When join queries specifies fromIndex result docset might be cached let's say in toIndex filter cache. Usually joins are heavy, take a while to execute. But after result docset is cached it respond fast.
if docs updated in fromIndex and commit occurs (in fromIndex) the next request (for the same join query) misses the toIndex filter cache and takes a while to regenerate this join query entry.
However, we might be ok to use old docset in toIndex if it doesn't harm query latency, and fine if changes in fromIndex will be joined little bit later.
This issue proposes to introduce
{!join cacheEventually=true ..}..
separate user cache for these join query docsets and
update processor to regenerate them explicitly /solr/fromIndex/update?stream.body=%3Ccommit/%3E&post-processor=refresh-join-caches
Improvement
Major