=== Partial Updates to a Document

The update API , as shown in <>, combines the read and((("updating documents", "partial updates")))((("documents", "partial updates"))) write patterns explained previously.

[[img-distrib-update]] .Partial updates to a document image::images/elas_0404.png["Partial updates to a document"]

Here is the sequence of steps used to perform a partial update on a document:

1. The client sends an update request to Node 1.

2. It forwards the request to Node 3, where the primary shard is allocated.

3. Node 3 retrieves the document from the primary shard, changes the JSON in the _source field, and tries to reindex the document on the primary shard. If the document has already been changed by another process, it retries step 3 up to retry_on_conflict times, before giving up.

4. If Node 3 has managed to update the document successfully, it forwards the new version of the document in parallel to the replica shards on Node 1 and Node 2 to be reindexed. Once all replica shards report success, Node 3 reports success to the requesting node, which reports success to the client.

The update API also accepts the routing, replication, consistency, and timeout parameters that are explained in <>.

.Document-Based Replication

When a primary shard forwards changes to its replica shards,((("primary shards", "forwarding changes to replica shards"))) it doesn't forward the update request. Instead it forwards the new version of the full document. Remember that these changes are forwarded to the replica shards asynchronously, and there is no guarantee that they will arrive in the same order that they were sent. If Elasticsearch forwarded just the change, it is possible that changes would be applied in the wrong order, resulting in a corrupt document.