sequence number vs version number in elasticsearch

Question

I am reading the concepts of elasticsearch-7.4 and I got to know about two fields. _seq_no and _version.

As per the documentation:

Version

Returns a version for each search hit.

Sequence Numbers and Primary Term

Returns the sequence number and primary term of the last modification to each search hit.

But it is not clearing anything related to when they both will be different or same for a document.

I created an index test

PUT /test/_doc/_mapping
{
  "properties": {
    "total_price" : {
      "type": "integer"
    },
    "final_price": {
      "type": "integer"
    },
    "base_price": {
      "enabled": false
    }
  }
}

I am updating the full document using PUT API.

PUT /test/_doc/2
{
  "total_price": 10,
  "final_price": 10,
  "base_price": 10
}

Both _seq_no and _version are increasing in this case.

On doing partial updates using UPDATE API,

POST /test/_doc/2/_update
{
    "doc" : {
        "base_price" : 10000
    }
}

Both _seq_no and _version are increasing in this case too.

So, I am unable to find the case when only one field is changing but the other is not.
When will both the fields be different?

Answer 1

Sequence numbers have been introduced in ES 6.0.0. Just before that release came out, they were very well explained in this blog article.

But in summary,

• version is a sequential number that counts the number of time a document was updated
• _seq_no is a sequential number that counts the number of operations that happened on the index

So if you create a second document, you'll see that version and _seq_no will be different.

Let's create three documents:

POST test/_doc/_bulk
{"index": {}}
{"test": 1}
{"index": {}}
{"test": 2}
{"index": {}}
{"test": 3}

In the response, you'll get the payload below.

{
  "took" : 166,
  "errors" : false,
  "items" : [
    {
      "index" : {
        "_index" : "test",
        "_type" : "_doc",
        "_id" : "d2zbSW4BJvP7VWZfYMwQ",
        "_version" : 1,
        "result" : "created",
        "_shards" : {
          "total" : 2,
          "successful" : 1,
          "failed" : 0
        },
        "_seq_no" : 0,
        "_primary_term" : 1,
        "status" : 201
      }
    },
    {
      "index" : {
        "_index" : "test",
        "_type" : "_doc",
        "_id" : "eGzbSW4BJvP7VWZfYMwQ",
        "_version" : 1,
        "result" : "created",
        "_shards" : {
          "total" : 2,
          "successful" : 1,
          "failed" : 0
        },
        "_seq_no" : 1,
        "_primary_term" : 1,
        "status" : 201
      }
    },
    {
      "index" : {
        "_index" : "test",
        "_type" : "_doc",
        "_id" : "eWzbSW4BJvP7VWZfYMwQ",
        "_version" : 1,
        "result" : "created",
        "_shards" : {
          "total" : 2,
          "successful" : 1,
          "failed" : 0
        },
        "_seq_no" : 2,
        "_primary_term" : 1,
        "status" : 201
      }
    }
  ]
}

As you can see:

• for all documents, version is 1
• for document 1, _seq_no is 0 (first index operation)
• for document 2, _seq_no is 1 (second index operation)
• for document 3, _seq_no is 2 (third index operation)