logstash-5.x gelf input multiline codec doesn't work

Question

I have a simple and straightforward config and I'm not sure what I'm doing wrong trying to get this multiline working.

input {
    gelf {
        codec => multiline {
            pattern => "^%{TIMESTAMP_ISO8601} "
            negate => true
            what => "previous"
        }
    }
}

filter {}

output {
    # I have the relevant ES hosts & index here
    elasticsearch { }
    stdout {
        codec => rubydebug
    }
}

And I'm testing it as below and I get single lines, multiline tag doesn't get added and I see it in logstash debug logs that events go through filter one by one.

docker run -it --log-driver gelf --log-opt gelf-address=udp://127.0.0.1:12201 \
    --log-opt tag=mline-test python:alpine \
    python -c 'print("[2017-10-18 00:00:00,000] Hello world");assert False'

• I tested this with both logstash versions 5.5.2 & 5.6.3 => elasticsearch 5.5
• I have logstash-codec-multiline installed
• I know that I can do this with the (now removed) multiline filter in logstash 2.4

Answer 1

I have discovered that the codecs are ignored in gelf input plugin: logstash-plugins/logstash-input-gelf#37

This means that the only multiline option with gelf is using logstash-2.4 and taking the single-thread performance hit.

Settings: Default pipeline workers: 4
Defaulting pipeline worker threads to 1 because there are some filters that might not work with multiple worker threads {:count_was=>4, :filters=>["multiline"], :level=>:warn}
Pipeline main started