It is not clear why templates with a default mapping created prior to 7.x fail in 7.x

[talevy]

Problem

It is not clear how to resolve templates with default mappings in 7.x.

Indices created in 7.x matching templates created with the default mapping in 6.x fail to create
with following exception:

{
  "error" : {
    "root_cause" : [
      {
        "type" : "illegal_argument_exception",
        "reason" : "The [default] mapping cannot be updated on index [my_index]: defaults mappings are not useful anymore now that indices can have at most one type."
      }
    ],
    "type" : "illegal_argument_exception",
    "reason" : "The [default] mapping cannot be updated on index [my_index]: defaults mappings are not useful anymore now that indices can have at most one type."
  },
  "status" : 400
}

This error makes it clear that there is something wrong with the mapping, which results in users exploring templates for default-mapping usage. The problem is that users will not see their templates defining the _default_ mapping since 7.x defaults to the typeless APIs. So include_type_name=true must be set on the GET Templates API to see this information.

A Few Possible Solutions

• specify how to take action in the error message
• document this breaking change better in the docs
• add better template upgrade support in the Upgrade Assistant (outside scope of this repository)

[elasticmachine]

Pinging @elastic/es-search (:Search/Mapping)

[praveenmak]

I am seeing the same error after upgrading to 7.4.2 from 6.4.2

I get this when I do a rollover. I am using alias and we rollover to new one using URL math.

May I know what is the fix ?

[freakingid]

Greetings. I ran in to this as well. Please see:
Goodbye, types. Hello, typeless

Under "Stop using default mappings"

You should update your index templates and your application code to add these mappings to the actual type, rather than include them in a default mapping. If the name of the type is not important for you, the recommendation is to use _doc, which will make the transition to typeless APIs easier in the future.

[jeethsam]

I ran into the same issue. Is there a fix for this apart from 'not using default mapping'?

[jtibshirani]

As @freakingid mentioned, in 7.x you must remove the default mapping from index templates. No other fix is necessary.

To clarify one aspect that can be confusing -- issuing a normal 'get templates' call in 7.x won't show the default mapping, even if it's defined. To see all mappings that are defined, including the default mapping, you should pass the include_type_name parameter:

GET /_template/my_template?include_type_name

[jtibshirani]

I merged #54915 to add debugging information to the migration docs. It also updates the error message to link to those docs.