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From Isaac Huang <he.hu...@intel.com>
Subject overhead of the Gossip protocol
Date Tue, 27 Nov 2012 09:14:46 GMT
Hi list,

I've been looking at the Cassandra implementation of the gossip
protocol, and it appeared to me that the size of each
GossipDigestSynMessage would grow linearly with the size of the
system:

makeRandomGossipDigest():
......
List<InetAddress> endpoints = new ArrayList<InetAddress>(endpointStateMap.keySet());
Collections.shuffle(endpoints, random);

So if there's N servers participating in the gossip protocol, at each
cycle, there's totally N GossipDigestSynMessage's, each of a size linear
to N - the aggregate protocol overhead would be O(N**2).

I'm totally new to both Cassandra and Java, so this understanding
could be very wrong. But if it's true, why hasn't it been a scalability
concern? Is it because Cassandra servers are often geographically
distributed so the protocol overhead doesn't hit a single site? Or
does Cassandra servers gossip in a hierarchy of groups that I failed
to see?

Also, what purpose does the Collections.shuffle() serve?

Thanks and please kindly CC me on replies.

- Isaac

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