initial commit: it works!

Gemfile

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+source 'https://rubygems.org'
+gemspec

Gemfile.lock

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+PATH
+  remote: .
+  specs:
+    logstash-codec-fieldselect (0.0.1)
+      logstash-core (>= 2.0.0.beta2, < 3.0.0)
+      logstash-mixin-aws
+      stud (~> 0.0.22)
+
+GEM
+  remote: https://rubygems.org/
+  specs:
+    aws-sdk (2.1.36)
+      aws-sdk-resources (= 2.1.36)
+    aws-sdk-core (2.1.36)
+      jmespath (~> 1.0)
+    aws-sdk-resources (2.1.36)
+      aws-sdk-core (= 2.1.36)
+    aws-sdk-v1 (1.66.0)
+      json (~> 1.4)
+      nokogiri (>= 1.4.4)
+    cabin (0.7.2)
+    clamp (0.6.5)
+    coderay (1.1.0)
+    concurrent-ruby (0.9.2-java)
+    diff-lcs (1.2.5)
+    ffi (1.9.10-java)
+    filesize (0.0.4)
+    gem_publisher (1.5.0)
+    gems (0.8.3)
+    i18n (0.6.9)
+    insist (1.0.0)
+    jmespath (1.1.3)
+    jrjackson (0.3.7)
+    jruby-openssl (0.9.12-java)
+    json (1.8.3-java)
+    kramdown (1.9.0)
+    logstash-codec-line (2.0.2)
+      logstash-core (>= 2.0.0.beta2, < 3.0.0)
+    logstash-codec-plain (2.0.2)
+      logstash-core (>= 2.0.0.beta2, < 3.0.0)
+    logstash-core (2.1.0.snapshot2-java)
+      cabin (~> 0.7.0)
+      clamp (~> 0.6.5)
+      concurrent-ruby (= 0.9.2)
+      filesize (= 0.0.4)
+      gems (~> 0.8.3)
+      i18n (= 0.6.9)
+      jrjackson (~> 0.3.7)
+      jruby-openssl (>= 0.9.11)
+      minitar (~> 0.5.4)
+      pry (~> 0.10.1)
+      rubyzip (~> 1.1.7)
+      stud (~> 0.0.19)
+      thread_safe (~> 0.3.5)
+      treetop (< 1.5.0)
+    logstash-devutils (0.0.18-java)
+      gem_publisher
+      insist (= 1.0.0)
+      kramdown
+      minitar
+      rake
+      rspec (~> 3.1.0)
+      rspec-wait
+      stud (>= 0.0.20)
+    logstash-input-generator (2.0.2)
+      logstash-codec-plain
+      logstash-core (>= 2.0.0.beta2, < 3.0.0)
+    logstash-mixin-aws (2.0.2)
+      aws-sdk (~> 2.1.0)
+      aws-sdk-v1 (>= 1.61.0)
+      logstash-codec-plain
+      logstash-core (>= 2.0.0.beta2, < 3.0.0)
+    method_source (0.8.2)
+    minitar (0.5.4)
+    nokogiri (1.6.6.3-java)
+    polyglot (0.3.5)
+    pry (0.10.3-java)
+      coderay (~> 1.1.0)
+      method_source (~> 0.8.1)
+      slop (~> 3.4)
+      spoon (~> 0.0)
+    rake (10.4.2)
+    rspec (3.1.0)
+      rspec-core (~> 3.1.0)
+      rspec-expectations (~> 3.1.0)
+      rspec-mocks (~> 3.1.0)
+    rspec-core (3.1.7)
+      rspec-support (~> 3.1.0)
+    rspec-expectations (3.1.2)
+      diff-lcs (>= 1.2.0, < 2.0)
+      rspec-support (~> 3.1.0)
+    rspec-mocks (3.1.3)
+      rspec-support (~> 3.1.0)
+    rspec-support (3.1.2)
+    rspec-wait (0.0.8)
+      rspec (>= 2.11, < 3.5)
+    rubyzip (1.1.7)
+    slop (3.6.0)
+    spoon (0.0.4)
+      ffi
+    stud (0.0.22)
+    thread_safe (0.3.5-java)
+    treetop (1.4.15)
+      polyglot
+      polyglot (>= 0.3.1)
+
+PLATFORMS
+  java
+
+DEPENDENCIES
+  logstash-codec-fieldselect!
+  logstash-codec-line
+  logstash-devutils
+  logstash-input-generator
+
+BUNDLED WITH
+   1.10.6

Rakefile

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+@files=[]
+
+task :default do
+  system("rake -T")
+end
+
+require "logstash/devutils/rake"

lib/logstash/codecs/fieldselect.rb

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+# encoding: utf-8
+require "logstash/codecs/base"
+require "logstash/codecs/line"
+
+class LogStash::Codecs::Fieldselect < LogStash::Codecs::Base
+
+  # This codec will select only the fields listed or default
+  # to only selecting message
+  #
+  # input {
+  #   stdin { codec => fieldselect { fields => [] }
+  # }
+  # 
+  # or
+  # 
+  # output {
+  #   stdout { codec => fieldselect { fields => [] }
+  # }
+  config_name "fieldselect"
+
+  # Select fields to output, concated with a comma
+  config :fields, :validate => :array, :default => ['message']
+
+  public
+  def register
+    @lines = LogStash::Codecs::Line.new
+    @lines.charset = "UTF-8"
+  end
+
+  public
+  def decode(data)
+    @lines.decode(data) do |line|
+      selected = field_selector(line)
+      yield LogStash::Event.new(selected)
+    end
+  end # def decode
+
+  public
+  def encode(event)
+    encoded = field_concater(event)
+    @on_event.call(event, encoded)
+  end # def encode
+
+  private
+  def field_selector(data)
+    selected = {}
+    @fields.each do |field|
+      selected[field] = data[field]
+    end
+    return selected
+  end
+
+  private
+  def field_concater(data)
+    concated = ""
+    @fields.each do |field|
+      concated += data[field] + ", "
+    end
+    concated.chomp!(", ")
+    return concated + "\n"
+  end
+end # class LogStash::Codecs::Fieldselect

logstash-codec-fieldselect.gemspec

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+Gem::Specification.new do |s|
+
+  s.name            = 'logstash-codec-fieldselect'
+  s.version         = '0.0.1'
+  s.licenses        = ['Apache License (2.0)']
+  s.summary         = "This codec was created to select certain fields for output"
+  s.description     = "This gem is a logstash plugin required to be installed on top of the Logstash core pipeline using $LS_HOME/bin/plugin install gemname. This gem is not a stand-alone program"
+  s.authors         = ["Elastic"]
+  s.email           = '[email protected]'
+  s.homepage        = "http://www.elastic.co/guide/en/logstash/current/index.html"
+  s.require_paths = ["lib"]
+
+  # Files
+  s.files = Dir['lib/**/*','spec/**/*','vendor/**/*','*.gemspec','*.md','CONTRIBUTORS','Gemfile','LICENSE','NOTICE.TXT']
+
+  # Tests
+  s.test_files = s.files.grep(%r{^(test|spec|features)/})
+
+  # Special flag to let us know this is actually a logstash plugin
+  s.metadata = { "logstash_plugin" => "true", "logstash_group" => "output" }
+
+  # Gem dependencies
+  s.add_runtime_dependency "logstash-core", ">= 2.0.0.beta2", "< 3.0.0"
+  s.add_runtime_dependency 'logstash-mixin-aws'
+  s.add_runtime_dependency 'stud', '~> 0.0.22'
+  s.add_development_dependency 'logstash-devutils'
+  s.add_development_dependency 'logstash-input-generator'
+  s.add_development_dependency 'logstash-codec-line'
+end
+

spec/codecs/fieldselect_spec.rb

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+# encoding: utf-8
+
+require "logstash/devutils/rspec/spec_helper"
+require "logstash/codecs/fieldselect"
+require "logstash/event"
+
+describe LogStash::Codecs::Fieldselect do
+  context "default settings" do
+    subject do
+      next LogStash::Codecs::Fieldselect.new
+    end
+
+    context "#encode" do
+      let (:event) {LogStash::Event.new({"message" => "hello world", "host" => "test", "leggo" => "my eggo"})}
+
+      it "should return only the default select field, message" do
+        expect(subject).to receive(:on_event).once.and_call_original
+        subject.on_event do |e, d|
+          insist {d} == event['message'] + "\n"
+        end
+        subject.encode(event)
+      end
+    end
+
+    context "#decode" do
+      it "should return only the default select field, message" do
+        decoded = false
+        subject.decode("hello world\n") do |e|
+          decoded = true
+          insist { e.is_a?(LogStash::Event) }
+          insist { e["message"] } == "hello world"
+          insist { e["host"] } == nil
+        end
+        insist { decoded } == true
+      end
+
+    end
+  end
+
+  context "select specific fields" do
+    subject do
+      next LogStash::Codecs::Fieldselect.new({'fields' => ['message', 'leggo']})
+    end
+
+    context "#encode" do
+      it "should return both selected fields, message and leggo" do
+        expect(subject).to receive(:on_event).once.and_call_original
+        subject.on_event do |e, d|
+          insist {d} == event['message'] + ", " + event['leggo'] + "\n"
+        end
+      end
+    end
+  end
+end
+