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List:       solr-user
Subject:    Re: Leader node on specific host machines?
From:       Erick Erickson <erickerickson () gmail ! com>
Date:       2019-10-28 11:51:03
Message-ID: F8086ED7-0EBB-462C-9E9F-A0C8FE18AA38 () gmail ! com
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There's the preferredLeader property, see: \
https://lucene.apache.org/solr/guide/6_6/collections-api.html

That said, this was put in for situations where there were 100s of shards with \
replicas from many shards hosted on any given machine, so it was possible in that \
setup to have 100 or more leaders on a single node.

In the usual case, the leader role doesn't do very much extra work, and the extra \
work is mostly distributing the incoming documents to the followers during indexing \
(mostly I/O). During query time, the leader has no extra duties at all. So if "heavy \
use" means heavy querying, it shouldn't make any appreciable difference.

I would urge you to have evidence that this was worth the effort before spending time \
on it. And, the "preferredLeader" property is just that, a preference all things \
being equal. It's still possible for a leader to be a different replica, otherwise \
you'd defeat the whole point of trying for HA.

For TLOG and PULL setups, the leader will always be a TLOG replica, so you could \
strategically place them to get what you want. In this case, the leader indeed has a \
lot more work to do than the follower so it makes more sense.

Best,
Erick

> On Oct 28, 2019, at 6:13 AM, Koen De Groote <koen.degroote@limecraft.com> wrote:
> 
> Hello,
> 
> I'm looking for a way to configure my collections as such that the leader
> nodes of specific collections never share the same host.
> 
> This as a way to prevent several large and/or heavy-usage collections on
> the same machine.
> 
> Is this something I can set in solrconfig.xml? Or are there rules for this?
> 
> Kind regards,
> Koen De Groote


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