membership change detection

Cluster object allows registering state listeners. It then gurantees that they will be notifies on cluster membership changes.

Background

Given

a running cassandra cluster

And

a file named “printing_listener.rb” with:

class PrintingListener
  def initialize(io)
    @out = io
  end

  def host_found(host)
    @out.puts("Host #{host.ip} is found")
  end

  def host_lost(host)
    @out.puts("Host #{host.ip} is lost")
  end

  def host_up(host)
    @out.puts("Host #{host.ip} is up")
  end

  def host_down(host)
    @out.puts("Host #{host.ip} is down")
  end
end

And

the following example:

require 'printing_listener'
require 'cassandra'

listener = PrintingListener.new($stderr)
cluster  = Cassandra.cluster

cluster.register(listener)

$stdout.puts("=== START ===")
$stdout.flush
$stdin.gets
$stdout.puts("=== STOP ===")
$stdout.flush

And

it is running interactively

And

I wait for its output to contain “START”

some existing hosts are terminated

When

node 3 stops

And

node 2 stops

And

I close the stdin stream

Then

its output should contain:

Host 127.0.0.3 is down
Host 127.0.0.2 is down

a new host joins and then leaves the cluster

When

node 4 joins

And

node 4 leaves

And

I close the stdin stream

Then

its output should contain:

Host 127.0.0.4 is found
Host 127.0.0.4 is up
Host 127.0.0.4 is down
Host 127.0.0.4 is lost