20_Retrieving.asciidoc

Retrieving a document

A document can be retrieved from a primary shard or from any of its replicas.

Figure 1. Retrieving a single document

Below we list the sequence of steps to retrieve a document from either a primary or replica shard, as depicted in Retrieving a single document:

1. The client sends a get request to Node 1.

2. The node uses the document’s _id to determine that the document belongs to shard 0. Copies of shard 0 exist on all three nodes. On this occasion, it forwards the request to Node 2.

3. Node 2 returns the document to Node 1 which returns the document to the client.

For read requests, the requesting node will choose a different shard copy on every request in order to balance the load — it round-robins through all shard copies.

It is possible that a document has been indexed on the primary shard but has not yet been copied to the replica shards. In this case a replica might report that the document doesn’t exist, while the primary would have returned the document successfully.