Execution information

Every result contains useful execution information.

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'}
);

execution information is accessible from execution result

Given

the following example:

require 'cassandra'

cluster = Cassandra.cluster

at_exit { cluster.close }

session   = cluster.connect("simplex")
execution = session.execute("SELECT * FROM songs", consistency: :one).execution_info

puts "coordinator: #{execution.hosts.last.ip}"
puts "keyspace: #{execution.keyspace}"
puts "cql: #{execution.statement.cql}"
puts "requested consistency: #{execution.options.consistency}"
puts "actual consistency: #{execution.consistency}"
puts "number of retries: #{execution.retries}"

When

it is executed

Then

its output should match:

coordinator: 127\.0\.0\.(1|2|3)

And

its output should contain:

keyspace: simplex
cql: SELECT * FROM songs
requested consistency: one
actual consistency: one
number of retries: 0

execution information reflects retry decision

Given

a file named “retrying_at_a_given_consistency_policy.rb” with:

class RetryingAtAGivenConsistencyPolicy
  include Cassandra::Retry::Policy

  def initialize(consistency_to_use)
    @consistency_to_use = consistency_to_use
  end

  def read_timeout(statement, consistency_level, required_responses,
                   received_responses, data_retrieved, retries)
    try_again(@consistency_to_use)
  end

  def write_timeout(statement, consistency_level, write_type,
                    acks_required, acks_received, retries)
    try_again(@consistency_to_use)
  end

  def unavailable(statement, consistency_level, replicas_required,
                  replicas_alive, retries)
    try_again(@consistency_to_use)
  end
end

And

the following example:

require 'cassandra'
require 'retrying_at_a_given_consistency_policy'

cluster   = Cassandra.cluster(retry_policy: RetryingAtAGivenConsistencyPolicy.new(:one))
session   = cluster.connect("simplex")
execution = session.execute("SELECT * FROM songs", consistency: :all).execution_info

puts "requested consistency: #{execution.options.consistency}"
puts "actual consistency: #{execution.consistency}"
puts "number of retries: #{execution.retries}"

When

node 3 stops

And

it is executed

Then

its output should contain:

requested consistency: all
actual consistency: one
number of retries: 1