Non-raft nodes need to be notifified when the metastore changes. For
example, a database could be dropped on node 1 (non-raft) and node 2
would not know. Since queries for that database would not be a cache
miss, node 2 would not get updated.
To propogate changes to non-raft nodes, each non-raft node maintains
a blocking connection to a raft node that blocks until a metadata
change occurs. When the change is triggered, the updated metadata
is returned to the client and the client idempotently updates its local
cache. It then reconnects and waits for another change. This is
similar watches in zookeeper or etcd. Since the blocking request is
always recreated, it also serves as a polling mechanism that will retry
another raft member if the current connection is lost.
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