update API combines the read and write patterns explained above.
[[img-distrib-update]] .Partial updates to a document image::images/04-04_update.png["Partial updates to a document"]
Below we list the sequence of steps used to perform a partial update on a
document, as depicted in <
The client sends an update request to
It forwards the request to
Node 3, where the primary shard is allocated.
Node 3 retrieves the document from the primary shard, changes the JSON
_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.
Node 3has managed to update the document successfully, it forwards the new version of the document in parallel to the replica shards on
Node 2to be reindexed. Once all replica shards report success,
Node 3reports success to the requesting node, which reports success to the client.
update API also accepts the
timeout parameters that are explained in <
When a primary shard forwards changes to its 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.