Is it possible for a search head cluster to search multiple index clusters?
A Solo Searchhead can do so, but I'm not sure with the Searchhead Cluster.
Yes you can.
Here is the doc that shows the configs to do so.
https://docs.splunk.com/Documentation/Splunk/8.2.6/Indexer/Configuremulti-clustersearch
https://docs.splunk.com/Documentation/Splunk/8.2.6/DistSearch/SHCandindexercluster
Yes, you can have single SH or SH-cluster searching multiple indexers and indexer clusters as well as single indexer/indexer cluster being searched by multiple SHs. Or any combination of those options.
Yes it should be able to. You can configure multiple multisite indexers to be searchable to a SH cluster.
https://docs.splunk.com/Documentation/Splunk/8.2.6/Indexer/Configuremulti-clustersearch
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