Hello!
Our logstash servers are the last servers to upgrade to elasticsearch 5.5.2. I'm planning to do this upgrade on Thursday December 7, between 9:00 and 10:00 UTC (10:00 to 11:00 CET, 01:00 to 02:00 PST).
Testing on labs went well, so I'm not expecting any issue. Now that all servers are behind LVS, we should not even loose a single message. That being said, there is a risk. If you see disapearing log messages during that period, please let me know!
Have fun!
Guillaume