Mappings: Ensure that reindexing is always possible

[jpountz]

Our mappings are very permissive in terms of what can be enabled or disabled, and in particular it is possible to disable the _source. This can be problematic if at some point you need to perform a change that requires reindexing (eg. an analyzer change) because this means that the data needs to be pulled again from another data source.

Instead, we could disable some mappings options in order to make sure that we always have all the information needed to reindex (eg. with client helpers). Here are the settings that we would need to enforce:

• _source is enabled and stored
• _timestamp, _ttl, _routing, _parent are stored

Relates to #8870

[clintongormley]

+1

[rmuir]

+1

[dadoonet]

I'm aware of some corner cases where people would definitely not store the source and want to get back only _id.
And as they send a massive amount of data, they don't want to store _source neither fields.
Same for users who only want to compute.

Not saying that we should not do it but that we need to be aware of the consequences for some users.
As I said, extreme corner cases though.

[rjernst]

+1

[Vineeth-Mohan]

+1

[clintongormley]

I can imagine there are certain expert cases where users actually do want to disable storing _source, so perhaps we should still maintain the ability to do so, but make it much harder. eg some non-dynamic cluster setting, the documentation for which also lists all the things you will lose if you disable _source.

[bleskes]

+1 to that latest suggestion.

On Thu, Feb 12, 2015 at 7:04 PM, Clinton Gormley notifications@github.com
wrote:

I can imagine there are certain expert cases where users actually do want to disable storing _source, so perhaps we should still maintain the ability to do so, but make it much harder. eg some non-dynamic cluster setting, the documentation for which also lists all the things you will lose if you disable _source.

Reply to this email directly or view it on GitHub:
#8142 (comment)

[jpountz]

Can we get more information about these expert use-cases? Having the _source stored is so valuable that I would like to make sure that we don't allow users to disable this without good reasons. And if we still decide to allow disabling the source, can we consider making backward compatibility best-effort only on such indices?

[clintongormley]

eg from an email I received:

A binary search engine? How are you indexing the binary fields to make them searchable?

The client uses Lucene to search over his binary data. In summary they insert and query data sending chunks in a binary format. Internally this binary format is broken into terms (base64 small strings) and offsets for each term. So, during the search they verify if the term and the offsets matches, returning the documents with the most matches of terms respecting the offsets. They have created new queries, scorers, collectors, refiners, etc.

This custom plugin wasn't able to keep up with indexing and query speed, but disabling the source allowed them to do that.

I realise that this is not typical, hence "expert case", but not storing the _source could be a tradeoff that some people would be willing to make. I just want to make it hard enough that the ordinary user won't do it without realising the consequences.

And if we still decide to allow disabling the source, can we consider making backward compatibility best-effort only on such indices?

Yes absolutely. That is one of the tradeoffs.

[colings86]

Maybe we should have a cluster level setting to allow indices to disable the _source field? this way we can put a big notice in the documentation warning about how this is an expert feature and what affect it has on backwards compatibility. Also with a cluster-wide setting, we can detect the setting on startup and put a warning in the logs stating that backwards compatibility is not guaranteed with this setting on. This way we will have made it very clear to the user that there are serious implications to turning on this setting

[brwe]

Disabling _source does reduce the size on disk:
https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/elasticsearch/cZ7tkx8HZ70/AOFAt42mNt0J
I think when people store the raw data outside es in some secondary store then we should not force them to store the source in es again or even make it hard to disable it.