Distributed indexing

Indexing process

1. When you index a document, the node that receives your HTTP request determines which shard should contain your document, based on a hash of the document’s ID.
2. Then, it forwards the document to the node holding that primary shard.
3. Once the shard is updated, the document is sent to be indexed in all replicas of that shard.
4. Finally, the indexing command returns after all replicas are updated.

This process keeps replicas in sync and allows them to serve searches and to be promoted to primary shards in case the original primary is unavailable.

Elasticsearch divides each index into shards, which are standalone Lucene indices. These can be either primary shards or replicas. 

The number of shards and replicas per shard is set when a new index is created. After that, Elasticsearch takes care of evenly distrib...