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From Vasileios Vlachos <vasileiosvlac...@gmail.com>
Subject Multi-DC Environment Question
Date Thu, 29 May 2014 22:22:22 GMT
Hello All,

We have plans to add a second DC to our live Cassandra environment. 
Currently RF=3 and we read and write at QUORUM. After adding DC2 we are 
going to be reading and writing at LOCAL_QUORUM.

If my understanding is correct, when a client sends a write request, if 
the consistency level is satisfied on DC1 (that is RF/2+1), success is 
returned to the client and DC2 will eventually get the data as well. The 
assumption behind this is that the the client always connects to DC1 for 
reads and writes and given that there is a site-to-site VPN between DC1 
and DC2. Therefore, DC1 will almost always return success before DC2 
(actually I don't know if it is possible for DC2 to be more up-to-date 
than DC1 with this setup...).

Now imagine DC1 looses connectivity and the client fails over to DC2. 
Everything should work fine after that, with the only difference that 
DC2 will be now handling the requests directly from the client. After 
some time, say after max_hint_window_in_ms, DC1 comes back up. My 
question is how do I bring DC1 up to speed with DC2 which is now more 
up-to-date? Will that require a nodetool repair on DC1 nodes? Also, what 
is the answer when the outage is < max_hint_window_in_msinstead?

Thanks in advance!

Vasilis

-- 
Kind Regards,

Vasileios Vlachos


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