I'm learning how Elasticsearch (version 5.3.0) works in order to try and use it. I've read documentation, Elasticsearch Reference and some ES blog posts too but I couldn't find how indices (shards?) recovery works.
Let assume a node A turn off and, then, become active again. If the cluster didn't stop its activity and some documents were indexed, how are those changes synchronized with the node A? Does ES replace all files or there is a mechanism to communicate only changes to that node?
References and documentation are welcomed.
Thank you in advance for the responses.
These days Elasticsearch is doing a diff between the segments (files) in primary shard and the ones in the replica shard. What is different is copied over new from the primary.
In future though (ES 6), there will be sequence IDs: https://github.com/elastic/elasticsearch/issues/10708
The advantage of having these is that ES will make a first attempt to compare the sequence IDs from the primary and replica and see how "far" they are apart. If the translog from the primary shard still has all the changes since the replica went offline, ES will simply replay the operations in the primary shard translog on the replica shard. If not all the operations are there anymore, then it will get back to the segments diffing (the current approach).