After adding new mapping half way through re-indexing fails

[NutaN_Shrivastava]

I used the below code to add new mapping to a new index

PUT _index_template/engage-logs
    {
      "index_patterns": [
        "engage-log-reindexed-*"
      ],
      "template": {
      "mappings": {
        "dynamic_templates": [
          {
            "strings": {
              "match_mapping_type": "string",
              "mapping": {
                "type": "keyword"
              }
            }
          }
        ],
        "properties": {
          "message" : {
            "type" : "text"      
          }
        }
            
      }
      }
    }

When I try to reindex the data of my original index to the new index, I get parse exception. Under Index management I do see the new index created and slowly after some time shows all the documents were added to it. But why do I get the exception am unable to understand, its difficult to find out if any data loss was there.

POST _reindex?slices=5&refresh
{
  "source": {
    "index": "engage-log-2020.11.18"
  },
  "dest": {
    "index": "engage-log-reindexed-2020.11.18"
  }
}

Response:

{
  "error" : {
    "root_cause" : [
      {
        "type" : "parse_exception",
        "reason" : "request body is required"
      }
    ],
    "type" : "parse_exception",
    "reason" : "request body is required"
  },
  "status" : 400
}

[warkolm (Mark Walkom)]

Welcome to our community!

It'd be helpful if you could post the entire exception you are seeing. Please also check your Elasticsearch logs for anything else at that time that might seem related.

[NutaN_Shrivastava]

Thank you Mark

I have added the response in the question itself. I don't see any logs generated for this on the master node.