What does Solr index do and how to handle its high avaliablity?

Hi, guys,

Can you give me an example of solr usage in dovecot? As far as I know, you can search email easily by MUA like outlook, so which role does solr play?

And based on https://dovecot.org/pipermail/dovecot/2019-April/115575.html

I'm going to use an VIP to host 2 mail servers. Currently, it works in fail over and fail back test except solr index, so how to resolve this?

Is it possible to put dovecot index on shared storage like NFS?

How to if it's a yes. It seems it's under /var/solr/data/dovecot/data/

How to rebuild if I don't put index data on shared storage in case of fail over?

Thanks,