OK, so does that mean that internally ES has some kind of queue for unprocessed shards, and it just so happened that it's running behind what's coming in, but it's only the formal shutdown/restart that exposes things in this queue to us via the /_cluster/health command? Is there an API call that would expose this backlog? I currently monitor /_cluster/health (which obviously isn't showing this) but would rather have a better option that actually told me a backlog was forming. We are currently on one node and will expand into a cluster - but I'd still like to know when this backlog starts happening, as it tells you something it going to go wrong soon if it only increases over time
Thanks
Jason