Reindex source types disregarded in 7.x

[henningandersen]

Clarify that types in source index are disregarded.

Closes #48460

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Reindex)

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[jrodewig]

I think there are two problems here:

1. The type parameters aren't documented.
2. It's not clear that this is chiefly for users trying to preserve their existing document types.

I pushed f669009 and a8087be to this PR to address both of those.

Feel free to change or revert this commits if wanted. Thanks!

[henningandersen]

I think I agree to 1, moving it to a definition of the type parameter makes sense.

About 2, while they might be trying to preserve their types, they were in reality just expecting an error in this case and I wanted to clarify that the source type is disregarded completely now. In older versions, ES would copy over the source type to the destination too, but now that dest have only one type (which in 7.x can still be freely specified) copying over the type makes little to no sense. The source could in principle still have multiple types if specifying multiple indices with different types.

I commented on this inline too.

[jrodewig]

Thanks for this PR @henningandersen.

I think the first commit was a bit unclear so I pushed f669009 and a8087be to try to address.

Feel free to revert or adjust those as you see fit.

[csenol]

user feedback: Thanks for this. This could have helped us.
I hope users in the future will be aware of this with the given Warning

[henningandersen]

I think we should leave out this part of the change, since it solely concerns the source type and this is just a filter on the source. I think adding it here increases the risk of it being used, so would kind of prefer not to add this now.

It is also unrelated to the issue, since regardless of specifying this filter, the type from source is disregarded when indexing into dest.

[jrodewig]

I'm okay with removing this. As you mentioned, this isn't the main problem.

[henningandersen]

The original problem here really was that the user expected an error. I think that is somewhat lost in this new version and I prefer the original text (here) instead. WDYT?

Suggested change

	By default, all reindexed documents are assigned the `_doc` document type. To
	Types in source indices are always ignored, also when not specifying a
	destination `type`. If explicitly specifying destination `type`, the specified
	type must match the type in the destination index or be either unspecified or
	the special value `_doc`. See <<removal-of-types>> for further details.

[jrodewig]

👍 That works for me.

[henningandersen]

I think I agree to 1, moving it to a definition of the type parameter makes sense.

About 2, while they might be trying to preserve their types, they were in reality just expecting an error in this case and I wanted to clarify that the source type is disregarded completely now. In older versions, ES would copy over the source type to the destination too, but now that dest have only one type (which in 7.x can still be freely specified) copying over the type makes little to no sense. The source could in principle still have multiple types if specifying multiple indices with different types.

I commented on this inline too.

[henningandersen]

Thanks for looking into this @jrodewig , I have a few comments to your changes. I do not mind carrying out the final changes, but thought we could agree on them here first.

[jrodewig]

Thanks @henningandersen. I agree with your suggestions. Thanks for seeing this one through.

[jrodewig]

LGTM. Thanks again for the iteration @henningandersen.

[henningandersen]

Thanks for reviewing @jrodewig