts coordinator_ips.sort

When

it is executed

Then

its output should contain:

127.0.0.1
127.0.0.2
127.0.0.3
127.0.0.4

Round Robin policy ignores datacenters

Given

the following example:

  require 'cassandra'

  policy  = Cassandra::LoadBalancing::Policies::RoundRobin.new
  cluster = Cassandra.cluster(load_balancing_policy: policy)
  session = cluster.connect('simplex')

  coordinator_ips = 4.times.map do
    info = session.execute("SELECT * FROM songs").execution_info
    info.hosts.last.ip
  end

  puts coordinator_ips.sort

When

it is executed

Then

its output should contain:

127.0.0.1
127.0.0.2
127.0.0.3
127.0.0.4