Elasticsearch 7.9 CCR changing the number of replicas on the leader index is not replicated by the follower index

Question

I have an issue/question

I have 2 es clusters. Cross cluster replication is configured. After reading replication mechanism from documentation I still have few questions.

1. What is the "some updates" in below quote? And what is "as needed"??? This is no clear.

The follower index automatically retrieves some updates applied to the leader index, while other updates are retrieved as needed

2. Right after this I see another quote from which I can't understand how I can change replicas on leader so as it will replicated on follower? Is it possible at all?

For example, changing the number of replicas on the leader index is not replicated by the follower index, so that setting might not be retrieved.

3. Next quote. How I can understand what is the static and what is the dynamic properties.

If you apply a non-dynamic settings change to the leader index that is needed by the follower index, the follower index closes itself, applies the settings update, and then re-opens itself. The follower index is unavailable for reads and cannot replicate writes during this cycle.

4. Num_of_replicas is it non-dynamic property? If yes why it is not updating it at the follower site as said in previous quote. And how I could get list of such not updatable parameters.

Answer 1

You have a lot of questions in a single question , But let me try to answer all of them.

1. What are the some updates in the below quote? And what is as needed??? This is no clear.

Answer: Updates which are not required for the read operation(follower serve only read aka search requests) on the follower index, will not be updated unless its required, for example, you update the mapping and add another field in your index, but unless your search query include that field, it might not be required to update this mapping on follower index. so as and when you required they pull the changes from the leader index.

2. Right after this I see another quote from which I can't understand how I can change replicas on the leader so as it will be replicated on followers? Is it possible at all?

Answer: As replicas are used for high availability, and you are already using cross-cluster replication, so when you again change the replica on leader index, it doesn't make sense to apply them on follower index, but if you delete all replica shards(Manually) than it might fetch the settings from leader index, you can try this yourself and my guess that time it will fetch the settings and apply it.

3. Next quote. How I can understand what is the static and what is the dynamic properties.

Answer: You can have a look at index module which explains the difference and list all static and dynamic settings of the index.

4. Num_of_replicas is it a non-dynamic property? If yes why it is not updating at the follower site as said in the previous quote. And how I could get a list of such not updatable parameters.

Answer: it's a dynamic property and mentioned in API doc as well, And other sub-questions are explained earlier