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From Karthik N <>
Subject Re: Multi datacenter, WAN hiccups and replication
Date Tue, 26 Jun 2012 17:14:31 GMT
Let me attempt to articulate my question a little better.

Say I choose LOCAL_QUORUM with a Replication Factor of 3. Cassandra
stores three copies in my local datacenter. Therefore the cost
associated with "losing" one node is not very high locally, and I
usually HH, and use read repair/nodetool repair instead.

However over the WAN network blips are quite normal and HH really
helps. More so because for WAN replication Cassandra sends only one
copy to a coordinator in the remote datacenter.

Therefore I was wondering if Cassandra already intelligently optimizes
for HH-over-WAN (since this is common) or alternately if there's a way
to enable HH for WAN replication?

Thank you.

On Tue, Jun 26, 2012 at 9:22 AM, Mohit Anchlia <> wrote:
> On Tue, Jun 26, 2012 at 8:16 AM, Karthik N <> wrote:
>> Since Cassandra optimizes and sends only one copy over the WAN, can I opt
>> in only for HH for WAN replication and avoid HH for the local quorum? (since
>> I know I have more copies)
> I am not sure if I understand your question. In general I don't think you
> can selectively decide on HH. Besides HH should only be used when the outage
> is in mts, for longer outages using HH would only create memory pressure.
>> On Tuesday, June 26, 2012, Mohit Anchlia wrote:
>>> On Tue, Jun 26, 2012 at 7:52 AM, Karthik N <> wrote:
>>>> My Cassandra ring spans two DCs. I use local quorum with replication
>>>> factor=3. I do a write in DC1 with local quorum. Data gets written to
>>>> multiple nodes in DC1. For the same write to propagate to DC2 only one
>>>> copy is sent from the coordinator node in DC1 to a coordinator node in
>>>> DC2 for optimizing traffic over the WAN (from what I have read in the
>>>> Cassandra documentation)
>>>> Will a Wan hiccup result in a Hinted Handoff (HH) being created in
>>>> DC1's coordinator for DC2 to be delivered when the Wan link is up
>>>> again?
>>> I have seen hinted handoff messages in the log files when the remote DC
>>> is unreachable. But this mechanism is only used for a the time defined in
>>> cassandra.yaml file.
>> --
>> Thanks, Karthik

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