This is for me one year later. I ran into this problem when trying to customize logstash-input-redis plugin, I added require 'redis-cluster-client', done the code, bundle execute rspec, gem build and logstash-plugin install ... successfully but logstash failed to start. 3 days of hair pulling later after, I enabled debug log with --log.level debug and found out that it could not load 'redis-cluster-client' and the evil thing here is this crash-causing issue is logged in debug level.

I had also tried to follow Logstash's document with external dependencies but it doesn't seem to do anything better.

Finally I get it to work by changing JRUBY_HOME env var from the binaries I have downloaded to JRUBY_HOME=$LOGSTASH_HOME/vendor/jruby, then I clean up the gem, remove then reinstall the plugin and it finally works.

if this may helped you, please give me a star https://github.com/tai-tran-tan/logstash-input-redis_cluster Good luck my friends!