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.

To ensure an older version of a document doesn’t overwrite a newer version, every operation performed to a document is assigned a sequence number by the primary shard that coordinates that change. The sequence number is increased with each operation and thus newer operations are guaranteed to have a higher sequence number than older operations. Elasticsearch can then use the sequence number of operations to make sure a newer document version is never overridden by a change that has a smaller sequence number assigned to it.

For example, the following indexing command will create a document and assign it an initial sequence number and primary term:

PUT products/_doc/1567
{
    "product" : "r2d2",
    "details" : "A resourceful astromech droid"
}

You can see the assigned sequence number and primary term in the _seq_no and _primary_term fields of the response:

{
    "_shards" : {
        "total" : 2,
        "failed" : 0,
        "successful" : 1
    },
    "_index" : "products",
    "_type" : "_doc",
    "_id" : "1567",
    "_version" : 1,
    "_seq_no" : 362,
    "_primary_term" : 2,
    "result" : "created"
}

Elasticsearch keeps tracks of the sequence number and primary term of the last operation to have changed each of the documents it stores. The sequence number and primary term are returned in the _seq_no and _primary_term fields in the response of the GET API:

GET products/_doc/1567

returns:

{
    "_index" : "products",
    "_type" : "_doc",
    "_id" : "1567",
    "_version" : 1,
    "_seq_no" : 362,
    "_primary_term" : 2,
    "found": true,
    "_source" : {
        "product" : "r2d2",
        "details" : "A resourceful astromech droid"
    }
}

Note: The Search API can return the _seq_no and _primary_term for each search hit by setting seq_no_primary_term parameter.

The sequence number and the primary term uniquely identify a change. By noting down the sequence number and primary term returned, you can make sure to only change the document if no other change was made to it since you retrieved it. This is done by setting the if_seq_no and if_primary_term parameters of either the Index API or the Delete API.

For example, the following indexing call will make sure to add a tag to the document without losing any potential change to the description or an addition of another tag by another API:

PUT products/_doc/1567?if_seq_no=362&if_primary_term=2
{
    "product" : "r2d2",
    "details" : "A resourceful astromech droid",
    "tags": ["droid"]
}