[jira] [Commented] (CASSANDRA-7667) Improve migration

["Brandon Williams (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-7667?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14082952#comment-14082952 ] 

Brandon Williams commented on CASSANDRA-7667:
---------------------------------------------

I'm not sure this makes a lot of sense.  Adding DC optimization like writes _might_ save on latency, but it's also adding an extra hop so maybe not, and the point of dc write optimization is bandwidth, of which little is used here and schema changes aren't that frequent.  Remember that broadcast is just best effort, if it fails it doesn't matter because schema propagation is two-pronged: broadcast if possible, but also passively announce via gossip and let the other side pull.

> Improve migration
> -----------------
>
>                 Key: CASSANDRA-7667
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7667
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Robert Stupp
>
> Idea is to optimize migration especially for large clusters with multiple DCs.
>  
> Currently {{MigrationManager.announce()}} simply broadcasts to all nodes.
>  
> Idea is to not announce schema migration to every node.
>  
> *Against remote DCs*: only contact a single node in each remote DC. This could make migration faster, because network latency comes into play. And it lowers cross-DC network requirements (since 1-to-all communication creates a huge amount of TCP connections). Drawback: remote-DC-announcement must be reliable (receiver must ACK - if it doesn't, sender must retry another node).
>  
> *Within the local DC*: use some gossip approach - e.g. only contact N nodes at once. Not sure about this one  - it might be nice, if there are really many (~100) nodes in one DC. But it's a bit difficult to guarantee, that all nodes will receive the announcement without giving up the "fire-and-forget" approach.



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