Deprecate the path option in the _timestamp field

[clintongormley]

This was suggested in #6730 (comment) but it feels like a separate discussion, so I've opened this issue.

Removing the path parameter from _routing and _id has real benefits: we don't have to parse the body of the document on the coordinating node just to figure out which node should handle an indexing request. However, the _timestamp field doesn't have the same issue.

Currently the _timestamp field has two uses:

• its value can be auto-generated
• it is used with the _ttl functionality

If you're not using _ttl then it makes no sense extracting the _timestamp from a field in your document. Just query the field directly instead of making a redundant copy of it.

For auto-generated timestamps, I think you probably always want them to be auto-generated, or you always want to provide them yourself. So having logic to extract the timestamp from a field, and auto-generate a value if one isn't provided feels pretty much redundant.

The options are:

1. Remove the path option from _timestamp

The _timestamp field will primarily be used with auto-generated timestamps. If the user wants to use the _ttl feature with a predefined timestamp, then they should specify that value manually at index/update time, with ?timestamp=...

1. Add a path option to the _ttl

The _ttl will use the auto-generated _timestamp field by default, but can access a field within the document body instead (and fall back to an auto-generated value if none is specified). This option rather complicates things: what if the user uses the _timestamp field AND a _ttl.path?

1. Leave things as they are

[jpountz]

I like removing the path option for two reasons:

• The more configuration metadata fields have, the harder they are to deal with.
• It removes questions such as "if there are both a timestamp to extract in my document, and a timpestamp in the indexing request (?timestamp=...), who wins?"

If we consider that the _timestamp field is only useful in the context of the _ttl feature (eg. if all other use-cases can be solved by using a regular field), then another option would be to remove the _timestamp field mapper and leave the responsibility to the _ttl field mapper to serialize the original timestamp if it needs it.

[clintongormley]

If we consider that the _timestamp field is only useful in the context of the _ttl feature (eg. if all other use-cases can be solved by using a regular field), then another option would be to remove the _timestamp field mapper and leave the responsibility to the _ttl field mapper to serialize the original timestamp if it needs it.

I think that this is probably true, but perhaps some users want auto-generated timestamps that are not related to _ttl?

[clintongormley]

Closed by #11794