The zeromq filter within logstash 1.4 is not working out as expected for me. I opened an issue on github to cope with that. For now I work around this issue by starting logstash 1.3 as a separate instance and let this version deal with zeromq.

Come again?!

For those of you asking WTF?... :) ZeroMQ is a message library that provides multiple patterns like PUB/SUB, PUSH/PULL and others. I got a use-case in which I want specific log events to be handled outside of logstash. And more outside then firing up the ruby filter. I want to process the event within a external daemon to check some things, update the JSON with additional information (lookup names, routes or alike) and after I am done I push it back into the logstash pipeline.

Bugfixed proposed

There is a bugfix provided by barravi which I am going to try out. Since I do not have a ruby or java strong suit in my closet, I figured I should blog about my experience first hand. :)

Set up the development environment

Since I like docker I created a branch which provides a docker file to spin up a fresh dev environment. To use it; just build and run it.

$ git clone -b docker_dev
$ cd logstash
$ fig build
$ fig up -d

Show error scenario

To verify that the problem still exists, I will reproduce the error. I connect to the host, and setup a small zmq test server.

$ docker exec -ti logstash_logstash_1 bash
container $ cat << \EOF > /tmp/
import zmq
context = zmq.Context()
consumer_receiver = context.socket(zmq.REP)
while True:
    work = consumer_receiver.recv_json()
    print work
container $ python /tmp/ &

The logstash config used is pretty basic. Stdin piped through zeromq and pushed out via stdout.

container $ cat << \EOF > /tmp/all.config
input {
    stdin {}

filter {
   zeromq {
        address => "tcp://localhost:5557"

output {
    stdout {}

If this is fired up with the installed logstash version it comes down to this:

container $ /opt/logstash/bin/logstash -f /tmp/all.config
Using milestone 1 filter plugin 'zeromq'. This plugin should work, but would benefit from use by folks like you. Please let us know if you find bugs or have suggestions on how to improve this plugin.  For more information on plugin milestones, see {:level=>:warn}
{u'host': u'0e6a6256aae8', u'message': u'Test', u'@version': u'1', u'@timestamp': u'2015-04-19T09:41:46.474Z'}
0mq filter exception {:address=>"tcp://localhost:5557", :exception=>#<NoMethodError: undefined method `[]' for nil:NilClass>, :backtrace=>["/opt/logstash/lib/logstash/event.rb:163:in `overwrite'", "/opt/logstash/lib/logstash/filters/zeromq.rb:197:in `filter'", "(eval):23:in `initialize'", "org/jruby/ `call'", "/opt/logstash/lib/logstash/pipeline.rb:262:in `filter'", "/opt/logstash/lib/logstash/pipeline.rb:203:in `filterworker'", "/opt/logstash/lib/logstash/pipeline.rb:143:in `start_filters'"], :level=>:warn}
NoMethodError: undefined method `[]' for nil:NilClass
        sprintf at /opt/logstash/lib/logstash/event.rb:224
           gsub at org/jruby/
        sprintf at /opt/logstash/lib/logstash/event.rb:216
           to_s at /opt/logstash/lib/logstash/event.rb:99
         encode at /opt/logstash/lib/logstash/codecs/line.rb:54
        receive at /opt/logstash/lib/logstash/outputs/stdout.rb:57
         handle at /opt/logstash/lib/logstash/outputs/base.rb:86
     initialize at (eval):35
           call at org/jruby/
         output at /opt/logstash/lib/logstash/pipeline.rb:266
   outputworker at /opt/logstash/lib/logstash/pipeline.rb:225
  start_outputs at /opt/logstash/lib/logstash/pipeline.rb:152

Test bugfix

Now I am going to fetch the bug fixed version of the filter, bootstrap my logstash branch and install the plugin.

container $ cd /opt
container $ git clone
container $ cd /logstash/
container $ echo 'gem "logstash-filter-zeromq", :path => "/opt/logstash-filter-zeromq/"' >> Gemfile
container $ rake bootstrap
container $ bin/plugin install --no-verify

Workaround (?)

Somehow I need to add some standard input/outputs and codecs to get it going... I guess the libs are not in the right ruby path or something like that. Maybe it's even nicer this way, because I made sure not to use the default plugins.

container $ cp /opt/logstash/lib/logstash/inputs/stdin.rb lib/logstash/inputs/ 
container $ cp /opt/logstash/lib/logstash/outputs/stdout.rb lib/logstash/outputs/
container $ cp /opt/logstash/lib/logstash/codecs/{json.rb,line.rb,plain.rb,json_lines.rb} lib/logstash/codecs/

Run the thing:

container $ ./bin/logstash -f /tmp/all.config
Logstash startup completed
{u'host': u'0e6a6256aae8', u'message': u'Test', u'@version': u'1', u'@timestamp': u'2015-04-19T09:49:25.928Z'}
2015-04-19T09:49:25.928Z 0e6a6256aae8 Test
It works! Yeah!!
{u'host': u'0e6a6256aae8', u'message': u'It works! Yeah!!', u'@version': u'1', u'@timestamp': u'2015-04-19T09:49:33.925Z'}
2015-04-19T09:49:33.925Z 0e6a6256aae8 It works! Yeah!!

Nice, the error is gone.