Few sources worth to study -

https://dzone.com/articles/solr-vs-elasticsearch-whos-the-leading-open-source
https://www.searchtechnologies.com/blog/solr-vs-elasticsearch-top-open-source-search
​https://www.searchtechnologies.com/blog/elasticsearch-solr-lucene
https://www.elastic.co/guide/en/elasticsearch/reference/6.8/get-started-built-in-users.html
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