Use the a mod number of shard function to route

[Tim-Brooks]

Using hash % number of shards to routing documents.

[Tim-Brooks]

Work in progress.

[github-actions]

ℹ️ Important: Docs version tagging

👋 Thanks for updating the docs! Just a friendly reminder that our docs are now cumulative. This means all 9.x versions are documented on the same page and published off of the main branch, instead of creating separate pages for each minor version.

We use applies_to tags to mark version-specific features and changes.

Expand for a quick overview

When to use applies_to tags:

✅ At the page level to indicate which products/deployments the content applies to (mandatory)
✅ When features change state (e.g. preview, ga) in a specific version
✅ When availability differs across deployments and environments

What NOT to do:

❌ Don't remove or replace information that applies to an older version
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❌ Don't forget that applies_to tags can be used at the page, section, and inline level

🤔 Need help?

• Check out the cumulative docs guidelines
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[Tim-Brooks]

I'm going to figure out how to mute that rest compat test which is tightly coupled to shard number tomorrow.

[lkts]

The routing piece looks good to me.

We'll need to change the documentation once this is merged.

[henningandersen]

Looks good. I left a few detailed comments.

Additionally, I wonder if we need a minimal unit test of old and new index-routing behavior.

[Tim-Brooks]

This is ready for another review @henningandersen. I'm opening a rest api test fix on 8.x to fix the BWC test.

[henningandersen]

LGTM.