[atombender]

Elasticsearch may not be fundamentally flawed, but it sure is flawed!

It's operationally unpredictable, even if you know all the corner cases (like field cache sizes) and JVM flag tuning voodoo. It's notoriously memory-hungry, and its networking is notoriously unstable. I've had issues where minor network blips throw the entire cluster into a weird quantum state where nodes are up but the cluster is down.

One particular annoyance with ES is that, once it starts having issues, it often becomes completely unresponsive, and its actual status becomes difficult to understand. You often can't access endpoints like /_cluster/health, /_cat/shards, etc. to diagnose, and meanwhile the logs are spewing inscutable Java stack traces that are of no help. There are clearly weird bottlenecks inside ES which fail in extreme circumstances.

It's gotten better. The consensus protocol was brittle and unsound for many years, and has slowly been patched for robustness, but I wouldn't say it's been fixed. ES is much more unreliable than many other clustered systems. The only one that comes to mind as being as unreliable is RabbitMQ.