how elastic search find document content by doc id

Ragav is correct. However, I do have a bit to add that may help you work with document Ids.

When you index documents that don't have an ID, and ID is generated for you by ElasticSearch. That field name is "_id".

If you know the Id value of the document you wish to find, you can simply perform the query like this:

GET my_index/_search
{
  "query": {
    "terms": {
      "_id": [ "1", "2" ] 
    }
  }
}

https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-id-field.html

The above query would return documents that have have _id equal to 1 OR 2.

As Ragav said in his answer, if you created documents in the way described with id 1 or 2, you would return them with that sample query I pulled from the ElasticSearch documentation.

Hope this helps.

Elasticsearch is built on top of Lucene. When you index a new document onto Elasticsearch, it indexes _index, _type and _id as a part of the document along with the actual content(_source).

So, when you try to get a document using the get API _index/_type/_id, it is basically converted into a query which searches for doc matching the _index, _type and the _id.

This is how Elasticsearch is able to return you the document.