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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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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] 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 to document values occur. In this configuration, documents indexed by {es} are
immutable. Clients are located in each datacenter alongside the {es} immutable. Clients are located in each datacenter alongside the {es}
cluster, and do not communicate with clusters in different datacenters. 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 follower shard pauses
<<ccr-pause-replication,until you resume it>>. <<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 When a follower shard receives operations from the leader shard, it places
those operations in a write buffer. The follower shard submits bulk write those operations in a write buffer. The follower shard submits bulk write
requests using operations from the write buffer. If the write buffer exceeds 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 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. 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 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 needed by the follower index, the follower index closes itself, applies the
settings update, and then re-opens itself. The follower index is unavailable settings update, and then re-opens itself. The follower index is unavailable