+1.
> On 29 Apr 2021, at 17:30, Ilan Ginzburg <[email protected]> wrote:
>
> Expliciting (I think) your suggestion from the Slack thread Jan:
> Add support for a new solr.xml config called something like
> forceDefaultLegacyPlacementStrategy
> Do not add anything in solr.xml
> At runtime:
> If a placement plugin is explicitly configured (existing plugin config in
> ZK), use it,
> If forceDefaultLegacyPlacementStrategy is defined in solr.xml, use LEGACY
> If forceDefaultLegacyPlacementStrategy is not defined in solr.xml, use
> AffinityPlacementFactory
> I like it!
>
> Ilan
>
> On Thu, Apr 29, 2021 at 5:23 PM Jan Høydahl <[email protected]
> <mailto:[email protected]>> wrote:
> Bringing over a discussion from Slack
> <https://the-asf.slack.com/archives/CEKUCUNE9/p1619692977151000>
>
> In 9.0, the old Autoscaling is gone, and instead we have cluster level
> "Placement Plugins", see
> https://nightlies.apache.org/Solr/Solr-reference-guide-main/replica-placement-plugins.html
>
> <https://nightlies.apache.org/Solr/Solr-reference-guide-main/replica-placement-plugins.html>
>
> The default behavour on main branch now is "Legacy", described like this in
> ref-guide:
>
>> Legacy placement simply assigns new replicas to live nodes in a round-robin
>> fashion: first it prepares a sorted list of nodes with the smallest number
>> of existing replicas of the collection. Then for each shard in the request
>> it adds the replicas to consecutive nodes in this order, wrapping around to
>> the first node if the number of replicas is larger than the number of nodes.
>> This placement strategy doesn’t ensure that no more than 1 replica of a
>> shard is placed on the same node. Also, the round-robin assignment only
>> roughly approximates an even spread of replicas across the nodes.
>
> From the Slack discussion there seems to be a willingness to default to one
> of the brand new placement plugins, the AffinityPlacementFactory, which is
> described as
>
>> This plugin implements replica placement algorithm that roughly replicates
>> this Solr 8.x autoscaling configuration defined here
>> <https://github.com/lucidworks/fusion-cloud-native/blob/master/policy.json#L16>:
>>
>> The autoscaling specification in the configuration linked above aimed to do
>> the following:
>> spread replicas per shard as evenly as possible across multiple availability
>> zones (given by a system property),
>> assign replicas based on replica type to specific kinds of nodes (another
>> system property), and
>> avoid having more than one replica per shard on the same node.
>> only after the above constraints are satisfied:
>> minimize cores per node, or
>> minimize disk usage.
>
> So the proposal is to make an instance of AffinityPlacementFactory the
> default, with some universally sane defaults for config - either configured
> in the default solr.xml or in java code.
>
> We can make the formal decision in this email thread - by lazy consensus.
>
> Jan
