Implementing custom load balancing policies

To implement a load balancing policy, you must implement all of the methods specified in Cassandra::LoadBalancing::Policy. Currently, load balancing policies are required to be thread-safe.

The object returned from the plan method must implement all methods of Cassandra::LoadBalancing::Plan

Plan will be accessed from multiple threads, but never in parallel and it doesn’t have to be thread-safe.

Background

Given

a running cassandra cluster with schema:

CREATE KEYSPACE simplex WITH replication = {'class': 'SimpleStrategy', 'replication_factor': 3};
USE simplex;
CREATE TABLE songs (
  id uuid PRIMARY KEY,
  title text,
  album text,
  artist text,
  tags set<text>,
  data blob
);
INSERT INTO songs (id, title, album, artist, tags)
VALUES (
   756716f7-2e54-4715-9f00-91dcbea6cf50,
   'La Petite Tonkinoise',
   'Bye Bye Blackbird',
   'Joséphine Baker',
   {'jazz', '2013'})
;
INSERT INTO songs (id, title, album, artist, tags)
VALUES (
   f6071e72-48ec-4fcb-bf3e-379c8a696488,
   'Die Mösch',
   'In Gold',
   'Willi Ostermann',
   {'kölsch', '1996', 'birds'}
);
INSERT INTO songs (id, title, album, artist, tags)
VALUES (
   fbdf82ed-0063-4796-9c7c-a3d4f47b4b25,
   'Memo From Turner',
   'Performance',
   'Mick Jager',
   {'soundtrack', '1991'}
);

A policy to ignore a certain keyspace

Given

a file named “ignoring_keyspace_policy.rb” with:

class IgnoringKeyspacePolicy
  class Plan
    def has_next?
      false
    end

    def next
      nil
    end
  end

  def initialize(keyspace_to_ignore, original_policy)
    @keyspace = keyspace_to_ignore
    @policy   = original_policy
  end

  def setup(cluster)
  end

  def plan(keyspace, statement, options)
    if @keyspace == keyspace
      Plan.new
    else
      @policy.plan(keyspace, statement, options)
    end
  end

  def distance(host)
    @policy.distance(host)
  end

  def host_found(host)
    @policy.host_found(host)
  end

  def host_lost(host)
    @policy.host_lost(host)
  end

  def host_up(host)
    @policy.host_up(host)
  end

  def host_down(host)
    @policy.host_down(host)
  end
end

And

the following example:

require 'cassandra'
require 'ignoring_keyspace_policy'

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

begin
  session.execute("SELECT * FROM songs")
  puts "failure"
rescue Cassandra::Errors::NoHostsAvailable
  puts "success"
end

When

it is executed

Then

its output should contain:

success

A policy to ignore certain hosts

Given

a file named “blacklist_policy.rb” with:

class BlackListPolicy
  def initialize(ips_to_ignore, original_policy)
    @ips    = ::Set.new
    @policy = original_policy

    ips_to_ignore.each do |ip|
      case ip
      when ::IPAddr
        @ips << ip
      when ::String
        @ips << ::IPAddr.new(ip)
      end
    end
  end

  def setup(cluster)
  end

  def plan(keyspace, statement, options)
    @policy.plan(keyspace, statement, options)
  end

  def distance(host)
    @policy.distance(host)
  end

  def host_found(host)
    @policy.host_found(host) unless @ips.include?(host.ip)
  end

  def host_lost(host)
    @policy.host_lost(host) unless @ips.include?(host.ip)
  end

  def host_up(host)
    @policy.host_up(host) unless @ips.include?(host.ip)
  end

  def host_down(host)
    @policy.host_down(host) unless @ips.include?(host.ip)
  end
end

And

the following example:

require 'cassandra'
require 'blacklist_policy'

policy  = BlackListPolicy.new(['127.0.0.2', '127.0.0.3'], Cassandra::LoadBalancing::Policies::RoundRobin.new)
cluster = Cassandra.cluster(load_balancing_policy: policy)
session = cluster.connect('simplex')

host_ips = cluster.hosts.map(&:ip).sort

coordinator_ips = 3.times.map do
  info = session.execute("SELECT * FROM songs").execution_info
  info.hosts.last.ip
end

puts "Cluster hosts:"
puts host_ips
puts ""
puts "Hosts used:"
puts coordinator_ips.sort.uniq

When

it is executed

Then

its output should contain:

Cluster hosts:
127.0.0.1
127.0.0.2
127.0.0.3

Hosts used:
127.0.0.1