Logstash script for ruby filter plugin to destructively merge two array fields

destructive-array-merge.logstash-filter-ruby.rb

###############################################################################
# destructive-array-merge.logstash-filter-ruby.rb
# ---------------------------------
# A script for a Logstash Ruby Filter to destructively merge one array field
# into another.
#
# This script has three parameters:
# - `source`: a field reference to the source array
# - `target`: a field reference to the target array
# - `coerce`: when encountering non-array values in either source or target
#             fields, setting `coerce => true` causes these fields to first
#             be converted to a single-entry array.
###############################################################################
#
# Copyright 2020 Ry Biesemeyer
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.
def register(params)
  params = params.dup # isolate

  @source = params.delete('source') || report_configuration_error("missing required param `source`")
  @target = params.delete('target') || report_configuration_error("missing required param `target`")

  coerce = params.delete('coerce') || 'false'
  @coerce = case coerce
            when 'true'  then true
            when 'false' then false
            else report_configuration_error("invalid value for param `coerce`: `#{coerce}` (expected `true` or `false`)")
            end

  params.empty? || report_configuration_error("unknown script parameter(s): #{params.keys}.")
end

def report_configuration_error(message)
  raise LogStash::ConfigurationError, message
end

def filter(event)
  timestamp = event.get('@timestamp')

  return [event] unless event.include?(@source)

  source = event.get(@source)
  unless @coerce || source.kind_of?(::Array)
    logger.debug("source field `#{@source}` not an array", :event => event.to_hash) if logger.debug?
    return [event]
  end
  target = event.get(@target)
  unless target.nil? || @coerce || target.kind_of?(::Array)
    logger.debug("target field `#{@target}` not an array", :event => event.to_hash) if logger.debug?
    return [event]
  end

  # to prevent partial failure, we clone the event and return the
  # modified clone on success or the original unmodified event
  # on failure
  clone_event = LogStash::Event.new(event.to_hash_with_metadata)
  source = clone_event.remove(@source)
  source = Array(source) if @coerce

  return [event] if source.empty?

  merge_target = clone_event.get(@target) || []
  merge_target = Array(merge_target) if @coerce
  source.each do |value|
    merge_target << value
  end
  clone_event.set(@target, merge_target)

  event.cancel

  return [clone_event]
rescue => e
  logger.error('failed to merge fields on event', exception: e.message, event: event.to_hash, source: @source, target: @target)
  event.tag('_arraymerge_error')
  [event]
end

USAGE.md

This script has three parameters:

• source: a field reference to the source array
• target: a field reference to the target array
• coerce: (optional: default false) when encountering non-array values in either source or target fields, setting coerce => true causes these fields to first be converted to a single-entry array.

filter {
  ruby {
    path => "${PWD}/destructive-array-merge.logstash-filter-ruby.rb"
    script_params => {
      source => '[path][to][source][field]'
      target => '[target][field]'
    }
  }
}