What is difference b/w modifying and updating in the Elasticsearch? - elasticsearch

Question

I am following Elasticsearch official docs where there is a section on Modifying Document: https://www.elastic.co/guide/en/elasticsearch/reference/6.2/_modifying_your_data.html
So I already have a document under /customer/_doc/1:
{
"_index" : "customer",
"_type" : "_doc",
"_id" : "1",
"_version" : 1,
"_seq_no" : 1,
"_primary_term" : 1,
"found" : true,
"_source" : {
"name" : "ajay"
}
}
Below is the request to "modify"
PUT /customer/_doc/1
{
"firstname": "ajay",
"lastname": "tanwar"
}
GET would return the updated document
{
"_index" : "customer",
"_type" : "_doc",
"_id" : "1",
"_version" : 2,
"_seq_no" : 2,
"_primary_term" : 1,
"found" : true,
"_source" : {
"firstname" : "ajay",
"lastname" : "tanwar"
}
}
On the next page of docs, Updating Documents https://www.elastic.co/guide/en/elasticsearch/reference/6.2/_updating_documents.html
Below is the request used to "update"
POST /customer/_doc/1/_update
{
"doc":{
"firstname": "ajay",
"lastname": "tanwar"
}
}
This also return the same result as "modify".
Two difference I noticed in both of these:
"modify" request updates the _version on each request. Whereas the
"update" request keeps the _version same
"modify" request's response contain "result" : "updated" whereas
the "update" request's response contain "result" : "noop"
But few doubts I have: first of all, why the "modify" returns "result" : "updated"? Docs itself says it's a modification operation. And why "modify" returns "result" : "noop"? What is noop BTW?
And if we go logically, modifying and updating are the same thing. What is the purpose of these two different APIs?

Answer 1

When you modify document, you delete the old document and insert an entirely new document in its place. This is similar to HTTP's PUT method, in that it simply replaces the old document with whatever is sent in the HTTP body.
When you update a document, you make changes to the old document. Internally, ElasticSearch will also delete the old document and insert a new (updated) document. However, this operation should be treated as if it just made changes to the old document. This is similar to HTTP's PATCH method, in that it will keep the old document and only apply the changes sent in the HTTP body.
"result" : "updated" means changes were made to the ElasticSearch database whereas "result" : "noop" (no operation) means nothing happened (probably because the end result after update would've been the same as before the update).