Simplify synonyms YAML tests after auto expand replicas changed

[carlosdelest]

Follow up to #115078

After changing auto-expand replicas to 0-1 for synonyms system index, we can now simplify the YAML tests.

Now we can wait for green on the synonyms index, knowing that:

• For bwc tests mixed cluster tests, there will be 2 updated nodes and 2 non-updated nodes.

  • If a updated node receives the put synonyms request, it will use auto-expand-replicas 0-1. That means that at least the other updated node will be able to create a replica (as non-updated nodes will be in a previous version), so the index will be green.
  • If a non-updated node receives the put synonyms request, it will use auto-expand-replicas 0-all. But as the updated nodes will be able to host replicas (index was created in a previous version), all nodes will be able to hold replicas, so the index will be green.

• For serverless, we will wait until the index is green with no timeout, so the tests will wait for a search replica to be available before proceeding.

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[benwtrent]

🤔 This might work. Mixed cluster behavior is still dubious to me.

[mayya-sharipova]

Not related to this PR, but for my educaiton: what is .synynoms-2? Why we added it?

[carlosdelest]

.synonyms-2 is the current version of the index - we changed the index format before releasing it, so it has a -2 suffix to it.

.synonyms is the alias for the system indices. I think it's more correct to refer to the alias instead of the specific index, so in case it changes we don't have to change all the YAML tests.

So, it's unrelated to this change, but I thought we could do some winter cleaning here 🙂

[mayya-sharipova]

Thanks, Carlos, makes sense

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.x

[carlosdelest]

BwC tests are failing in the backport to 8.x . So it seems that the following is not true:

If a updated node receives the put synonyms request, it will use auto-expand-replicas 0-1. That means that at least the other updated node will be able to create a replica (as non-updated nodes will be in a previous version), so the index will be green.

I'm confused about why an upgraded node (with the auto-expand replicas set to 0-1) would create an index with auto-expand replicas set to 0-all. 🤔

I'll watch test failures on main. That seems to be working as bwc tests on 8.18 already include the auto expand replica change.

Worst case scenario, we don't backport this test simplification to 8.x, and keep it in main. And eventually, we'll stop supporting 8.17 and 8.16 so we'll forget about this. I won't, but others will.