20_Retrieving.asciidoc

Retrieving a Document

A document can be retrieved from a primary shard or from any of its replicas, as shown in Retrieving a single document.

Figure 1. Retrieving a single document

Here is the sequence of steps to retrieve a document from either a primary or replica shard:

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 coordinating 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, while a document is being indexed, the document will already be present on the primary shard but not yet 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. Once the indexing request has returned success to the user, the document will be available on the primary and all replica shards.