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That's kind of my point, Solr and Lucene put the index in memory.

Can't the index be shared across a cluster? Possibly with multiple replicas for a share to allow for servers to easily go up and down?

It can, but that would be slow and cost would be prohibitive.

Really? It's just a bit of text. RAM is fairly cheap these days.

not the entire index, only what is needed for each query.