[ElasticSearch] Refresh/Flush/Optimize - by elasticsearch.org

원본) http://www.elasticsearch.org/guide/en/elasticsearch/guide/current/inside-a-shard.html

1. near real-time search

refresh apiedit

In Elasticsearch, this lightweight process of writing and opening a new segment is called a refresh. By default, every shard is refreshed automatically once every second. This is why we say that Elasticsearch has near real-time search: document changes are not visible to search immediately, but will become visible within one second.

2. making changes persistent

flush apiedit

The action of performing a commit and truncating the translog is known in Elasticsearch as a flush. Shards are flushed automatically every 30 minutes, or when the translog becomes too big. See thetranslog documentation for settings that can be used to control these thresholds.

3. segment merging 

optimize apiedit

The optimize API is best described as the forced merge API. It forces a shard to be merged down to the number of segments specified in the max_num_segments parameter. The intention is to reduce the number of segments (usually to 1) in order to speed up search performance.