When I set the "workers" parameter to any value bigger than 1 on the elasticsearch plugin (with embedded => true in my case), no data gets written to elasticsearch:
... and obviously kibana and friends fail to fetch data from elasticsearch.
When I set "workers => 1", everything work fine.
To add confusion, when I start with 1 worker and later change to something > 1 and restart logstash, http://localhost:9200/_status?pretty shows the index is there, but a closer look shows that nothing is getting written to it anymore (various *size counters aren't incremented anymore). And obviously no new events show up in kibana.
FTR, relevant parts of my config:
running with "-w 8"