New write_behavior setting.

This setting introduces a new behavior to allow each event to overwrite
the entire file.

The default behavior (append) is unchanged.

Fixes #24

Author: jordansissel

lib/logstash/outputs/file.rb

@@ -1,6 +1,7 @@
 # encoding: utf-8
 require "logstash/namespace"
 require "logstash/outputs/base"
+require "flores/random"
 require "logstash/errors"
 require "zlib"
 
@@ -66,6 +67,16 @@ class LogStash::Outputs::File < LogStash::Outputs::Base
   # Example: `"file_mode" => 0640`
   config :file_mode, :validate => :number, :default => -1
 
+
+  # How should the file be written?
+  #
+  # If `append`, the file will be opened for appending and each new event will
+  # be written at the end of the file.
+  #
+  # If `overwrite`, the file will be truncated before writing and only the most
+  # recent event will appear in the file.
+  config :write_behavior, :validate => [ "overwrite", "append" ], :default => "append"
+
   default :codec, "json_lines"
 
   public
@@ -130,7 +141,14 @@ def multi_receive_encoded(events_and_encoded)
     @io_mutex.synchronize do
       encoded_by_path.each do |path,chunks|
         fd = open(path)
-        chunks.each {|chunk| fd.write(chunk) }
+        if @write_behavior == "overwrite"
+          fd.truncate(0)
+          fd.seek(0, IO::SEEK_SET)
+          fd.write(chunks.last)
+        else
+          # append to the file
+          chunks.each {|chunk| fd.write(chunk) }
+        end
         fd.flush
       end
 

logstash-output-file.gemspec

@@ -25,5 +25,6 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency 'logstash-codec-line'
 
   s.add_development_dependency 'logstash-devutils'
+  s.add_development_dependency 'flores'
   s.add_development_dependency 'logstash-input-generator'
 end

spec/outputs/file_spec.rb

@@ -2,6 +2,7 @@
 require "logstash/devutils/rspec/spec_helper"
 require "logstash/outputs/file"
 require "logstash/codecs/line"
+require "logstash/codecs/json_lines"
 require "logstash/event"
 require "logstash/json"
 require "stud/temporary"
@@ -111,6 +112,49 @@
 
   describe "receiving events" do
 
+    context "when write_behavior => 'overwrite'" do
+      let(:tmp) { Stud::Temporary.pathname }
+      let(:config) {
+        { 
+          "write_behavior" => "overwrite",
+          "path" => tmp,
+          "codec" => LogStash::Codecs::JSONLines.new
+        }
+      }
+      let(:output) { LogStash::Outputs::File.new(config) }
+
+      let(:count) { Flores::Random.integer(1..10) }
+      let(:events) do 
+        Flores::Random.iterations(1..10).collect do |i|
+          LogStash::Event.new("value" => i)
+        end
+      end
+
+      before do
+        output.register
+      end
+
+      after do
+        File.unlink(tmp) if File.exist?(tmp)
+      end
+
+      it "should write only the last event of a batch" do
+        output.multi_receive(events)
+        result = LogStash::Json.load(File.read(tmp))
+        expect(result["value"]).to be == events.last.get("value")
+      end
+
+      context "the file" do
+        it "should only contain the last event received" do
+          events.each do |event|
+            output.multi_receive([event])
+            result = LogStash::Json.load(File.read(tmp))
+            expect(result["value"]).to be == event.get("value")
+          end
+        end
+      end
+    end
+
     context "when the output file is deleted" do
 
       let(:temp_file) { Tempfile.new('logstash-spec-output-file_deleted') }