[jira] [Created] (CASSANDRA-16856) Prevent broken schema pulls

["Berenguer Blasi (Jira)" <ji...@apache.org>]

Berenguer Blasi created CASSANDRA-16856:
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             Summary: Prevent broken schema pulls
                 Key: CASSANDRA-16856
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-16856
             Project: Cassandra
          Issue Type: Bug
          Components: Cluster/Gossip
            Reporter: Berenguer Blasi
            Assignee: Berenguer Blasi


There's a race condition around pulling schema changes, that can occur
in case the schema changes push/propagation mechanism is not
immediately effective (e.g. because of network delay, or because of
the pulling node being down, etc.).

If schema changes happen on node 1, these changes do not reach node 2
immediately through the SCHEMA.PUSH mechanism, and are first
recognized during gossiping, the corresponding SCHEMA.PULL request
from node 2 can catch the node 1 schema in the middle of it being
modified by another schema change request. This can easily lead to
problems (e.g. if a new table is being added, and the node 2 request
reads the changes that need to be applied to system_schema.tables, but
not the ones that need to be applied to system_schema.columns).

This PR addresses that by synchronizing the SCHEMA.PULL "RPC call"
executed in node 1 by a request from node 2 with the method for
applying schema changes in node 1.



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