Michaël Figuière created CASSANDRA-8345:
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Summary: Client notifications should carry the entire delta of the
information that changed
Key: CASSANDRA-8345
URL: https://issues.apache.org/jira/browse/CASSANDRA-8345
Project: Cassandra
Issue Type: Improvement
Reporter: Michaël Figuière
Currently when the schema changes, a {{SCHEMA_CHANGE}} notification is sent to
the client to let it know that a modification happened in a specific table or
keyspace. If the client register for these notifications, this is likely that
it actually cares to have an up to date version of this information, so the
next step is logically for the client to query the {{system}} keyspace to
retrieve the latest version of the schema for the particular element that was
mentioned in the notification.
The same thing happen with the {{TOPOLOGY_CHANGE}} notification as the client
will follow up with a query to retrieve the details that changed in the
{{system.peers}} table.
It would be interesting to send the entire delta of the information that
changed within the notification. I see several advantages with this:
* This would ensure that the data that are sent to the client are as small as
possible as such a delta will always be smaller than the resultset that would
eventually be received for a formal query on the {{system}} keyspace.
* This avoid the Cassandra node to receive plenty of query after it issue a
notification but rather to prepare a delta once and send it to everybody.
* This should improve the overall behaviour when dealing with very large
schemas with frequent changes (typically due to a tentative of implementing
multitenancy through separate keyspaces), as it has been observed that the the
notifications and subsequent queries traffic can become non negligible in this
case.
* This would eventually simplify the driver design by removing the need for an
extra asynchronous operation to follow up with, although the benefit of this
point will be real only once the previous versions of the protocols are far
behind.
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