According to Elasticsearch: The Definitive Guide, when OCC is used by Elasticsearch:

This approach assumes that conflicts are unlikely to happen and doesn't block operations from being attempted. However, if the underlying data has been modified between reading and writing, the update will fail. It is then up to the application to decide how it should resolve the conflict. For instance, it could reattempt the update, using the fresh data, or it could report the situation to the user.

Elasticsearch is distributed. When documents are created, updated, or deleted, the new version of the document has to be replicated to other nodes in the cluster. Elasticsearch is also asynchronous and concurrent, meaning that these replication requests are sent in parallel, and may arrive at their destination out of sequence. Elasticsearch needs a way of ensuring that an older version of a document never overwrites a newer version.

Every document has a _version number that is incremented whenever a document is changed. Elasticsearch uses this _version number to ensure that changes are applied in the correct order. If an older version of a document arrives after a new version, it can simply be ignored.

We can take advantage of the _version number to ensure that conflicting changes made by our application do not result in data loss. We do this by specifying the version number of the document that we wish to change. If that version is no longer current, our request fails.

Let's create a new blog post:

    PUT /website/blog/1/_create
    { 
       "title": "My first blog entry",
       "text": "Just trying this out..." 
    } 

The response body tells us that this newly created document has _version number 1. Now imagine that we want to edit the document: we load its data into a web form, make our changes, and then save the new version.

First we retrieve the document:

    GET /website/blog/1

The response body includes the same _version number of 1:

{ 
    "index": "website", 
    "type": "blog", 
    "id": "1", 
    "version": 1, 
    "found": true, 
    "_source": { 
        "title": "My first blog entry", 
        "text": "Just trying this out..." 
    } 
}

Now, when we try to save our changes by reindexing the document, we specify the version to which our changes should be applied. We want this update to succeed only if the current _version of this document in our index is version 1:

   PUT /website/blog/1?version=1
   { 
      "title": "My first blog entry", 
      "text": "Starting to get the hang of this..."  
   }

This request succeeds, and the response body tells us that the _version has been incremented to 2:

   { 
      "index": "website", 
      "type": "blog", 
      "id": "1", 
      "version": 2,
      "created": false 
   }

However, if we were to rerun the same index request, still specifying version=1, Elasticsearch would respond with a 409 Conflict HTTP response code, and a body like the following:

{
    "error": {
        "root_cause": [{ 
             "type": "version_conflict_engine_exception",
             "reason":
                 "[blog][1]: version conflict,current[2],provided [1]",   
             "index": "website",
             "shard": "3"
        }],
        "type": "version_conflict_engine_exception" ,
        "reason":
            "[blog][1]:version conflict,current [2],provided[1]",
        "index": "website",
        "shard": "3"
    },
    "status": 409
}

This tells us that the current _version number of the document in Elasticsearch is 2, but that we specified that we were updating version 1.

What we do now depends on our application requirements. We could tell the user that somebody else has already made changes to the document, and to review the changes before trying to save them again. Alternatively, as in the case of the widget stock_count previously, we could retrieve the latest document and try to reapply the change.

All APIs that update or delete a document accept a version parameter, which allows you to apply optimistic concurrency control to just the parts of your code where it makes sense.