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[DOCS] Update bi-directional CCR note (#72266)

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* [DOCS] Update bi-directional CCR note

* Integrating note with a new sub-section

* Add "particularly"
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Adam Locke 2021-05-25 14:23:58 -04:00 • committed by GitHub
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16
docs/reference/ccr/index.asciidoc
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@ -118,7 +118,7 @@ comes online, replication resumes between the clusters.
image::images/ccr-arch-bi-directional.png[Bi-directional configuration where each cluster contains both a leader index and follower indices]
NOTE: This configuration is useful for index-only workloads, where no updates
This configuration is particularly useful for index-only workloads, where no updates
to document values occur. In this configuration, documents indexed by {es} are
immutable. Clients are located in each datacenter alongside the {es}
cluster, and do not communicate with clusters in different datacenters.
@ -189,6 +189,18 @@ failure), the follower shard enters into a retry loop. Otherwise, the
follower shard pauses
<<ccr-pause-replication,until you resume it>>.
[discrete]
[[ccr-update-leader-index]]
==== Processing updates
You can't manually modify a follower index's mappings or aliases. To make
changes, you must update the leader index. Because they are read-only, follower
indices reject writes in all configurations.
For example, you index a document named `doc_1` in Datacenter A, which
replicates to Datacenter B. If a client connects to Datacenter B and attempts
to update `doc_1`, the request fails. To update `doc_1`, the client must
connect to Datacenter A and update the document in the leader index.
When a follower shard receives operations from the leader shard, it places
those operations in a write buffer. The follower shard submits bulk write
requests using operations from the write buffer. If the write buffer exceeds
@ -214,8 +226,6 @@ h| Update type h| Automatic h| As needed
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.
NOTE: You cannot manually modify a follower index's mappings or aliases.
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