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https://issues.apache.org/jira/browse/SOLR-15209?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17463986#comment-17463986
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Houston Putman commented on SOLR-15209:
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Currently the default placement uses the legacy implementation for 8.11 and
9.0, this issue is about changing that.
However the legacy implementation was changed in SOLR-15803, and while the
change is important, it is quite small. Basically for non-ReplaceNode commands,
placement preferences are recalculated after placing replicas for each shard.
So when you are creating a collection with 4 shards and 3 replicas, the code
used to basically used to calculate an ordering of preferred nodes for these
collections, then go through the 12 replicas (3 shard-1 replicas, then 3
shard-2 replicas, etc) and, in order, place replicas on the sorted list of
nodes. Now it will sort the nodes, place the 3 replicas for shard-1, in order,
then re-sort the nodes, with the shard-1 replica information added, then place
the 3 replicas for shard-2, etc.
This is a small change but it does make a big difference in practice. It's
certainly not perfect, and the affinity placement plugin is likely better, but
the legacy implementation is certainly lighter weight. I think I would still
default to using the legacy implementation given how simple it is and how it
would certainly solve most use cases for solr users, but I could be convinced
otherwise.
{quote}I propose to re-package the "legacy" strategy as a
{{PlacementPluginFactory}} so that it can be configured in the same way as
other placement plugins.
{quote}
I do whole-heartedly support this. There should be no difference in how the
logic between legacy and new-style placement strategies are defined. This could
be a blocker, and would require a lot less work than changing the default
placement plugin.
What are your thoughts [~ilan] ?
> Make the AffinityPlacementFactory the default placement plugin
> --------------------------------------------------------------
>
> Key: SOLR-15209
> URL: https://issues.apache.org/jira/browse/SOLR-15209
> Project: Solr
> Issue Type: Improvement
> Reporter: Andrzej Bialecki
> Assignee: Ilan Ginzburg
> Priority: Blocker
> Fix For: main (9.0)
>
>
> Currently there's a lot of code in {{Assign}} dealing with the fact that we
> still support the old "legacy" replica assignment as well as the new
> plugin-based placement strategies.
> Furthermore, the "legacy" assignment is now the default even though it's
> neither robust nor optimal, except in the very simple and small clusters.
> Also, providing another plugin-based placement as the default impl. runs into
> a small complication - in the absence of any plugin config the code reverts
> to the "legacy".
> In order to promote the adoption of the new plugin-based placements we should
> make the {{AffinityPlacementFactory}} the new default placement strategy,
> selected when the explicit configuration is missing (and then create it as a
> default configuration in {{PlacementPluginFactoryLoader}}).
> I propose to re-package the "legacy" strategy as a {{PlacementPluginFactory}}
> so that it can be configured in the same way as other placement plugins.
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