[DOCS] How to migrate to node roles from node attrs. Closes #65855

[debadair]

Closes #65855

Preview: https://elasticsearch_71160.docs-preview.app.elstc.co/guide/en/elasticsearch/reference/master/migrate-index-allocation-filters.html

In addition to the basic steps outlined here:

• Are there upgrade/order implications that need to be called out, other than you should complete the upgrade before modifying the node roles, policies, and templates?

• What is necessary on cloud deployments wrt template changes? Do they need to remove the old default template?

• Are there any interactions with autoscaling or restores that need to be called out?

[elasticmachine]

Pinging @elastic/es-core-features (Team:Core/Features)

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[andreidan]

Thanks for working on this @debadair

I think this is a good start for the guide. I've left a few comments.

We should elaborate on the motivations behind needing these steps, either inline or maybe in a new troubleshooting section?

I'm still working on summarising the scenarios and proposed solutions and will comment here or send them over a gdoc when finished

[Leaf-Lin]

Can we also include explicit settings and examples that require change?
For example,

• allocate uses classic allocation filter index.routing.allocation.require.<some_node_attribute> was the setting to be removed. When one migrate to node roles, all node attributes should be migrated to node roles.
  For example, you can see the following index setting were required to be allocated on the hot (attribute) nodes:

    "settings" : {
      "index" : {
        "routing" : {
          "allocation" : {
            "require" : {
              "data" : "hot"
            }
          }
        }
      }
    },

This should be converted to include routing with _tier_preference:

        "routing" : {
          "allocation" : {
            "include" : {
              "_tier_preference" : "data_hot"
            }
          }

---

• migrate leverages the new "data tiers" support in 7.10+, and uses data tier allocation filtering (i.e. index.routing.allocation.include._tier_preference). This works using node roles

• ILM automatically injects the migrate action in the warm and cold phases if no allocation options are specified with the allocate action. This will still only apply the role based filtering, not the attribute based filtering.

• You might have to manually remove the template .cloud-hot-warm-allocation-0 to avoid any index routing conflicts.

[andreidan]

Thanks for working on this @debadair

This is looking great!

I've left just a few more minor questions and suggestions.

[andreidan]

LGTM (assuming the docs-checks need some work to get them passing?), thanks for iterating on this Deb

This will be greatly appreciated by our users!

[andreidan]

This is only true for the hot tier (data_hot node role). New indices are always allocated to the hot (or content) tier irrespective of the user adding the warm or cold tier. Shall we be more specific here?

[debadair]

The point I was trying to make was that when you add a tier to your deployment, nodes are configured with the corresponding role automatically. I tweaked the wording to make that clearer.