anchor.diff:  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
  
positive.code: 
  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
  
negative.code: 
  image::images/ccr-arch-bi-directional.png[Bi-directional configuration where each cluster contains both a leader index and follower indices]
  
  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.
  