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From Sandeep Tikoo <>
Subject RE: SolrCould read-only replicas
Date Thu, 02 Oct 2014 14:07:12 GMT

Thank you for your response. Yup, when I said it is not possible to have a cross continent
data center replica, I meant that we never ever want to do that because of the latency.

What I was hoping is that  I could have Solr cloud in my DataCentre A (DC-A) and get all the
benefits of sharding ( scaling/parallel computing) and failover redundancy within the same
data center. If I could then have a read-only replica (with no guaranteed consistency of course
) of this entire cloud in my DataCenter B (DC-B), that would make my reads over DC-B faster
without making my writes slow. To clarify, all the writes were going to go against DC-A only.
The read-only cluster in DC-B could  also be made the master in case the entire DC-A went
down.  The DC-B wouldn't be guaranteed to be in sync with the DB-A master but in my use case
I could live with that. Seems like that is no possible out-of-the-box if I am using Solr 4.0+
in the cloud mode. It is either Solr Coud or cross data center read only replica. Can't do
both at the same time.
I think that is what you confirmed as well. If I have it wrong, please let me know. Also,
any thoughts on the most easy way to accomplish the read-only replica of the entire solr cloud


From: Sandeep Tikoo
Sent: Saturday, September 27, 2014 9:43 PM
To: ''
Subject: SolrCould read-only replicas


I have been reading up on SolrCloud and it seems that it is not possible to have a cross-datacenter
read-only slave anymore but wanted to ask here to be sure.
We currently have a pre Solr 4.0 installation with the master instance in our US mid-west
datacenter. The datacenter in Europe has read-replicas which pull data using solr.ReplicationHandler.
We wanted to upgrade to SolrCloud. As far as I have been able to figure out, with SolrCloud
you cannot have a read-only replica anymore. A replica has to be able to become a leader and
writes against all replicas for a shard have to succeed. Because of the a strong consistency
model across replicas, it seems that replicas cannot be across datacenters anymore.

So my question is, how can we have a read-ony replica in a remote datacenter in Solr 4.0+
similar to pre Solr 4.0? Is it not possible anymore without doing it all yourself?


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